Phenolic Composition, Antioxidant Potential and Health Benefits of Citrus T Peel ⁎ Balwinder Singha, Jatinder Pal Singhb, Amritpal Kaurb, , Narpinder Singhb a P.G

Food Research International 132 (2020) 109114

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Food Research International

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Review Phenolic composition, antioxidant potential and health benefits of citrus T peel ⁎ Balwinder Singha, Jatinder Pal Singhb, Amritpal Kaurb, , Narpinder Singhb a P.G. Department of Biotechnology, Khalsa College, Amritsar 143002, Punjab, India b Department of Food Science and Technology, Guru Nanak Dev University, Amritsar 143005, Punjab, India

ARTICLE INFO ABSTRACT

Keywords: Citrus peel (CP) forms around 40–50% of the total fruit mass but is generally thought to be a waste. However, it Citrus peel is a substantial source of naturally occurring health enhancing compounds, particularly phenolic compounds and Phenolic compounds carotenoids. Phenolic compounds in CP mainly comprise phenolic acids (primarily caffeic, p-coumaric, ferulic Antioxidant activity and sinapic acid), flavanones (generally naringin and hesperidin) and polymethoxylated flavones (notably no- Heath benefits biletin and tangeretin). It has also been noted that CP’s contain more amounts of these compounds than cor- Bioavailability responding edible parts of the fruits. Phenolic compounds present in CP act as antioxidants (by either donation of protons or electrons) and protect cells against free radical damage as well as help in reducing the risk of many chronic diseases. Owing to the more abundance of polyphenols in CP’s, their antioxidant activity is also higher than other edible fruit parts. Therefore, peels from citrus fruits can be used as sources of functional compounds and preservatives for the development of newer food products, that are not only safe but also have health- promoting activities. The present review provides in-depth knowledge about the phenolic composition, antioxidant potential and health benefits of CP.

1. Introduction Fino, 2016). Citrus peel (CP) possibly becomes a source of economic and environmental problems because of fermentation and microbial Citrus fruits are the most abundant fruits grown throughout the spoilage processes (Casquete et al., 2015; Satari & Karimi, 2018). world containing valuable beneficial phytochemicals (Hou et al., 2019; However, CP is a valuable by-product of a citrus industry that can be Satari & Karimi, 2018). They belong to the family Rutaceae and are used in food, pharmaceutical and cosmetic industries (Mahato, Sharma, considered as one of the largest plant species (consisting of 40 different Sinha, & Cho, 2018). The major advantages of utilizing CP industrial species) broadly dispersed in the tropical, subtropical and temperate waste are that it is readily available and a cheap source of biomass areas of earth. Many different varieties and hybrids of citrus have been which can be renewed (Chavan, Singh, & Kaur, 2018). It is a promising produced as a result of natural or artificial crossbreeding. Oranges, source of natural flavonoids (flavanones, flavanone glycosides and grapefruits, mandarins, lemons, and limes are popular for nutritional polymethoxylated flavones) (Cheigh, Chung, & Chung, 2012). More- value and are the main industrialized citrus crops (Satari & Karimi, over, CP is easily available, economical and cost-effective plant-based 2018). The global total production of citrus was estimated at 124.24 source to address lifestyle associated diseases. It is also an excellent million tons in 2016 statistical bulletin (Food and Agriculture source of dietary fiber and minerals. Therefore, CP can be used infood Organization, 2017). Brazil, China, United States, Mexico, India, Spain, products as a functional ingredient for potential health properties and/ Iran, Italy, Nigeria, and Turkey are in the top ten citrus producing or as a substitute for chemical preservatives. countries of the world (Food and Agriculture Organization, 2017). Ci- Many important pharmaceuticals can be produced from agricultural trus fruits are consumed worldwide freshly or in the form of juice. resources, especially from fruit waste. CP is easily available, cost-ef- These fruits also are known for their antioxidant properties that have fective and attractive source of bioactive ingredients of pharmaceutical beneficial effects on the human health (Chen, Tait, & Kitts, 2017). importance (Al-Ashaal & El-Sheltawy, 2011). Bioactive constituents and One-third of the total citrus fruits are processed and thousands of minerals present in CP have a potential to be investigated for their tons of peels produced during citrus juice processing are commonly health enhancing activities in foods (de Moraes Barros, de Castro considered as an agro-industrial waste (Negro, Mancini, Ruggeri, & Ferreira, & Genovese, 2012). It can be primarily used as a drug or as a

⁎ Corresponding author. E-mail address: [email protected] (A. Kaur). https://doi.org/10.1016/j.foodres.2020.109114 Received 12 November 2019; Received in revised form 24 January 2020; Accepted 18 February 2020 Available online 19 February 2020 0963-9969/ © 2020 Elsevier Ltd. All rights reserved. B. Singh, et al. Food Research International 132 (2020) 109114 food supplement. It constitutes nearly about 50 to 65% of the total present in oil glands of CP) (Ballistreri et al., 2019). D-Glucose and L- weight of the fruit. The addition of CP in soy sauce has been effective in rhamnose are the general glyosidic substituents, which affect the taste increasing its functional as well as bioactive properties (Peng et al., of citrus fruits. 2018). Moreover, in an innovative approach, pomegranate peel phe- Waste biomass such as CP can be utilized for the isolation of poly- nolic compounds were encapsulated in orange peel waste and then phenolic compounds by solid–liquid as well as liquid-liquid methods of successfully utilized (without having much sensorial quality loss) in extraction (Banerjee et al., 2017). As citrus fruits are consumed cookies as sources of functional compounds (Kaderides, Mourtzinos, & worldwide, they are viable options for valorization. Scoma, Bertin, Goula, 2020). CP is separated into epicarp or flavedo (colored outer- Zanaroli, Fraraccio, and Fava (2011) documented that digestion was most surface) and mesocarp or albedo (white and soft inner layer). CP done prior to the recovery of polyphenolic compounds from waste contains phenolic compounds (phenolic acids, flavanones, and poly- biomass as it leads to disruption of their structures. It is generally ac- methoxylated flavones), carotenoids and ascorbic acid. Phenolic com- cepted that fruit peel extracts are more effective (because of synergism) pounds show various bioactivities such as antimicrobial, antioxidant, than isolating individual polyphenolic compounds. The conventionally anticancer and anti-inflammatory, antimutagenic and antiallergic used solvents for their extraction include alcohols. However, novel properties (Ferreira, Silva, & Nunes, 2018; Kurup, Nair, & Baby, 2018; methods have been designed for better recoveries (such as microwave, Shetty et al., 2016; Singh et al., 2016; Sridharan, Mehra, Ganesh, & ultrasound and enzyme assisted ones) (Dahmoune et al., 2013; Singh Viswanathan, 2016). In particular, polymethoxylated flavones (tan- et al., 2018a). Dahmoune et al. (2013) reported that microwave assisted geretin and nobiletin) present in CP are of commercial interest due to extraction was more effective in terms of recoveries of polyphenols their pharmacological potential and a wide range of applications in present in CP than ultrasound assisted one. Castro-Muñoz, Yáñez- food industries (Duan et al., 2017; Gao, Gao, Zeng, Li, & Liu, 2018; Fernández, and Fíla (2016) suggested the most novel method for re- Hagenlocher, Feilhauer, Schäffer, Bischoff, & Lorentz, 2017). The re- covering polyphenolic compounds was using membrane technology. trieval of phenolic compounds contained in CP is a valid and an fasci- Valorization of CP waste as a renewable biological resource has a future nating option of citrus by-products valorization (Ferreira et al., 2018; relevance as it kind of reduces the harmful influences on the environ- Satari & Karimi, 2018). Development of reliable methods for extraction ment by the citrus processing industry. Moreover, utilization of en- of high value and useful compounds from citrus peels are of consider- vironmental friendly methods of extraction of bioactive compounds able interest for food processing industry. The present review provides nowadays from peel wastes also has a lot of value (Satari & Karimi, comprehensive information about the phenolic composition, anti- 2018). Additionally, the unit where valorization is to done should be in oxidant potential and health benefits of CP. close proximity to the processing industries owing to the challenging issue of transportation of peel wastes. 2. Methodology The contents and various phenolic compounds identified in CP is presented in Table 1. The structures of these major compounds are The keywords “citrus peel”, “citrus phenolics” and “citrus peel shown in Figs. 1 and 2. Moreover, their three dimensional (3D) ball and phenolics” were analyzed using Scopus and Web of Science online da- stick diagrams are illustrated in Supplementary Fig. 1. The high content tabases. The results showed that since 1950, the number of publications of phenolic compounds reported in CP makes it a potential source of for “citrus peel”, “citrus phenolics” and “citrus peel phenolics” were beneficial phytochemicals and functional food ingredient (Ferreira 4640, 1333 and 353, respectively using Scopus, while 5248, 1663, 506, et al., 2018). respectively using Web of Science up till January 2020. There was a significant increase in the number of publications on citrus species in 3.1. Phenolic acids the recent years. It was found that some review articles were published just recently concerning CP functional ingredients (Chavan et al., 2018; Peels of citrus family fruits are a major source of phenolic acids. The Rafiq et al., 2018; Sharma, Mahato, & Lee, 2019). However, these ar- amounts of free and bound phenolic acids was documented significantly ticles did not provide a comprehensive information on CP phenolic higher in peels than in peeled fruits (Gorinstein et al., 2001). Four compounds, antioxidant activity and health benefits, which necessi- hydroxycinnamic acids (caffeic, p-coumaric, ferulic and sinapic acids) tated the designing of this review article. In addition, recently pub- in free and bound form were reported in the methanolic extracts of lished studies have also been taken into consideration. calamansi (Citrus microcarpa) peel. Calamansi peel contains high concentrations of bound phenolic acids (bound to cell walls in fruit by ester 3. Phenolic compounds in citrus peel and glycosidic linkages) than free phenolic acids (Cheong et al., 2012). The primary phenolic compounds reported in the peel extract of bitter The protective nature of fruits is primarily due to the presence of orange (C. aurantium L.) were phenolic acids (73.8%; 1.03 mg/g) and phytonutrients or phytochemicals. CP contains abundant polyphenolic the most abundant were p-coumaric (24.68%) and ferulic (23.79%) compounds for use as traditional and medicinal purposes. These com- acids (Kurowska & Manthey, 2004). Ferulic acid was quantified as a pounds act as antioxidants and protect cells against free radical damage major phenolic acid and caffeic acid as minor in peels of citrus fruits as well as help in reducing the risk of cancer by inhibiting tumor for- (lemons, oranges and grapefruits) and their levels were significantly mation. Flavonoids and phenolics are dominant groups of bioactive larger than those of peeled fruits (Gorinstein et al., 2001). Ferulic acid compounds that act as primary antioxidants or free radical scavengers was the primary bound phenolic acid and p-coumaric acid was the (Singh, Singh, Kaur, & Singh, 2018a). CP is an abundant source of major free phenolic acid identified in calamansi peel from Malaysia, polyphenols (phenolic acids, flavanones, flavanol, and flavones). In Philippines and Vietnam (Cheong et al., 2012). Ferreira et al. (2018) particular, CP is rich in polymethoxylated flavones that are rarely found ascertained chlorogenic, caffeic and ferulic acids as the major phenolic in other plants. Phenolic compounds are not only present in edible parts acids in mandarin peel extracts. Phenolic acids have also been extracted of citrus fruit, but they have also been reported in non-edible parts using deep eutectic solvents such as choline-chloride, glycerol and (especially citrus peels) with multiple biological functions. Apart from ethylene glycol in orange peels. Ferulic acid was the primary phenolic CP, pulp fraction also contains phenolic compounds but these are acid in this peel, with lesser levels of ρ-coumaric acid and gallic acid comparatively lesser in quantity. In addition, there is also a variation in (Ozturk, Parkinson, & Gonzalez-Miquel, 2018). the composition of different phenolic compounds in the two. The pulp The level of caffeic, p-coumaric, ferulic and sinapic acids reported in fraction primarily contains flavonoids in the form of glycosides, while sour orange peel was 0.229, 0.193, 1.580 and 0.954 mg/g dry weight CP is abundant in the less polar flavanone as well as flavone aglycons basis (DW) and in bergamot peel, the level reported was 0.006, 0.071, and polymethoxyflavones, (most hydrophobic among flavonoids and 0.036 and 0.030 mg/g DW, respectively (Bocco, Cuvelier, Richard, &

2 .Snh tal. et Singh, B. Table 1 Total phenolic and flavonoid content reported and main phenolic compounds identified in fruit peel of different citrus species.

Peel source Botanical Name Extract Phenolic content Flavonoid content Main identified compounds References

Sweet orange C. sinensis EAE 66.9 mg GAE/100 g – – Anagnostopoulou et al. (2006) Sour orange C. aurantium ME 2.95 mg/g DW – Caffeic, p-coumaric, ferulic and sinapinic acids, naringin, neohesperidin and Bocco et al. (1998) Bergamot C. bergamia 0.41 mg/g DW neoeriocitrin Lemon C. limon AE 222.76 mg GAE/ – – Casquete et al. (2015) 100 g Lime C. latifolia 362.98 mg GAE/ 100 g Mandarin C. reticulata 530.05 mg GAE/ 100 g Sweet orange C. sinensis 284.19 mg GAE/ 100 g Limau purut C. hystrix EE 1291.8 mg GAE/ – – Chan et al. (2009) 100 g DW Orange (fresh peel) C. sinensis ME 39.45 mg GAE/g DW 12.95 mg CE/g DW Caffeic acid, p-coumaric acid, naringin, neohesperidin , kaempferol andrutin Chen et al. (2011) Orange (peel dried at100 oC) 65.72 mg GAE/g DW 13.79 mg CE/g DW Orange (California) C. reticulata AE 51.8 mg GAE/g 31.9 mg/g Narirutin, hesperidin, nobiletin, and tangeritin Chen et al. (2017) Orange (Guangxi) 42.0 mg GAE/g 26.0 mg/g Orange (Zhejiang) 46.3 mg GAE/g 23.2 mg/g Orange (Sichuan) 43.8 mg GAE/g 14.0 mg/g Orange (Xinhui) 50.2 mg GAE/g 25.0 mg/g Novel orange C. sinensis EE 559.32 mg TAE/ 84.03 mg RE/100 g – El-aal and Halaweish (2010) 100 g FW FW Baldi orange 560.55 mg TAE/ 87.72 mg RE/100 g 100 g FW FW

3 Mandarin orange C. reticulata HEE 12.2 mg GAE/g – Hesperidin, naringin, tangeritin, and rutin, quercetin, chlorogenic acid, Ferreira et al. (2018) AE 10.4 mg GAE/g caffeic acid, ferulic acid, naringenin and hesperitin SPE-HEE 53.5 mg GAE/g SPE-AE 56.4 mg GAE/g Mandarin (fresh peel) C. reticulata 2.91 g CA/100 g DM – – Ghanem et al. (2012) Lemon (fresh peel) C. limon 2.45 g CA/100 g DM Thompson navel (fresh peel) C. sinensis 1.89 g CA/100 g DM Sweet orange (Washington Navel) C. sinensis ME 160.3 mg GAE/g 23.3 mg QE/g – Ghasemi et al. (2009) Orange (Ponkan) C. reticulata 172.1 mg GAE/g 5.2 mg QE/g Satsuma Mandarin (Sugiyama) C. unshiu 195.5 mg GAE/g 19.8 mg QE/g Grapefruit C. paradisi 222.2 mg GAE/g 23.2 mg QE/g Sour orange C. aurantium 223.2 mg GAE/g 7.7 mg QE/g Lemon C. limon 131.0 mg GAE/g 16.2 mg QE/g Lemon C. limon EE 190 mg ChA/100 g – Ferulic, sinapic, p-coumaric and caffeic acids Gorinstein et al. (2001) FW

Orange C. reticulata 179 mg ChA/ 100 g Food ResearchInternational132(2020)109114 FW Grapefruit C. paradisi 155 mg ChA/100 g FW Grapefruit C. paradisi ME 55.88 mg GAE/g 2.29 mg CE/g – Guimarães et al. (2010) Lemon C. limon 87.77 mg GAE/g 15.96 mg CE/g Lime C. aurantifolia 124.63 mg GAE/g 13.61 mg CE/g Sweet orange C. sinensis 79.75 mg GAE/g 3.97 mg CE/g Kinnow (non-heated) C. nobilis × C. deliciosa ME 3583.5 µg/g DW 5037.1 µg/g DW Gallic, p-hydroxybenzoic, vanillic, p-coumaric and ferulic acids, catechin, Hayat et al. (2010) naringin,, naringenin, hesperidin, rutin and kaempferol Kinnow (microwave heated, 250 W, 3482.5 µg/g DW 6375.9 µg/g DW 10 min) (continued on next page) .Snh tal. et Singh, B. Table 1 (continued)

Peel source Botanical Name Extract Phenolic content Flavonoid content Main identified compounds References

Baladi orange C. sinensis ME 165.38 mg/g 28.36 µg/g – Hegazy and Ibrahium (2012) EE 169.56 mg/g 29.75 µg/g EAE 58.27 mg/g 24.92 µg/g ACE 49.20 mg/g 18.21 µg/g HE 21.76 mg/g 8.27 µg/g Ponkan mandarin C. reticulata ME 40.8 mg GAE/g 7.62 mg CE/g Nobiletin, tangeretin, naringin, neohesperidin, and hesperidin Ho and Lin (2008) Ponkan mandarin (peel heated at 54.1 mg GAE/g 9.07 mg CE/g 100 °C for 180 min) Ponkan C. reticulata ME – 11.1 mg RE/g Hesperidin naringin neohesperidin, narirutin,nobiletin sinensetin and Huang and Ho (2010) Tonkan C. tankan 10.5 mg RE/g tangeretin Lemon C. limon 11.9 mg RE/g Satsuma Mandarin C. unshiu EE 71.8 µM – p-cinnamic, ferulic, isoferulic, 5-hydroxyvaleric, vanillic 5-hydroxyvaleric Jeong et al. (2004) Satsuma Mandarin (peel heated at EE 171.0 µM and 2-oxybenzoic acids, 2,3-diacetyl-1-phenyl naphthalene and p- 150 °C for 40 min) WE 84.4 µM hydroxybenzal doxime Satsuma Mandarin (peel heated at WE 177.6 µM 150 °C for 40 min Orange C. sinensis EE-UAE 275.8 mg GAE/100 g – Naringin and hesperidin Khan et al. (2010) FW Yuzu C. junos EE 47.8 mg/100 g – Rutin, naringin, hesperidin, quercetin and tangeretin Kim et al. (2013) Sweet orange (Washington Navel) C. sinensis MWE 9.61 mg GAE/g DW 1.29 mg QE/g DW – Lagha-Benamrouche and Sweet orange (Thomson Navel) 25.60 mg GAE/g DW 1.28 mg QE/g DW Madani (2013) Sweet orange (Sanguinelli) 14.95 mg GAE/g DW 0.91 mg QE/g DW Sweet orange (Double fine) 12.28 mg GAE/g DW 0.71 mg QE/g DW Sweet orange (Portugaise) 14.94 mg GAE/g DW 0.29 mg QE/g DW Sweet orange (Jaffa) 14.31 mg GAE/g DW 0.56 mg QE/g DW

4 Sour orange 31.62 mg GAE/g DW 1.17 mg QE/g DW (Bigarade) Tahiti lime C. latifolia AE-UAE 74.80 mg GAE/g – hesperidin, neohesperidin, diosmin, nobiletin and tangeritin Londoño-Londoño et al. (2010) Sweet orange C. sinensis 66.36 mg GAE/g Tangerine C. reticulata 58.68 mg GAE/g Penggan C. reticulata ME-UAE 19.12 mg GAE/g DW – – Ma, Chen, et al. (2008) Orange C. sinensis AAE-MAE 12.09 mg GAE/g DW – Gallic acid, chlorogenic acid, caffeic acid p-coumaric acid, ferulic acid, rutin, Nayak et al. (2015) AAE-UAE 10.35 mg GAE/g DW quercetin, catechin AAE-ASE 6.26 mg GAE/g DW AAE-CSE 10.21 mg GAE/g DW Meyer lemon C. meyeri ME 1882 µg/g FW – Poncirin, didymin, rutin, diosmin, isorhoifolin, neohesperidin, hesperidin, Ramful et al. (2010) Tangor C. reticulata × C. sinensis 7667 µg/g FW neoeriocitrin and narirutin Kinnow Mandarin C. nobilis × C. deliciosa ME-CSE 28.40 mg GAE/g – Gallic, chlorogenic, ferulic, coumaric and caffeic acid, catechins epicatechins Safdar et al. (2017) ME-UAE 32.48 mg GAE/g hesperidin naringenin quercetin and kaempferol Orange Citrus sinensis L. Osbeck HEE-UAE 3.9 mg GAE/g DW 17.6 mg QE/g DW Gallic acid, p-coumaric acid, trans-ferulic acid, naringin, hesperidin, rutin, Gómez-Mejía et al. (2019)

myricetin, resveratrol and quercetin Food ResearchInternational132(2020)109114 Lemon Citrus lemon L. HEE-UAE 5.9 mg GAE/g DW 18 mg QE/g DW

Clementine Citrus × clementina HEE-UAE 5.5 mg GAE/g DW 16.5 mg QE/g DW

AE: aqueous extract; ACE: acetone extract; AAE: aqueous acetone extract; HE: hexane extract; EAE: ethyl acetate extract; EE: ethanol extract; ME: methanol extract; WE: Water extract; HEE: hydro-ethanolic extract; SPE: Solid phase extraction; ASE: accelerated solvent extraction; CSE: conventional solvent extraction; MWE: methanol water extract; MAE: microwave-assisted extraction; UAE: ultrasound-assisted extraction; CA: caffeic acid; CE: Catechin equivalents; ChA: chlorogenic acid,; GAE: gallic acid equivalents; QE: Quercetin equivalents; RE: Rutin equivalents; FW: fresh weight basis; DW: dry weight basis. B. Singh, et al. Food Research International 132 (2020) 109114

Fig. 1. Chemical structures of gallic acid (a), vanillic acid (b), caffeic acid (c), p-coumaric acid (d), ferulic acid (e), sinapic acid (f), chlorogenic acid (g), kaempferol (h), quercetin (i), rutin (j), naringenin (k) and naringin (l) present in citrus peels.

Berset, 1998). The level of chlorogenic, caffeic and ferulic acid in citrus quantified from kinnow peel extract (Safdar et al., 2017). Ferulic acid hybrids peels from China were reported in the range of 8.8–18.7, was identified as the most abundant (102.13 µg/g) and caffeic acidas 4.5–29.9, and 14.4–97.8 µg/g, respectively (He et al., 2011). Kurowska the least abundant (2.43 µg/g) phenolic acid in kinnow peel extract. and Manthey (2004) identified ferulic, p-coumaric, chlorogenic, ros- Phenolic acid content and composition varies among different citrus marinic, trans-2-hydroxycinnamic, gallic, vanillic, syringic and trans- species. Phenolic acid content in hydrolyzed sour orange and bergamot cinnamic acids at a level of 0.33, 0.34, 0.12, 0.08, 0.04, 0.03, 0.02, peel extracts was reported to be 2.95 and 0.41 mg/g DW, respectively 0.02, 0.02 mg/g, respectively in the peel extract of bitter orange. Gallic, (Bocco et al., 1998). The highest amount of total phenolic acids was chlorogenic, ferulic, coumaric and caffeic acid were identified and reported in Ponkan (C. reticulata Blanco), Tonkan (C. tankan Hayata)

Fig. 2. Chemical structures of hesperitin (a), hesperidin (b), neohesperidin (c), neoeriocitrin (d), poncirin (e), nobiletin (f), tangeritin (g), sinensetin (h) and narirutin (i) present in citrus peels.

5 B. Singh, et al. Food Research International 132 (2020) 109114 and Murcott (C. reticulate × C. sinensis) peels with a level of 914, 946 citrus fruits. Total phenolic content (TPC) is higher in peels than that in and 852 µg/g, respectively (Wang, Chuang, & Hsu, 2008). Lemon peel pulp or juice of citrus fruits. TPC was reported in the range of contained higher amounts of ferulic, sinapic, p-coumaric and caffeic 131.0–223.2 mg gallic acid equivalents [GAE]/g for fruit peels and it is acids (44.9, 42.1, 34.9 and 14.2 mg/100 g) than orange (39.2, 34.9, higher than those of edible fruit tissues of different citrus species 27.9 and 9.5 mg/100 g) and grapefruit (32.3, 31.9, 13.1 and 5.6 mg/ (Ghasemi, Ghasemi, & Ebrahimzadeh, 2009). Citrus peels of lima or- 100 g) peels (Gorinstein et al., 2001). They reported that the sour orange, pera orange, tahiti lime, sweet lime and ponkan mandarin pre- ange peel contained ten-fold more sinapic and ferulic acid and five-fold sented 2.5 to 4 times higher Folin–Ciocalteu reducing capacity (values more p-coumaric and caffeic acids than bergamot peel. Caffeic ranging from 310.18 to 575.06 mg CE/100 g fresh weight basis [FW]) (0.31 mg/g) and p-coumaric (0.67 mg/g) were the two phenolic acids compared to pulps (109.16 to 118.94 mg CE/100 g FW) (de Moraes reported in fresh orange peel (Chen, Yang, & Liu, 2011). The contents of Barros et al., 2012). The Folin–Ciocalteu reducing capacity is connected chlorogenic, p-coumaric, ferulic, sinapinic and caffeic acids were re- with the amounts of phenolic compounds and ascorbic acid. ported in the range of 145–339, 41.7–346, 30.3–150, 10.1–178 and Peels of lemons, oranges and grapefruits contained TPC of 190, 179 3.06–80.0 mg/g, respectively in fruit peels of eight citrus varieties and 155 mg chlorogenic acid (ChA)/100 g FW, respectively, while the (Wang et al., 2008). Ferulic, sinapic acids and their ester derivatives edible portion (peeled fruits) of lemons, oranges and grapefruits con- (dihydroxycoumarin, dihydroxycoumarin-O-sinapoyl-glucose ester, and tained TPC of 164, 154 and 135 mg ChA/100 g FW, respectively feruloyl glucoside ester) were identified in orange peel (Kanaze et al., (Gorinstein et al., 2001). Methanol extract of grapefruit, lemon, lime 2009). In case of kumquat peels, the primary identified phenolic acids and sweet orange peels contained TPC of 55.88, 87.77, 124.63 and were p-hydroxybenzoic acid, vanillic, protocatechuic, chlorogenic and 79.75 mg GAE/g, respectively, whereas methanol extract of juice con- sinapic acid (Al-Saman, Abdella, Mazrou, Tayel, & Irmak, 2019). tained TPC of 8.93, 8.43, 7.51 and 13.43 mg GAE/g, respectively Drying temperature changes the level of phenolic acids. After drying (Guimarães et al., 2010). TPC in ethanol peel extracts of Baladi and of orange peels at 100 °C, the amounts of caffeic and p-coumaric acids Novel orange was reported as 559.3 and 591.7 mg tannic acid enhanced by 5 (1.53 mg/g) and 2 (1.38 mg/g) folds, respectively (Chen equivalents [TAE]/100 g FW, respectively (El-aal & Halaweish, 2010). et al., 2011). Storage of citrus peels also enhances the level of phenolic TPC in fresh mandarin (C. reticulata), lemon (C. limon) and Thompson acids. The level of eight phenolic acids (caffeic, p-coumaric, m-cou- navel (C. sinensis) peel was reported as 2.91, 2.45 and 1.89 g caffeic maric, ferulic, vanillic, trans-cinnamic syringic and salicylic) increased acid equivalents [CAE]/100 g DW, respectively (Ghanem, Mihoubi, in Chenpi (peels of C. unshiu, C. reticulate and C. tachibana) after 3 years Kechaou, & Mihoubi, 2012). TPC in peels of six sweet orange (C. sinensis of extended storage (Choi et al., 2011). Gallic, protocatechuic and β- L. cv. Washington Navel, Thomson Navel, Sanguinelli, Double fine, hydroxybenzoic acids were identified in long-term (3 years) stored Portugaise and Jaffa) and one sour orange (C. aurantium L. cv. Bi- Chenpi, whereas they were not detected in regular (1 year) stored garade) varieties cultivated in Algeria was reported as 9.61, 25.60, chenpi. Extraction method significantly affects the level of phenolic 14.95, 12.28, 14.94, 14.31 and 31.62 mg GAE/g DW, respectively acids in CP extracts. The amounts of seven phenolic acids in Satsuma (Lagha-Benamrouche & Madani, 2013). Ramful, Bahorun, Bourdon, mandarin (C. unshiu Marc.) peel extract obtained by UAE were sig- Tarnus, and Aruoma (2010) reported TPC ranging from 188.2 to nificantly larger than the extracts obtained by the conventional ma- 766.7 mg GAE/100 g FW in peels of 21 citrus varieties. Lime (C. lati- ceration extraction (Ma, Chen, Liu, & Ye, 2008). The content of ferulic, folia) peel contained a higher content of phenolic compounds as com- p-coumaric, sinapic, caffeic, p-hydroxybenzoic, vanillic and proto- pared to sweet orange (C. sinensis) and tangerine (C. reticulata) peel catechuic acids reported in Satsuma mandarin (C. unshiu Marc.) peel (Londoño-Londoño et al., 2010). These authors reported that CP ob- extracts obtained by ultrasonic treatment of 8 W at 30 °C for 10 min tained from lime, sweet orange and tangerine (C. reticulata) contained were 1513.2, 140.8, 132.7, 64.2, 34.1, 34.1 and 15.8 µg/g DW, re- TPC of 74.80, 66.36 and 58.68 mg GAE/g, respectively. TPC in lime, spectively, while with maceration extraction (40 °C for 8 h) were 763.5, lemon, sweet orange and mandarin peel was reported as 362.98, 63.15, 132.2, 31.7, 23.5, 29.9 and 20.68 µg/g DW, respectively (Ma, 222.76, 284.19 and 530.05 mg GAE/100 g FW peel extracts, respec- Chen, et al., 2008). The optimal ultrasound conditions for each phenolic tively (Casquete et al., 2015). acids is different and it is attributed to differences in chemical structure CP dried at higher temperatures (90 and 100 °C) contained around and stability of phenolics and combined effects of ultrasonic variables two-fold more TPC than the fresh peel. TPC reported in methanol ex- (Ma, Chen, Liu, & Ye, 2009). tracts of fresh and dried (at 100 °C) orange peel was 39.45 and Microwave-assisted extraction is a fast as well as a reliable method 65.72 mg GAE/g, respectively (Chen et al., 2011). Chan, Lee, Yap, for identification and quantification of phenolic acids. Microwaves re- Mustapha, and Ho (2009) optimized conditions for extraction of poly- duce extrication time, solvent requirement as well as energy utilization phenols from limau purut (C. hystrix) peel using response surface in the extraction process. Total content of free, ester-bound, glycoside- methodology (RSM). The results of the study showed that ethanol bound and insoluble-bound phenolic acid compounds in kinnow peel concentration, extraction temperature and extraction time had a sig- extract obtained by microwave-assisted extraction method were nificant effect on TPC of limau purut peels. Pressure treatment 1162.84, 2137.06, 265.53 and 213.95 µg/g DW, respectively (Hayat (300 MPa for 3 min) of citrus peels before extraction increased the et al., 2009). The total content of gallic, p-hydroxybenzoic, vanillic, p- values of TPC to 397.21, 266.23, 288.16 and 587.28 mg GAE/100 g for coumaric and ferulic acids in the microwave-assisted Kinnow mandarin lime, lemon, sweet orange and mandarin peel, respectively (Casquete peel extract is 175.22, 72.19, 315.69, 830.27 and 2386.0 µg/g DW, et al., 2015). TPC in 3 different portions (free phenolic acids, soluble respectively (Hayat et al., 2009). The amount of phenolic acids in- phenolic acid esters and insoluble-bound phenolic acids) of regularly creases in free fraction and decreases in ester and glycoside-bound stored chenpi was reported to be 1.77, 1.71 and 0.64 GAE g/100 g and fractions of Kinnow peel with the enhancement in microwave power in long-term stored chenpi, it was 3.22, 2.21 and 0.60 GAE g/100 g, (Hayat et al., 2010). This indicated that the bound phenolic acids are respectively (Choi et al., 2011). Higher TPC reported in free phenolic liberated in microwave-assisted extraction process due to heating. The acids and soluble phenolic acid esters fractions of long-term stored content of gallic acid, chlorogenic acid, caffeic acid, p-coumaric acid chenpi might be due to hydrolysis by extracellular enzymes (cellulases, and ferulic acid in the microwave assisted C. sinensis peel extract was laccases, peroxidases) or degradation by enzymatic processes of non- 142.69, 1388.13, 815.95, 124.95 and 1455 µg/g, respectively (Nayak extractable bound polyphenols over the extended time of storage. et al., 2015). TPC varies among peels of different citrus fruits grown at different geographical locations. The variability in TPC of CP is attributed to 3.1.1. Total phenolic content agro-climatic conditions, harvesting time and different solvents used in CP contains more phenolic compounds than the edible parts of the extraction process. TPC reported in dried orange peels from USA

6 B. Singh, et al. Food Research International 132 (2020) 109114

(California) and China (Guangxi) was 51.8 and 42.0 mg GAE/g, re- DW (Ma, Ye, et al., 2008). The optimized ultrasonic-assisted extraction spectively (Chen et al., 2017). The solvents used for extraction also conditions significantly increase the amounts of polyphenols in CPex- have a considerable effect on the yield of phenolic compounds from tracts (Ma, Chen, et al., 2008). peels. Hegazy and Ibrahium (2012) evaluated the efficiency of different organic solvents (methanol, ethanol, dichloromethane, acetone, hexane 3.2. Flavonoids and ethyl acetate) in extraction of phenolic compounds from the orange peel and reported highest TPC in ethanol extract (169.5 mg/g), fol- Flavonoids are naturally occurring low molecular weight phenolic lowed by methanol (165.3 mg/g) extracts. Heating of peel powder compounds containing 2 aromatic rings (A & B) bound by a 3-carbon before solvent extraction increases the level of phenolic compounds in bridge (C6–C3–C6 structure). These compounds are one of the major CP extract. Ho and Lin (2008) reported a significant increase in TPC of bioactive substances present in citrus fruits. Citrus fruits accumulate a Ponkan mandarin (Citrus reticulata) peel from 40.8 mg GAE/g of extract substantial amount of flavonoids. The flavonoids include flavonols, (non-heated control) to 54.1 mg GAE/g of the extract by heating peel flavanones, flavones and anthocyanins (only in blood red oranges). The powder at 100 °C for 180 min before solvent extraction. TPC in ethanol level and composition of flavonoid changes with development and and water extract of C. unshiu peels was reported as 71.8 and 84.4 µM, maturation of citrus fruits and it varies among different citrus species. respectively in non-heated control and the content was increased to As already mentioned, CP represents nearly about 40–50% of the fruit 171.0 and 177.6 µM, respectively by heating peel powder at 150 °C for mass and is a rich source of naturally occurring flavonoids. Flavones-O- 40 min prior to extraction (Jeong et al., 2004). Electrolyzed water was glycosides, flavones-C-glycosides, flavonols, and flavanones are the four utilized as an extraction solvent for phenolic compounds in tangerine main groups of flavonoids identified in lemon peel(Baldi, Rosen, peel as opposed to conventional solvents such as ethanol and methanol. Fukuda, & Ho, 1995). The flavonoids of a sweet orange peel include TPC of tangerine peel estimated using this technique was 4.23 mg/g flavanone glycosides, flavones and flavonols (Anagnostopoulou, DW of extract and was higher than conventional extraction methods Kefalas, Papageorgiou, Assimopoulou, & Boskou, 2006). The flavonoids (Soquetta et al., 2019). found in sweet orange peel (C. sinensis) are polymethoxylated flavones, Microwave heating increases the level of free phenolic acids in CP C-glycosylated flavones, O-glycosylated flavones, O-glycosylated fla- by liberating the bound phenolic compounds. TPC in microwave de- vanones and flavonols (Anagnostopoulou, Kefalas, Kokkalou, hydrated Thompson navel peels at a power level of 600 W was 2.86 g Assimopoulou, & Papageorgiou, 2005). Flavanones are contained in the caffeic acid/100 g DM compared to 1.89 g caffeic acid/100 gDMin glycoside (hesperidin and narirutin) or aglycone (hesperetin and nar- fresh peel (Ghanem et al., 2012). Aqueous-ethanolic mixtures and ingenin) forms. Flavanone glycosides (poncirin, dydimin, naringin, aqueous extracts of mandarin (C. reticulata Blanco) peel contained TPC hesperidin, neohesperidin, neoeriocitrin and narirutin), flavonol gly- of 12.2 and 10.4 mg GAE/g FW, respectively and their solid phase coside (rutin) and flavone glycosides (rhoifolin, isorhoifolin, and extraction enriched phenolic fractions contained TPC of 53.5 and diosmin) were reported in the flavedo extracts of different citrus fruits 56.4 mg GAE/g FW, respectively (Ferreira et al., 2018). Solid phase (Ramful et al., 2010). Flavonoids are expressed by 2 main categories of extraction proved efficient as it increased the concentration of phenolic compounds (polymethoxylated flavones and glycosylated flavanones) compounds by 5.14 and 4.62 times in hydro-ethanolic and aqueous in most of the citrus fruit peels (Gao et al., 2018). extracts. TPC in Kinnow mandarin peel extracts obtained by using mi- The flavonoid contents in CP changes with the maturation of citrus crowave-assisted, ultrasonic and rotary extraction techniques was re- fruits. The amounts of flavanones (naringin, naringenin, hesperidin, ported as 3779.37, 3796.21 and 2816.82 µg/g DW, respectively (Hayat hesperetin), flavonol glycoside (rutin) and polymethoxylated flavones et al., 2009). TPC increased significantly in free phenolic fraction of (nobiletin, and tangeretin) were determined by high performance liquid kinnow peel from 1147.6 µg/g DW (untreated kinnow peel powder) to chromatography (HPLC) analysis in peels of mature and immature 1345.1 µg/g DW (microwave heated, 250 W, 15 min), whereas in fruits of 20 Citrus plants grown on Jeju Island, Korea (Choi, Hwang, Ko, glycoside-bound phenolic fraction the value decreased from 257.1 to Park, & Kim, 2007). They reported that the peels of immature citrus 126.3 µg/g DW under same treatment process (Hayat et al., 2010). fruits contain a higher content of hesperidin, naringin, nobiletin and Microwave-assisted extraction enhances the yield of TPC which is at- tangeretin than the peels of mature citrus fruits. The main flavonoids tributed to its penetration ability and interaction of microwaves with identified in mandarin (C. reticulata Blanco) peel extracts were he- polar molecules present in the cell matrix, thereby resulting into speridin, naringin, tangeritin, and rutin, which represented approxi- heating and breakdown of cell walls and release of phenolic compounds mately 86% of hydro-ethanolic and aqueous extracts and 71% of the (Nayak et al., 2015). Microwave-assisted extraction as a green tech- solid phase extraction improved extracts (Ferreira et al., 2018). The nology provides better recovery of phenolic compounds from CP ex- other components identified include flavonol (quercetin) and flava- tracts. TPC in C. sinensis peels using microwave-assisted, ultrasound- nones (naringenin and hesperitin). In C. unshiu fruit peel, the total assisted, conventional solvent and accelerated solvent methods were amount of flavonoids measured by HPLC–UV method was 2456.1 mg/ reported as 12.09, 10.35, 6.26 and 10.21 mg GAE/g DW, respectively kg fruit peel weight and it included flavanones (89.1%) and flavones (Nayak et al., 2015). (10.9%) as major and minor components (Kim et al., 2011). The fla- The ultrasound-assisted techniques are environment-friendly and vonoids represented 23.02% (0.33 mg/g) of the Tunisian bitter orange very effective for extraction of phenolic compounds from CP incom- peel extract and the main flavonoids are rutin (9.91%), naringin parison to conventional methods. Khan, Abert-Vian, Fabiano-Tixier, (5.23%), catechin (3.17%) and epicatechin (2.77%) (Kurowska & Dangles, and Chemat (2010) optimized conditions for ultrasound-as- Manthey, 2004). It was found that hesperidin was the most abundant sisted (temperature 40 °C, sonication power 150 W and 80% v/v flavonoid identified in CP of different species, and most ubiquitous was ethanol) extraction of polyphenols from orange peel and documented naringin (Gómez-Mejía, Rosales-Conrado, León-González, & Madrid, high yield (TPC: 275.8 mg GAE/100 g FW) in a shorter period of time 2019). In an another study, eighteen flavonoids were identified ina compared to a conventional procedure. Ultrasonic-assisted extraction recent study carried on sweet lime peel, with hesperidin being the most reduces extraction time and significantly improves extraction efficiency abundant flavonoid. In addition, procyanidin B and luteolin were also of phenolic compounds from CP (Londoño-Londoño et al., 2010; Ma, elucidated in the study (Buyukkurt, Guclu, Kelebek, & Selli, 2019). Ye, et al., 2008; Ma et al., 2009). Ultrasonic-assisted extraction conditions were optimized (ultrasonic power of 42–45 W, ultrasonic time of 3.2.1. Flavanones 23–25 min and extraction temperature of 31–34 °C) for extraction of CP contains more flavanone glycosides (naringin, neohesperidin, phenolic compounds from Penggan (C. reticulata) peel and the mean and neoeriocitrin) than seeds (Bocco et al., 1998). The composition of values of highest TPC reported in the experiments was 19.12 mg GAE/g glycosylated flavanones in peels varies among different citrus species.

7 B. Singh, et al. Food Research International 132 (2020) 109114

The total glycosylated flavanone content in methanol extracts of sour 34.65, 19.49, 18.85, 11.34 mg/g FW, respectively in flavedo extracts of orange (C. aurantium), lemon (C. lemon) and bergamot (C. bergamia) mandarin (Ramful et al., 2010). Lu, Zhang, Bucheli, and Wei (2006) peels was reported as 22.30, 16.55 and 13.55 mg/g whereas in seeds it studied the content of citrus flavonoids in peels of many Chinese citrus was 1.02, 2.15 and 3.28 mg/g, respectively (Bocco et al., 1998). fruits and reported that the peels of C. paradisi (1.04%) and C. aur- Grapefruit, lemon, Ponkan (C. reticulata Blanco) and Tonkan (C. tankan antium (2.76%) were the best source of neohesperidin. The level of Hayata) peel extracts contained the total flavanone glycosides at a level neohesperidin was reported in the range of 0.02–0.34 mg/g in peels of of 106, 103, 68.4 and 63.7 mg/g, respectively (Huang & Ho, 2010). eight varieties of citrus fruits (Wang et al., 2008). The level of naringin Orange peel contained a complex mixture of flavanones and included and neohesperidin in citrus hybrids peels were found in the range of hesperetin (aglycone), hesperidin and neohesperidin, while tangerine 20.2–1588 and 13.1–984 µg/g, respectively (He et al., 2011). peel contained hesperidin and neohesperidin (Londoño-Londoño et al., Hesperidin is the most abundant flavonoid noted in dried peels of 2010). Narirutin, hesperidin, isosakuranetin, and eriocitrin are the orange collected from USA (California) and China (Guangxi) with a flavanone glycosides identified in orange peel(Manthey & Grohmann, level of 26.81 and 20.99 mg/g, respectively (Chen et al., 2017). The 2001). Kanaze et al. (2009) identified six flavanone-7-O-glycosides mean levels of hesperidin and naringin in the orange peel was reported (hesperetin-7-O-rutinoside, hesperetin-7-O-neohesperidoside, nar- as 2.16 and 0.05 mg/g dry peel, respectively (Kanaze et al., 2009). ingenin-7-O-biglucoside, naringenin-7-O-neohesperidoside, 5-methox- Naringin, hesperidin and poncirin (1328.4, 800.7 and 58.5 mg/kg FW, yflavanone-7-O-rhamno-glucoside, and isosakuranetin-7-O-rutinoside) respectively) are the major flavanones reported in C. unshiu fruit peel in the orange peel. Li, Gu, Dou, and Zhou (2007) identified two flava- (Kim et al., 2011). The level of naringin and hesperidin reported in gold nones (naringenin-7-O-α-glucoside and hesperetin-7-O-α-glucoside) in lotion (peel extract of six citrus fruits marketed in Japan for cosmetic the peel of C. changshan-huyou collected from China by HPLC–mass purpose) was 253.6 and 104.7 mg/ml, respectively (Lai, Li, Liu, et al., spectrometry (MS) and Nuclear magnetic resonance spectroscopy 2013). The levels of hesperidin and naringin in Yuzu (C. junos) peel (NMR). Hesperitin-7-rutinoside (hesperidin), eriodictyol-7-rutinoside ethanol extract is 36.3 and 11.6 mg/100 g, respectively (Kim et al., (eriocitrin) and naringenin-7-rutinoside (narirutin) were the main fla- 2013). Hesperidin is the dominant flavanone glycoside in the methanol vanone glycosides identified in ethyl acetate extract of lemon peel extracts of lemon, Ponkan and Tonkan (94.0, 65.5 and 58.5 mg/g, re- (Baldi et al., 1995). The flavanones identified in bergamot (C. bergamia spectively) peels and naringin (98.0 mg/g) is the main flavanone gly- Risso) peel were naringin, neoeriocitrin, neohesperidin, hesperetin coside identified in methanol peel extract of Grapefruit (Huang & Ho, mono-rhamnoside, naringenin mono-rhamnoside, eriodictyol mono- 2010). Hesperidin was found to be the most abundant (837.4–7995 µg/ rhamnoside, narirutin and eriocitrin having content of 1104.6, 953.9, g) flavanone in eight citrus hybrids peels (He et al., 2011). Hesperidin 919.6, 455.3, 260.5, 137.9, 66.8 and 31.2, mg/100 g, respectively was present in a considerable amount (5.97%) in orange peel extract (Mandalari et al., 2006). (Al-Ashaal & El-Sheltawy, 2011). The highest content of hesperidin Polymethoxy flavanones identified in sweet orange peel were (5.86–6.25%) was quantified in the fresh fruit peels of three C. unshiu 5,6,7,4′-tetramethoxyflavanone and 5-hydroxy-6,7,8,3′,4′-penta- cultivars among different Chinese citrus fruits (Lu et al., 2006). He- methoxyflavanone (Li, Lo, & Ho, 2006). Sawalha, Arráez-Román, speridin was the most widely distributed flavanone glycoside in the Segura-Carretero, and Fernández-Gutiérrez (2009) characterized and peels of mature and immature citrus fruits (Choi, Hwang, et al., 2007). quantified flavanone glycosides (naringin, narirutin, hesperidin and Hesperidin was the major flavanone glycoside identified with contents neohesperidin) from methanol extracts of sweet and bitter orange peels ranging from 83.4 to 234.1 mg/g FW in flavedo extracts of different using capillary electrophoresis coupled to mass spectrometry. They citrus species (Ramful et al., 2010). The level of hesperidin and naringin reported narirutin (26.9 mg/g) and hesperidin (35.2 mg/g) as the major (flavonoid glycosides) was reported in the range of 50.13–100.52 and flavanone glycosides in sweet orange peels and naringin (5.1 mg/g)and 0.21–4.29 mg/g, respectively in twelve samples of C. reticulatae Peri- neohesperidin (7.9 mg/g) as the major flavanone glycosides in bitter carpium (dried ripe peel of C. reticulata Blanco) gathered from the orange peels. Wang et al. (2008) studied flavanone (hesperidin, neo- major citrus growing regions in China (Liu et al., 2013). In an another hesperidin and naringin) content of eight citrus peels and reported study, hesperidin was identified as the most abundant (92.94 µg/g) higher levels of hesperidin in Ponkan (29.5 mg/g), Tonkan (23.4 mg/g) flavonoid in kinnow peel extract (Safdar et al., 2017). and Liucheng (20.7 mg/g) peels compared with the rest. Moreover, they The temperature used for drying of citrus peels has an impact on the reported higher level of naringin in Peiyou (29.9 mg/g) and Wendun level of flavanones. The flavanone glycosides reported in fresh orange (23.9 mg/g) peels as compared others. Rutin (0.27–0.89 mg/g), narir- peel were neohesperidin and naringin with a level of 58.39 and utin (9.70–16.31 mg/g) and hesperidin (48.20–77.08 mg/g) were the 3.49 mg/g and their content increased to 68.89 and 4.64 mg/g, re- major flavanone glycosides identified in Jinpi (unripe peelof C. unshiu spectively after drying of orange peel at 100 °C (Chen et al., 2011). Mark and/or C. reticulata Blanco) samples collected from Korea (Cho Cheigh et al. (2012) successfully extracted flavanones from C. unshiu et al., 2014). Jinpi and Quinpi are the popular herbal medicines pro- peel by utilizing subcritical water extraction method and altering the duced from CP in Korea and China. Narirutin (14.46–16.95 mg/g) and extraction temperature (110–200 °C) and time (5–20 min) under high hesperidin (36.25–75.49 mg/g) were contained at considerably high pressure (100 ± 10 atm). They reported high yields of narirutin levels in Quinpi (unripe peel of C. unshiu Mark and/or C. reticulata (11.7 mg/g) and hesperidin (72 mg/g) at an extraction temperature of Blanco) samples collected from China (Cho et al., 2014). 160 °C for an extraction time of only 10 min. Microwave heating in- The levels of flavanone glycosides differed in peels of orange grown creases the yield of flavanone glycosides from kinnow peels. The con- at different agroclimatic conditions or geographical locations (Chen, tent of naringin, naringenin and hesperidin in microwave heated Chu, Chyau, Chu, & Duh, 2012). The level of narirutin in dried orange (250 W, 10 min) kinnow peel powder was 4975.4, 820.7 and 53.0 µg/g peel collected from California and China (Guangxi) was reported as DW, respectively, while in non-heated kinnow peel powder the content 4.43 and 4.52 mg/g, respectively (Chen et al., 2012). The peel of ma- reported was 3915.8, 655.3 and 42.5 µg/g DW, respectively (Hayat ture and immature C. aurantium fruit is a very good source of naringin et al., 2010). The level of hesperidin and naringin in Ponkan mandarin (103.6 and 112.5 mg/g respectively) that can be utilized in foods and (C. reticulata) peel extract prepared from heated peel powder (100 °C pharmaceutical industries (Choi, Hwang, et al., 2007). The orange peel for 180 min) were 83.5 and 7.79 mg/g extract, respectively, while in contained a high content of hesperidin (48 mg/g of dry peel) as a major non-heated control the level was 68.8 and 5.87 mg/g extract, respec- flavonoid glycoside and it has a potential for commercial exploitation as tively (Ho & Lin, 2008). Microwave-assisted extraction (140 °C for a source material for the generation of hesperidin (Kanaze et al., 2009). 8 min) followed by low-temperature storage (5 °C, 24 h) was success- The content of flavanone glycosides viz. hesperidin, neoeriocitrin, fully demonstrated by Inoue, Tsubaki, Ogawa, Onishi, and Azuma naringin, poncirin, dydimin, and narirutin were reported as 170.5, (2010) to improve the yield of hesperidin by 27 times (compared to

8 B. Singh, et al. Food Research International 132 (2020) 109114 conventional extraction method) from C. unshiu peels. The optimized vone, 3-methoxynobiletin (3,5,6,7,8,3′,4′-heptamethoxyflavone) and conditions (40 °C, 150 W and 80% ethanol) for ultrasound-assisted tangeretin (5,6,7,8,4′-pentamethoxyflavone) (Li et al., 2006). Green, extraction of flavonoids from orange peel resulted in higher quantities Wheatley, Osagie, Morrison, and Asemota (2007) reported six major of hesperidin (205.2 mg/100 g FW) and naringin (70.3 mg/100 g FW) PMFs (sinensetin, nobiletin, tangeretin, heptamethoxyflavone, tetra- than those obtained from conventional solvent extraction procedure methylscutellarein and hexamethyl-o-quercetagetin) in peels of sweet (144.7 and 50.9 mg/100 g FW, respectively) (Khan et al., 2010). The oranges, mandarin, tangerine, limes and in citrus hybrids like ortanique extraction yields of hesperidin by pressurized liquid extraction from (Citrus reticulata × Citrus sinensis), tangelo ugli (C. maxima × C. re- peels of C. reticulata ‘Chachi’ (Guangchenpi) is reported as 58.40 mg/g ticulata), king orange (C. reticulata × C. sinensis) and mexican tangor (C. (Li et al., 2012). The optimized ultrasound-assisted extraction condi- reticulata × C. sinensis). tions can improve the yields of flavonoids from CP extracts. The con- Nobiletin and tangeretin are the primary PMF’s for their bioactiv- tents of narirutin and hesperidin in Satsuma mandarin (C. unshiu Marc.) ities and tetramethoxyflavone is the most common PMF in citrus plants peel extracts obtained by ultrasound-assisted extraction (8 W at 30 °C (Gao et al., 2018). Nobiletin and tangeretin were identified as the most for 10 min) were 296.7 and 1077.6 µg/g DW, respectively and with widely distributed PMFs in the peels of mature and immature fruits of maceration extraction (40 °C for 8 h) the contents were 152.3 and twenty different Citrus species (including cultivars) grown in Jeju, 601.2 µg/g DW, respectively (Ma, Chen, et al., 2008). Korea (Choi, Hwang, et al., 2007). Wang, Wang, Huang, Tu, and Ni (2007) isolated and characterized PMF’s (nobiletin, 5-demethylnobi- 3.2.2. Flavones letin, tangeretin, 5-demethyl tangeretin, sinensetin, isosinensetin, tet- Flavone glycosides (diosmin and isorhoifolin), C-glycosylated fla- ramethyl-o-scutellarein, tetramethyl-o-isoscutellarein and hepta- vones (6,8-di-C-glucosylapigenin), and polymethoxylated flavones (si- methoxyflavone) from green tangerine (Pericarpium Citri Reticulatae nensetin, nobiletin, tangeritin, hexa-O-methylquercetagetin, hexa-O- Viride) peel. Four polymethoxylated flavones (sinensetin, nobiletin, methylgossypetin, tetra-O-methylscutellarein, 3,5,6,7,8,3′,4′-hepta- 5,6,7,8,3′,4′,5′-hexamethoxyflavone and 5,6,7,8,3′,4′,5′-hepta methox- methoxyflavone and 5-hydroxy-3,7,8,3′,4′- pentamethoxyflavone) are yflavone) were identified in orange peel(Kanaze et al., 2009). Liu, Xu, the main flavonoids identified in orange peel(Manthey & Grohmann, Cheng, Yao, and Pan (2012) separated and identified three PMFs (no- 2001). Sweet orange peel contained a high concentration of poly- biletin, tangeretin and 5-demethylnobiletin) from Ponkan (C.reticulata methoxylated flavones (PMFs), C-glycosylated flavones and low con- Blanco cv. Ponkan) peel by HPLC–MS and NMR. Duan et al. (2017) centration of flavanones (hesperidin and naringin) (Anagnostopoulou isolated and identified eight polymethoxyflavones (PMFs) from the peel et al., 2005). The C-glycosylated flavones of a sweet orange peel include of C. reticulata ‘Chachi’ by NMR and mass spectroscopic analysis. The 6-C-β-glucosyldiosmin, 6,8-di-C-β-glucosyldiosmin, and 6,8-di-C-glu- identified PMF’s were nobiletin, 5-hydroxy-6,7,8,3′,4′-pentamethoxy- copyranosyl apigenin (Anagnostopoulou et al., 2005). Flavones-C-glu- flavone (5-demethylnobiletin), tangeretin, 5-hydroxy-6,7,8,4′-tetra- cosides (6,8-di-C-glucopyranosyl-luteolin, 6,8-di-C-glucopyranosyl-api- methoxyflavone (5-demethyltangeretin), sinensetin, isosinensetin, genin) were identified in petroleum ether extract and flavones-O- 3,5,6,7,8,3′,4′- heptamethoxyflavone and 6,7,8,3′,4′-pentamethoxy- glucosides (diosmetin-7-rutinoside) were identified in ethyl acetate flavanone. 6,7,8,3′,4′-pentamethoxyflavanone was isolated and re- extract of lemon peel (Baldi et al., 1995). Kanaze et al. (2009) identified ported for the first time from the peel of C. reticulata ‘Chachi'. The seven three flavone-7-O-glycosides (diosmetin-7-O-rutinoside, chrysoeriol-7- PMF’s identified and quantified from dried peels of hallabong ([C. un- O-rutinoside, and luteolin-7-O-rutinoside) in the orange peel. Flavone shiu Marcov. × C. sinensis Osbeck] × C. reticulata Blanco) were compounds (diosmin, luteolin, and sinensetin) are present in a very low 3,6,7,4′-tetramethoxyflavone (15.38 mg/g), 5,6,7,8,4′-pentamethoxy- amount in peels of eight citrus fruits varieties. The higher amounts of flavone (5.16 mg/g), 6,7,8,3′,4′-pentamethoxyflavone (3.26 mg/g), 3- diosmin (0.12–1.17 mg/g), luteolin (0.08–0.21 mg/g) and sinensetin hydroxy-5,6,7,4′-tetramethoxyflavone (1.35 mg/g), 5,6,7,8,3′,4′-hex- (0.42 mg/g) were reported in Kumquat, Ponkan and Liucheng peels amethoxyflavone (0.61 mg/g), 3,5,6,7,8,3′,4′-heptamethoxyflavone (Wang et al., 2008). Mean levels of diosmin in orange peel were re- (0.53 mg/g), and 5,6,7,3′,4′-pentamethoxyflavone (0.19 mg/g) (Han, ported as 47.78 mg/g DW (Kanaze et al., 2009). The highest content of Kim, Lee, Mok, & Lee, 2010). flavone glycosides (rhoifolin, isorhoifolin and diosmin at a levelof The amount of PMF’s varies in peels of same citrus species grown at 10.39, 14.14 and 18.06 mg/g FW, respectively) were quantified from different geographic locations. The level of nobiletin and tangeritin was flavedo extracts of Mauritian citrus fruits (Ramful et al., 2010). The 0.43 and 0.19 mg/g, respectively in dried orange peel collected from flavones identified in bergamot (C. bergamia Risso) peel were apigenin California, while orange peel collected from China (Xinhui) contained a mono-glucoside/mono-rhamnoside, apigenin 6,8-di-C-glucoside, dios- higher level of nobiletin (7.79 mg/g) and tangeritin (3.37 mg/g) (Chen metin 6,8-di-C-glucoside, diosmetin mono-rhamnoside, diosmetin et al., 2017). The level of nobiletin, tangeritin, isosinensetin, sinensetin, mono-glucoside isomer 1, diosmetin mono-glucoside isomer 2 and lu- 5,6,7,4′-tetramethoxyflavone and 5,7,8,4′-tetramethoxyflavone was teolin mono-glucoside/mono-rhamnoside with their level of 112.2, reported in the range of 1576–6453, 1053–3116, 273–2804, 121–984, 50.4, 32.2, 25.4, 14.6, 41.2, 63.1 mg/100 g, respectively (Mandalari 129–2022 and 28.3–1673 µg/g, respectively in seven Chinese C. re- et al., 2006). ticulata cultivars and in the range of 421–1008, 67.3–147.1, 21.6–63.8, Citrus PMF’s have two or more (maximum up to seven) methoxy 334–887, 8.6–21.7 and 83.26–245.9 µg/g, respectively in seven Chi- groups on their basic benzo-γ-pyrone (15-carbon, C6–C3–C6) skeleton nese C. sinensis cultivars (Xing, Zhao, Zhang, & Li, 2017). The content of with carbonyl group at C4 position (Gao et al., 2018). The ten poly- most of the PMF’s was reported higher in C. reticulata than in the C. methoxylated flavonoids isolated and characterized from tangerine (C. sinensis. The nobiletin (25.1%) and tangeretin (16.9%) were observed to tangerina) peel were 5,6,7,3′,4′-pentamethoxyflavone (sinensetin), be the two most abundant compounds in PMF-rich extract of C. re- 5,6,7,8,3′,4′-hexamethoxyflavone (nobiletin), 5,6,7,8,4′-pentamethox- ticulata peel by HPLC analysis (Duan et al., 2017). Among the selected yflavone (tangeretin), 5,7,8,4′-tetramethoxyflavone (tetra-O-methyli- Jamaican and Mexican citrus cultivars, Ortanique peel extract con- soscutellarein), 5,6,7,4′-tetramethoxyflavone (tetra-O-methylisoscu- tained the highest total PMF content with high levels of tangeretin tellarein), 7-hydroxy-3,5,6,3′,4′-pentamethoxyflavone, 5-hydroxy- (9807 µg/g), nobiletin (8253 µg/g), tetramethylscutellarein (7802 µg/ 6,7,8,3′,4′-pentamethoxyflavone, 7-hydroxy-3,5,6,8,3′,4′-hexamethox- g), sinensetin (3612 µg/g) and hexamethyl-o-quercetagetin (3439 µg/ yflavone, 3,5,6,7,8,3′,4′-heptamethoxyflavone and 5,7,8,3′,4′-penta- g). Tangerine peel extract from Jamaica contained a high level of no- methoxyflavone (Chen, Montanari, & Widmer, 1997). PMFs identified biletin (8663 µg/g), while mandarin peel extract from Mexico con- in sweet orange peel were sinensetin (5,6,7,3′,4′-pentamethoxy- tained a high content of heptamethoxyflavone (7084 µg/g) (Green flavone), 3-methoxysinensetin (3,5,6,7,3′,4′-hexamethoxyflavone), no- et al., 2007). The tangors, C. reticulata and C. sinensis contained highest biletin (5,6,7,8,3′,4′-hexamethoxyflavone), 5,6,7,4′-tetramethoxyfla- levels of PMFs and C. aurantium, C. medica, C. maxima, and C. paradisi

9 B. Singh, et al. Food Research International 132 (2020) 109114 contained lowest levels of PMFs (Green et al., 2007). Methanolic extract The amount of nobiletin and tangeretin in peel extract of Ponkan of Ponkan and Tonkan contained total PMF’s at a level of 12.8 and mandarin prepared from heated peel powder (100 °C for 180 min) was 9.01 mg/g, respectively (Huang & Ho, 2010). The total content of PMF’s 37.1 and 29.4 mg/g extract, respectively, whereas in non-heated was reported as 51.06% in PMF-rich extract of C. reticulata peel (Duan powder the content was 30.3 and 25.9 mg/g extract, respectively (Ho & et al., 2017). Lin, 2008). Flavones were more efficaciously extracted by pressurized Orange peel represented the most diverse source of flavonoids with liquid extraction from peels of C. reticulata ‘Chachi’ (Guangchenpi) than nobiletin and tangeritin as the main PMF’s and diosmin as the main the other extraction (ultrasonic-assisted, soxhlet and heat-reflux ex- flavone (Londoño-Londoño et al., 2010). The contents of three PMF’s traction) methods (Li et al., 2012). Extraction yields of nobiletin and (heptamethoxyflavone, tangeretin, and nobiletin) in orange peel extract tangeretin obtained by pressurized liquid extraction (by using extrac- was 55.04, 20.34 and 10.41 mg/g, respectively (Lai et al., 2011). Me- tion pressure of 1500 psi, extraction time of 20 min, extraction tem- thanolic peel extract of Ponkan contained nobiletin, tangeretin, and perature of 160 °C) is reported as 14.05 and 7.99 mg/g, respectively (Li sinensetin at the level of 5.89, 6.41 and 0.47 mg/g, respectively, while et al., 2012). It is less time-consuming extraction method and has Tonkan contained these compounds at a level of 6.93, 1.43 and higher extraction efficiency as compared to conventional extraction 0.64 mg/g, respectively (Huang & Ho, 2010). Nobiletin (92.5 mg/kg techniques. FW) and tangeretin (55.9 mg/kg FW) were the major flavones identified and quantified by HPLC–UV method in C. unshiu fruit peel (Kim 3.2.3. Flavonols et al., 2011). The other flavone identified and quantified in fruit peelof Limocitrol, isolimocitrol, limocitrin, and rutin were the main fla- this fruit were 3,3′,4′, 5,6,7,8-heptamethoxyflavone, sinensetin, hex- vonols identified in ethyl acetate extract of lemon peel(Baldi et al., amethoxyflavones, isosinensetin, 3-hydroxypentamethoxyflavone, 3- 1995). The amounts of flavonols (rutin, quercetin, and kaempferol) hydroxy-hexamethoxyflavone and tetramethyl-O-isoscutellarein having were reported in the range of 0.09–0.29, 0.14–0.78 and 0.13–0.38 mg/ a level of 40.1, 24.6, 15.5, 11.6, 11.1, 10.9 and 6.3 mg/kg FW, re- g, respectively in fruit peels of eight citrus varieties (Wang et al., 2008). spectively (Kim et al., 2011). The highest content of nobiletin (0.59%) The content of rutin (a flavonol glycoside) present in flavedo extracts of was quantified in the fresh fruit peels of C. subcompressa among dif- mandarin (C. reticulata), clementine (C. clementina), tangelo (C. re- ferent Chinese citrus fruits (Lu et al., 2006). The gold lotion from CP is a ticulata × C. paradisis), tangor (C. reticulata × C. sinensis) and orange rich source of flavonoids and contains high PMF content. The PMFs (C. sinensis) was reported as 42.13, 33.13, 13.09, 10.62 and 8.16 mg/g composition of gold lotion included nobiletin, sinesetin, 3,5,6,7,8,3′,4′- FW, respectively (Ramful et al., 2010). The level of rutin reported in hepta-methoxyflavone, tangeretin, 3,5,6,7,3′,4′-hexamethoxyflavone , ethanol extract of Yuzu peel is 2.7 mg/100 g (Kim et al., 2013). Rutin 5,6,7,4′-tetramethoxyflavone with their level of 50.8, 21.3, 19.2, 10.6, was identified as the most abundant flavonoid constituting 0.14mg/g 3.1 and 1.1 mg/ml, respectively (Lai, Li, Miyauchi et al., 2013). The (9.91%) of the Tunisian bitter orange peel extract (Kurowska & level of nobiletin, tangeretin, 5-hydroxy-6,7,8,3′,4′-pentamethoxy- Manthey, 2004). flavone and 3,5,6,7,8,3′,4′-heptamethoxyflavone were reported inthe Drying or heating of citrus peels at high temperature increases the range of 1.35–14.01, 0.56–11.54, 0.12–2.76 and 1.02–4.39 mg/g, re- level of flavonols. The level of kaempferol and rutin reported infresh spectively in 12 samples of C. Reticulatae Pericarpium collected from orange peel was 5.13 and 3.14 mg/g, respectively and their content was citrus-producing regions of China (Liu et al., 2013). The level of tan- increased to 6.63 and 5.31 mg/g, respectively after drying of orange geretin in Yuzu peel extract was reported as 0.7 mg/100 g (Kim et al., peels at 100 °C (Chen et al., 2011). The content of rutin and kaempferol 2013). Mandarin peel is rich in nobiletin as compared to other citrus in non-heated kinnow peel powder was 165.4 and 222.0 µg/g DW, species (oranges, grapefruit, and limes). The concentration of isolated respectively, while in microwave heated (250 W, 10 min) kinnow peel nobiletin varied significantly among hexane peel extracts of mandarin powder higher content (194.5 and 287.1 µg/g DW, respectively) was (C. reticulata Blanco cv. Egyptian), sweet orange (C. sinensis (L.) Osbeck reported (Hayat et al., 2010). The amount of rutin in C. sinensis peel cv. Olinda Valencia), white grapefruit (C. paradisi Macfad. cv. Duncan) extract obtained by using microwave-assisted extraction method was and lime (C. aurantiifolia Swingle cv.Mexican) with a reported value of 589.13 µg/g and by conventional solvent extraction methods was 202.91, 73.15, 18.13 and 0.09 µg/ml, respectively (Fayek et al., 2017). 3037.51 µg/g (Nayak et al., 2015). Hydroxylated PMF’s are mainly found in aged or long term stored CPs and their one or more methoxy (OCH3) groups is substituted with hy- 3.2.4. Total flavonoid content droxyl (OH) group (Gao et al., 2018; Li et al., 2009). Hydroxylated PMF’s Total flavonoid content (TFC) varies among fruits of different citrus of CP are mostly 5-hydroxylated PMFs. The common hydroxylated PMFs species and it is mainly concentrated in peels. TFC was reported higher identified in CPs are 5-hydroxy-6,7,8,3′,4′-pentamethoxyflavone (5-de- in peels (5.2–23.3 mg QE/g) when compared with fruit tissues methylnobiletin), 5-hydroxy-6,7,8,4′-tetramethoxyflavone (5-de- (0.3–3.3 mg QE/g) of different citrus species (Ghasemi et al., 2009). methyltangeretin), 5-hydroxy-3,7,8,3′,4′-pentamethoxyflavone, 7-hy- The polar fractions of grapefruit, lemon, lime and sweet orange peels droxy-3,5,6,3′,4′-pentamethoxyflavone and 7-hydroxy-3,5,6,8,3′,4′- contained TFC of 2.29, 15.96, 13.61 and 3.97 mg catechin equivalents hexamethoxyflavone (Chen et al., 1997; Duan et al., 2017). Eight hydro- [CE]/g, respectively, whereas polar fractions of juice extract from these xylated PMF’s identified in sweet orange peel are 5-hydroxy-6,7,4′-tri- citrus species contained TFC of 1.96, 1.43, 2.36 and 0.56 mg CE/g, methoxyflavone, 5-demethyltangeretin (5-hydroxy-6,7,8,4′-tetramethoxy- respectively (Guimarães et al., 2010). TFC of 29.75, 28.36, 21.87, flavone), 3-hydroxy-5,6,7,4′-tetramethoxyflavone, 3-hydroxytangeretin 18.36, 17.39 and 13.89 µg/g was reported in ethanol, methanol, (3-hydroxy-5,6,7,8,4′-pentamethoxyflavone), 5-hydroxy 3,6,7,8,3′,4′-hex- acetone, ethyl acetate, dichloromethane and hexane extracts of orange amethoxyflavone, 5-hydroxy-3,7,3′,4′-tetramethoxyflavone, 5-hydroxy- peel, respectively (Hegazy & Ibrahium, 2012). The peels obtained from 3,7,8,3′,4′-pentamethoxyflavone, 5-demethylnobiletin and 5-hydroxy- six sweet orange (C. sinensis L. cv. Washington Navel, Thomson Navel, 6,7,8,3′,4′-pentamethoxyflavone (Li et al., 2006). The total content of Sanguinelli, Double fine, Portugaise and Jaffa) and one sour orange(C. hydroxylated PMFs reported in orange peel extract is 893.25 mg/g (Lai aurantium L. cv. Bigarade) varieties cultivated in Algeria contained TFC et al., 2011). The six hydroxylated PMF’s identified in orange peel extract of 1.29, 1.28, 0.91, 0.71, 0.29, 0.56 and 1.17 mg QE/g DW, respectively are 5-hydroxy-6,7,8,3′,4′-pentamethoxyflavone (396.42 mg/g), 5-hy- (Lagha-Benamrouche & Madani, 2013). TFC in peels of Ponkan (C.re- droxy-3,6,7,8,3′,4′- hexamethoxyflavone (254.78 mg/g), 5-hydroxy-6,7,4′- ticulata Blanco), Peiyou (C. grandis Osbeck CV), Wendun (C. grandis trimethoxyflavone (115.52 mg/g), 5-hydroxy-6,7,8,4′-tetramethoxy- Osbeck), Kumquat (C. microcarpa), Murcott (C. reticulate × C. sinensis), flavone (74.96 mg/g), 5-hydroxy-6,7,3′,4-tetramethoxyflavone (44.5 mg/ Tonkan (C. tankan Hayata), Liucheng (C. sinensis (L.) Osbeck) and g) and 5-hydroxy- 3,6,7,3′,4′-pentamethoxyflavone (7.08 mg/g) (Lai et al., Lemon (C. limon (L.) Bur) was reported as 49.2, 48.7, 46.7, 41.0, 39.8, 2011). 39.6, 35.5 and 32.7 mg/g, respectively (Wang et al., 2008).

10 B. Singh, et al. Food Research International 132 (2020) 109114

Total anthocyanin content (TAC) in peels of 3 sweet orange (Blood: pummelo using high-speed countercurrent chromatography. In a recent C. sinensis L. cv. Sanguinelli, Double fine and Portugaise) varieties study, 5-geranyloxy-7-methoxycoumarin, citropten, 8-geranyloxypsor- cultivated in Algeria was reported as 32.91, 28.28 and 15.35 µg mal- alen, biacangelicin, bergamottin and oxypeucedanin were character- vidin-3-O-glucoside equivalents (MVE)/100 g DW, respectively (Lagha- ized from citrus industry waste (Costa, Albergamo, Arrigo, Gentile, & Benamrouche & Madani, 2013). In addition, TFC varies in peels of or- Dugo, 2019). In peels of C. auramtium, osthole and isogeijerin, known to ange sourced from different geographic locations. TFC reported in dried have health benefits, were identified using supercritical extraction peels of orange grown at USA (California) and China (Sichuan) was (Trabelsi et al., 2016). 31.9 and 14.0 mg/g, respectively (Chen et al., 2017). TFC reported in the methanol extract of C. sinensis (L.) Osbeck fresh peel was 12.95 mg 4. Essential oils CE/g DW and it was increased (13.79 mg CE/g DW) by drying peels at high (90 & 100 °C) temperature before extraction (Chen et al., 2011). Apart from phenolic compounds, CP is also rich in other types of Moreover, TFC was reported to be increased by microwave heating of antioxidants namely, essential oils which are mixtures of volatile kinnow peels. TFC in microwave treated (250 W for 10 min) and non- compounds (such as aldehydes, esters, alcohols, acids and ketones) treated kinnow peel powder was reported as 6375.9 and 5037.1 µg/g, (Bustamante et al., 2016). CP essential oils are economical and en- respectively (Hayat et al., 2010). Their study suggested that reasonable vironment-friendly alternative to chemical food preservatives (sodium microwave treatment at 250 W for 10 min was ideal for the release of nitrate or benzoate) (Mahato et al., 2019). Essential oil is a con- flavonoids from citrus peels. TFC in Ponkan mandarin (Citrus reticulata) centrated hydrophobic liquid that is present in oil cells of CP. It is about peel extract increased from 7.62 mg CE/g (non-heated control) to 0.5–5% of the fresh weight of CP and consist of volatile aromatic 9.07 mg /g extract by heating peel powder at 100 °C for 180 min before compounds. Chemically, monoterpene hydrocarbons, sesquiterpene extraction (Ho & Lin, 2008). hydrocarbons, oxygenated monoterpenes and oxygenated sesquiterpenes are present in most CP. Citrus fruits having thick peel such as 3.3. Coumarins sour orange, grapefruit and bergamot contain a good content of essential oils compared to citrus species with thin peels. The primary Coumarins are 1,2-benzopyrones (containing fused benzene and α- essential oils were non-terpenoid ester and aldehyde derivatives in pyrone rings) which are derivatives of the phenylpropanoid pathway. mandarin orange, pummelo, sweet orange, grapefruit and bitter orange These can be simple coumarins (benzo-pyrones), furanocoumarins (7- peels. In contrast, mono-and sesquiterpene hydrocarbons were most oxygenated coumarins), phenylcoumarins (benzo-benzopyrones) and abundant in yuzu, citron, key lime, and bergamot orange peels pyranocoumarins (Boysen & Hearn, 2010). The structures of some (González-Mas, Rambla, López-Gresa, Blázquez, & Granell, 2019). Li- coumarins found in citrus peels are shown in Fig. 3. Citrus species monene (nonoxygenated cyclic monoterpene) is a colorless aliphatic mainly contain coumarins and furanocoumarins which are mainly hydrocarbon identified as the major component in essential oilsof linked to defense against pathogenic organisms (Li, Wu, Wang, Hung, & different citrus species. In orange peels, the primary essential oilcom- Rouseff, 2019). Among these, scoparone, scopoletin, umbelliferone and ponents reported were monoterpene hydrocarbons that included limo- xanthyletin present in citrus fruit peels have been associated with re- nene (92.6%), γ-terpinene (3.39%), β-pinene (1.55%) and α-pinene sistance against fungi (Ramírez-Pelayo, Martínez-Quiñones, Gil, & (0.61%). The oxygenated monoterpenes (linalool 0.31%), sesquiterpene Durango, 2019). It has been observed that citrus peels from certain hydrocarbons (α-humulene 0.08%) and oxygenated sesquiterpenes species such as limes and mandarins contained abundant coumarins (cubebol 0.06% and α -sinensal 0.06%) were identified as other EO and furanocoumarins. Specifically, these were identified as limettin, 5- components (Hosni et al., 2010). Monoterpene hydrocarbons (primarily geranyloxy-7-methoxycoumarin, isopimpinellin, oxypeucedanin hy- d-limonene, γ-terpinene and β-pinene), fatty alcohol esters, sesqui- drate, bergaptene, and bergamottin (Ramírez-Pelayo et al., 2019). De- terpenes and oxygenated monoterpenes (primarily α-terpineol, nerol, rivatives of coumarin, specifically 7-geranyloxycoumarin, 6′,7′- dihy- and geraniol) were identified in lemon peels. It was also reported that droxybergamottin, 5-hydroxypsoralen and 8-hydroxypsoralen were amount of essential oil lessened during the ripening process (Di Rauso isolated from mature citrus fruits (Hirata et al., 2009). Zhang et al. Simeone, Di Matteo, Rao, & Di Vaio, 2020). (2017) isolated coumarins such as marmin, epoxybergamottin, auraptene, 5-[(6′,7′-dihydroxy-3′,7′-dimethyl-2-octenyl) oxy] psoralen, 8- 5. Antioxidant activity (3-hydroxy-2,2-dimethylpropyl)-7-methoxy-2H-chromen-2-one, 5- [(7′,8′-dihydroxy-3′,8′-dimethyl-2-nonadienyl)oxy] psoralen from Oxidation reactions (involving the shifting of electrons among

Fig. 3. Chemical structures of bergamottin (a), 5-geranyloxy-7-methoxycoumarin (b), auraptene (c) and limettin (d).

11 B. Singh, et al. Food Research International 132 (2020) 109114

Table 2 Antioxidant activity of fruit peels of different citrus species.

Peel source Botanical name Extract Antioxidant activity References

Sweet orange C. sinensis EAE 0.5 mg DE/mg DPPH IC50 Anagnostopoulou et al. (2006)

DEE 0.7 mg DE/mg DPPH IC50

DME 3.0 mg DE/mg DPPH IC50 Ponkan mandarin C. reticulata MWE 825.4 µmol TE/100 g FW de Moraes Barros et al. (2012) Lemon C. limon AE 80.93 mg Trolox/100 g Casquete et al. (2015) Lime C. latifolia 53.11 mg Trolox/100 g Mandarin C. reticulata 69.02 mg Trolox/100 g Sweet orange C. sinensis 102.39 mg Trolox/100 g

Orange (fresh peel) C. sinensis ME 2.05 mg/ml IC50 Chen et al. (2011)

Orange (peel dried at 100 °C) 0.57 mg/ml IC50 Orange (California) C. reticulata AE 236.2 µmol Trolox/g Chen et al. (2017) Orange (Guangxi) 189.5 µmol Trolox/g Orange (Zhejiang) 219.3 µmol Trolox/g Orange (Sichuan) 246.1 µmol Trolox/g Orange (Xinhui) 256.2 µmol Trolox/g Novel orange C. sinensis EE 69% El-aal and Halaweish (2010) Baldi orange 59% Mandarin orange C. reticulata HEE 3.22 mmol Trolox/100 g FW Ferreira et al. (2018) AE 3.10 mmol Trolox/100 g FW SPE-HEE 1.64 mmol Trolox/100 g FW SPE-AE 1.23 mmol Trolox/100 g FW

Sweet orange (Washington Navel) C. sinensis ME 1.1 mg/ml IC50 Ghasemi et al. (2009)

Orange (Ponkan) C. reticulata 0.6 mg/ml IC50

Satsuma Mandarin (Sugiyama) C. unshiu 1.3 mg/ml IC50

Grapefruit C. paradisi 2.1 mg/ml IC50

Sour orange C. aurantium 1.9 mg/ml IC50

Lemon C. limon 1.4 mg/ml IC50

Grapefruit C. paradisi ME 5.15 mg/ml IC50 Guimarães et al. (2010)

Lemon C. limon 3.77 mg/ml IC50

Lime C. aurantifolia 1.72 mg/ml IC50

Sweet orange C. sinensis 4.99 mg/ml IC50 Baladi orange C. sinensis ME 73.42% Hegazy and Ibrahium (2012) EE 78.14% EAE 68.99% ACE 65.38% HE 58.78%

Ponkan mandarin C. reticulata ME 371 μg extract/ml IC50 Ho and Lin (2008)

Ponkan mandarin (peel heated at 100 °C for 180 min) 17 μg extract/ml IC50

Bitter orange C aurantium ME 190 μg/ml IC50 Marzouk (2013) Satsuma Mandarin C. unshiu EE 29.64% Jeong et al. (2004) Satsuma Mandarin (peel heated at 150 °C for 60 min) EE 63.25% Satsuma Mandarin (peel heated at 150 °C for 60 min) WE 15.81% WE 54.70%

Orange C. sinensis ME 10.26 mg /mg IC50 Kanaze et al. (2009) Citrus peel AE 43.0% Kang et al. (2006) Orange C. sinensis EE-UAE 54% Khan et al. (2010) EE 42%

Sweet orange (Washington Navel) C. sinensis MWE 0.901 mg/ml IC50 Lagha-Benamrouche and Madani (2013)

Sweet orange (Thomson Navel) 0.612 mg/ml IC50

Sweet orange (Sanguinelli) 0.692 mg/ml IC50

Sweet orange (Double fine) 0.796 mg/ml IC50

Sweet orange (Portugaise) 0.743 mg/ml IC50

Sweet orange (Jaffa) 0.759 mg/ml IC50

Sour orange (Bigarade) 0.568 mg/ml IC50 Penggan C. reticulata ME-UAE 49.01% Ma, Chen, et al. (2008)

Orange C. sinensis AAE-MAE 337.16 ml/l IC50 Nayak et al. (2015)

AAE-UAE 433.08 ml/l IC50

AAE-ASE 450.44 ml/l IC50

AAE-CSE 358.45 ml/l EC50 Meyer lemon C. meyeri ME 13.3 µmol Trolox/g FW Ramful et al. (2010) Tangor C. reticulata × C. sinensis 46.1 µmol Trolox/g FW Washington Navel Orange C. sinensis 31.2 µmol Trolox/g FW Dancy mandarin C. reticulata 44.0 µmol Trolox/g FW Kinnow C. nobilis × C. deliciosa ME 72.83% Safdar et al. (2017)

AE: aqueous extract; ACE: acetone extract, AAE: aqueous acetone extract; DEE: diethyl ether extract; DE: dry extract; DME: dichloromethane extract; EE: ethanol extract; HE: hexane extract; WE: water extract; ME: methanol extract; ACE: acetone extract; EAE: ethyl acetate extract; HE: hexane extract; MWE: methanol water extract; HEE: hydro-ethanolic extract; SPE: solid phase extraction; EAE: ethyl acetate extract; ASE: accelerated solvent extraction; CSE: conventional solvent extraction; MWE: methanol water extract; MAE: microwave-assisted extraction; UAE: ultrasound-assisted extraction; IC50: half maximal inhibitory concentration, FW: fresh weight; TE: trolox equivalents. electron rich species or molecules) produces free radicals mostly re- be responsible for degenerative illnesses such as multiple sclerosis and active oxygen species. These species may damage many biomolecules of cancer in human beings (Singh, Singh, Kaur, & Singh, 2017; Thériault, the cell including DNA, RNA, lipids and proteins and additionally might Caillet, Kermasha, & Lacroix, 2006). Antioxidants terminate free radical

12 B. Singh, et al. Food Research International 132 (2020) 109114 generation that is harmful to biomolecules mainly proteins and nucleic (Casquete et al., 2015). Trolox equivalent antioxidant capacity (TEAC) acids. Primary antioxidants reduce oxidation reactions in an active values ranged from 11.0 to 46.1 µmol/g FW for the flavedo extracts of state, whereas secondary antioxidants reduce oxidation in an indirect different citrus fruits with lowest for value for calamondin (Citrus mitis) way (mostly by binding of pro-oxidants) (Craft, Kerrihard, Amarowicz, and highest for Tangor (C. reticulata × C. sinensis)(Ramful et al., 2010). & Pegg, 2012). Table 2 provides information regarding the antioxidant DPPH scavenging activities of ethanol peel extracts of Baladi and Novel activity of fruit peels of different citrus species. orange was reported as 69 and 59%, respectively (El-aal & Halaweish, Bioactive compounds present in CP possess high antioxidative po- 2010). The methanol extracts of fresh orange peels showed EC50 values tential towards free radicals. CP is a rich source of naturally occurring of 2.05 and 1.99 mg/ml in DPPH and ABTS free radical scavenging antioxidants (de Moraes Barros et al., 2012). Antioxidant capacity of CP methods (Chen et al., 2011). The antioxidant activity values for extracts is owing to the abundance of phenolic acids, flavonoids and ascorbic obtained from peels of sour orange (Bigarade) and sweet orange (Wa- acid (Kurowska & Manthey, 2004). CP contains phenolics and flavo- shington Navel, Thomson Navel, Sanguinelli, Double fine, Portugaise noids (hesperidin, narirutin, nobiletin, and tangeritin) that are involved and Jaffa) varieties cultivated in Algeria were 88.0, 55.5, 81.7, 72.2, in donating protons or electrons for stabilizing free radicals (Chen et al., 62.8, 67.3 and 65.8%, respectively (Lagha-Benamrouche & Madani, 2017). The flavonoids (flavanone and flavone glycosides or aglycones) 2013). The study presented a strong correlation between the antiradical of orange peel have a more selective antioxidant capacity and it concurs activity and TPC of the peels. The peels of Bigarade variety had high with a hydroxyl than hydrogen-donating radical removal process TPC (31.62 mg GAE/g DW) and it presented the most pronounced re-

(Kanaze et al., 2009). The polar fractions (includes phenolics and fla- ducing power (EC50 value of 0.568 mg/ml) compared to peels of other vonoids) of lime, lemon, sweet orange and grapefruit peels showed the orange varieties. highest antioxidant potential than volatile fractions (includes essential Storage conditions and extraction processes may change the level of oils) of peels (Guimarães et al., 2010). Ethyl acetate fraction of navel phenolic compounds and antioxidant potential of peel extract (Choi sweet orange peel showed considerable anti-oxidative potential due to et al., 2011; Hayat et al., 2009). DPPH activity of long-term stored the high content of flavonoids (polymethoxylated flavones, C-glycosy- Chenpi was reported higher than regularly stored Chenpi due to in- lated flavones, O-glycosylated flavones, O-glycosylated flavanones) and crease in the level of total phenolic compounds (Choi et al., 2011). It is esters of phenolic acids (Anagnostopoulou et al., 2005). Ethyl acetate recognized that Chenpi stored over an extended period of time (around fraction of lemon peel was reported to have flavonols, flavones-O-gly- three years) was a good source of antioxidants that were naturally cosides, and flavonones responsible for high antioxidant activity (Baldi present. It is used as medicine in China, Korea and Japan for the cure of et al., 1995). Hesperidin is an active antioxidant agent present in the inflammation, indigestion and respiratory disorders. Different organic ripe orange peel with a 2,2-Diphenyl-1-(2,4,6-trinitrophenyl)hydrazy solvents used for extraction of phenolic compounds from peels have (DPPH) value of 36.65% (Al-Ashaal & El-Sheltawy, 2011). The orange significant effects on antioxidant activity. Ethanol and methanol ex- peel collected from China (Xinhui) contained the highest content of tracts of orange peel showed high antioxidant capacity owing to high polymethoxylated flavones (nobiletin and tangeritin) compared toor- TPC and TFC as compared to dichloromethane, acetone, hexane and ange peel collected from USA (California) and also showed highest ethyl acetate peels extracts (Hegazy & Ibrahium, 2012). Hydro-etha- antioxidant capacity in 2,2′-azino-bis(3-ethylbenzothiazoline-6-sul- nolic and aqueous extracts of mandarin (C. reticulata Blanco) peel phonic acid) (ABTS) and (DPPH) free radical scavenging methods (Chen showed antioxidant activity of 3.22 and 3.10 mmol TE/100 g FW, re- et al., 2017). spectively and their solid phase extraction enriched phenolic fractions Antioxidant activity of peel is reported significantly higher than the showed activity of 1.64 and 1.23 mmol TE/100 g FW, respectively edible portion of citrus fruits (Gorinstein et al., 2001). CP contains (Ferreira et al., 2018). Enriched phenolic extracts achieved by solid highest content of phenolic compounds, vitamin C, carotenoids and phase extraction showed 3.98 and 5.09 times higher antioxidant ca- reducing sugars than juice as reported in different citrus species and pacity due to higher level of TPC than that of hydro-ethanolic and this certainly contributes to the high antioxidant capacities of peels aqueous extracts. The differences in antioxidant activity of mandarin (Guimarães et al., 2010). The total radical-trapping antioxidative po- peel reported are due to the differences in the level of TPC obtained by tential (TRAP) values in the peels of lemons, oranges and grapefruits various extraction methods (Ferreira, Martins, & Barros, 2017). were 6720, 3183 and 1667 nmol/ml, respectively, and in the peeled Drying temperature effects the level of polyphenol compounds and fruits the values were 4480, 2111 and 1111 nmol/ml, respectively antioxidant activities of citrus peels. The EC50 values for the DPPH (Gorinstein et al., 2001). The DPPH scavenging activity half maximal radical scavenging effect of the orange peel dried at 50 and60°C effective concentration (EC50) values of lime, lemon, sweet orange and temperature were reported higher (2.75 and 3.22 mg/ml) than those grapefruit peels polar fraction was reported as 1.71, 3.77, 4.99 and dried at 100 °C (0.57 mg/ml) (Chen et al., 2011). The DPPH-scavenging 5.15 mg/ml, respectively, whereas for juice polar fraction the values capacity of CP increased with heating of peel powder before solvent were 15.92, 11.15, 5.55 and 12.78 mg/ml, respectively (Guimarães extraction due to the increased liberation of phenolics and flavonoids. et al., 2010). The in vitro antioxidant capacity of peels from four citrus DPPH-scavenging capacity of Ponkan mandarin peel extract prepared species (Lima orange, Pera orange, Tahiti lime, Sweet lime and Ponkan from heated (100 °C for 180 min) peel powder was 117 μg extract/ml mandarin) in DPPH free radical scavenging capacity and ferric reducing and for non-heated control, it was 371 μg extract/ml (Ho & Lin, 2008). antioxidant power (FRAP) assay was reported higher than of pulps (de DPPH scavenging activity of ethanol and water extract of C. unshiu Moraes Barros et al., 2012). Among four citrus species, the peels of peels significantly increased from 29.64 to 63.25% and 15.81to Ponkan mandarin presented the largest antioxidant activity in DPPH 54.70%, respectively by heat treatment (150 °C for 60 min) of peel (825.4 µmol trolox equivalents [TE]/100 g FW) and FRAP powder when compared with non-heated control (Jeong et al., 2004). (3897.9 µmol TE/100 g FW) assays. However, in an another study, heating and gamma irradiation treat- Antioxidant capacity varies among fruit peels of different citrus ment had no considerable effect on the antioxidant potential ofCP species and it might be due to the dissimilarities in composition of extract (Kang, Chawla, Jo, Kwon, & Byun, 2006). DPPH free radical polyphenols (Gorinstein et al., 2001). Methanolic extracts of orange (C. scavenging activities of CP aqueous extract were 43.0, 41.5, 43.3, and reticulata var. Ponkan) peel showed the largest antioxidant capacity 44.3% for control, heated (in boiling water bath for 15 min), irradiated

(EC50: 0.6 mg/ml) and sour orange (C. aurantium) peel showed lowest (20 kGy) and irradiated (20 kGy) and heated, respectively. antioxidant activity (EC50: 2.1 mg/ml) among different citrus species Oxygen radical absorbance capacity (ORAC) values for the orange (Ghasemi et al., 2009). The antioxidant capacity of lime, lemon, sweet peel extracts obtained by ultrasound-assisted and conventional solvent orange and mandarin peel determined by DPPH was 53.11, 80.93, extraction were 712 and 509 mmol TE/100 g FW and free radical- 102.39 and 69.02 mg TE/100 g fresh peel extracts, respectively scavenging activity (FRSA) values were 54 and 42%, respectively (Khan

13 B. Singh, et al. Food Research International 132 (2020) 109114 et al., 2010). Kinnow peel extracts obtained by microwave-assisted Insoluble fiber-rich fractions (insoluble dietary fiber, water-in- extraction method showed higher antioxidant activities as compared to soluble solid and alcohol-insoluble solid) of C. sinensis peel could ef- extracts obtained by ultrasonic and rotary extraction methods (Hayat fectively adsorb glucose, retards diffusion of glucose and inhibits the et al., 2009). Microwave treatment (250 W for 10 min) liberated bound activity of α-amylase (Chau, Huang, & Lee, 2003). The hypoglycemic phenolic compounds and increased the level of free phenolic acids and potential of insoluble fiber-rich portions of CP can be exploited asa flavonoids in kinnow peel extract and thereby also increased theanti- functional ingredient in high-fiber foods to control serum glucose level oxidant activity (Hayat et al., 2010). It is generally accepted that free and to reduce calorie level. The incorporation of insoluble fiber-rich phenolic compounds have greater antioxidant potential than the bound fractions in the diet has also shown a favorable effect on gastro- forms. C. sinensis peel extract obtained by microwave-assisted extrac- intestinal function and health (Chau, Sheu, Huang, & Su, 2005). It tion showed lower IC50 (337.16 ml/l) compared to extract obtained by improves serum, intestinal, cecal and fecal parameters and decreases ultrasound-assisted (IC50: 437.45 ml/l), conventional solvent (IC50: the amount of ammonia formation in the digestive tract. CP extract 357.36 ml/l) and accelerated solvent (IC50: 450.44 ml/l) extraction ameliorated adverse effects of hyperthyroidism with its antioxidative, methods indicating higher scavenging of DPPH radicals by microwave antithyroid and high-density lipoprotein cholesterol stimulating prop- assisted peel extract (Nayak et al., 2015). CP subjected to pressure erties. Oral administration of C. sinensis peel extract (25 mg/kg) in treatments at 300 MPa for 3 min proved to be a rich source of natural hyperthyroid mice has significantly decreased the serum levels of antioxidants (Casquete et al., 2015). The pressure treatment (300 MPa thyroxine, triiodothyronine, total cholesterol, high-density lipoprotein for 3 min) of citrus peels before extraction further increased DPPH cholesterol, glucose and α-amylase activity (Parmar & Kar, 2008). The values (60.51, 189.85,114.21 and 73.70 mg TE/100 g for lime, lemon, administration of nobiletin rich peel extract of C. aurantium (200 mg/ sweet orange and mandarin peel, respectively). kg) for 4 weeks efficiently regulated cholestatic liver fibrosis induced by bile duct obstruction in mice having anti-inflammatory, antioxidant and 6. Health benefits anti-apoptotic activities (Lim et al., 2016). Peel extract significantly reduced the enhanced amounts of serum aspartate transaminase, ala- Several studies on CP have explored its nutritional and health nine transaminase, gamma-glutamyl transferase, total cholesterol and benefitting properties. It is a good source of medicinally important total bilirubin by 35.2, 38.9, 38.4, 27.0 and 18.2%, respectively and bioactive compounds. Health benefits of CP have been mainly ascribed effectively increased phosphorylation of cytoprotective proteins ina to phenolic acids and flavonoids. The health benefits of fruit peelsof mouse model system (Lim et al., 2016). CP extract and powder de- different citrus species are shown in Table 3 and also in Supplementary creased the total cholesterol, triglycerides, low density lipoprotein Fig. 2. Citrus flavonoids are the powerful antioxidants and potent free (LDL) and glucose amounts without imparting any harmful effect on radical scavengers that help in the prevention of diseases that occur due hematological parameters of hypercholesterolemic rats (Ashraf et al., to reactive oxygen species (Ashraf, Butt, Iqbal, & Suleria, 2017). Fla- 2017). Flavonoids present in CP have a role in decreasing plasma vonoids of citrus show health benefitting properties including anti- cholesterol level by preventing oleic acid conjugation in triglycerides. oxidant, cardioprotective, anticancer and anti-inflammatory activities. Diet enriched with CP extract (600 mg/kg) regulated glycemic and li- Flavanone glycosides (hesperidin and naringin) present in CP possess pidemic parameters in hypercholesterolemic subjects more effectively strong anti-oxidant, anti-inflammatory and anticancer activities (Al- compared to CP powder (Ashraf et al., 2017). Ashaal & El-Sheltawy, 2011). PMF’s and hydroxylated PMFs present in The hexane extract of lemon peel possessed blood glucose lowering CP are known for their broad range of biological activities like neuro- effect comparable to that of glimepiride and is effective in controlling protective, cardioprotective, anti-atherosclerosis, antimutagenic, anti- diabetes (Naim et al., 2012). Lemon peel extract stimulated β-cell of inflammatory, antiallergic, anti-oxidative and antitumor properties islets of Langerhans to secrete insulin and decreased the blood glucose (Chen et al., 2017; Duan et al., 2017; Gao et al., 2018; Li et al., 2009). level. Citron (Citrus medica L.) peels extract possesses potent antioxidant Orange peel is used as traditional medicine in skin inflammation, and antidiabetic properties. Treating streptozotocin-induced diabetic digestive problems, respiratory tract infections, muscle pain, hy- rats with ethanolic extract (400 mg/kg) of Citron peels for 8 weeks pertension and ringworm infections (Li et al., 2009). Mandarin orange considerably lowered the elevated amounts of blood glucose, glycosy- peel has been particularly shown to have antibacterial activity against lated hemoglobin and thiobarbituric acid reactive substances by in- acne causing microorganism (Cutibacterium acnes)(Hou et al., 2019). hibiting the lipid peroxidation (Kabra, Bairagi, & Wanare, 2012). Yuzu Hot alcoholic extract of orange (C. reticulata) peel showed strong in vitro (C. junos) peel contained certain flavonoids useful in preventing dis- anti-enzymatic (inhibition of collagenase and elastase up to 76 and eases associated with oxidative stress and inflammation. Ethanol ex- 80%, respectively) and antioxidant activities indicating the potent anti- tract (5%) of yuzu peel have significantly reduced weight gain, total aging ability of peel that can be used in skin care formulations (Apraj & cholesterol, serum triacylglycerol, liver fat content and insulin re- Pandita, 2016). Ethanol extract of orange peel can be used as natural sistance in mice fed on a high-fat diet (Kim et al., 2013). The study remedy for dental caries pathogens (Streptococcus mutans and Lactoba- reported that yuzu peel extract had produced anti-diabetic effects by cillus acidophilus) due to their therapeutic and antimicrobial potential reducing the secretion of high-fat diet induced adipocytokines (leptin (Shetty et al., 2016). Lemon peel extract helps in prevention and and resistin), increasing phosphorylation of AMP-activated protein ki- management of calcifications in the urinary system by inhibiting the nase (AMPK) in muscle tissues and by stimulating the transcriptional formation of calcium oxalate stony concretions and provides protection activity of peroxisome proliferator-activated receptor gamma (PPAR-γ) to urinary tract from stone induced damage (Sridharan et al., 2016). gene in a dose-dependent manner. Narirutin and hesperidin are the potential therapeutic agents in CP that CP extract can be used as a potent candidate in the treating fatty dynamically improve the angiogenic functions in vascular-related dis- liver disorder caused by excessive alcohol consumption. Ethanol con- eases. Aqueous extract of C. unshiu peel showed proangiogenic effects sumption elevated hepatic lipid content to 227 mg/g in the liver of by increased the phosphorylation of FAK and ERK1/2 through integrin- Sprague Dawley rats and co-administration of water-soluble peel ex- related signaling pathway in human umbilical vein endothelial cells tract (7.94 g/l of diet) from C. unshiu significantly suppressed total (Lee et al., 2016). CP extract is easily accessible and affordable for hepatic lipid to 187 mg/g of the liver (Park et al., 2012). Narirutin treatment of diarrhea. The hexane extract of C. limon peel reduces the present in C. unshiu peel extract suppressed hepatic fat accumulation in frequency of passage of wet feces, inhibits the accumulation of in- alcoholic liver disease and inhibited necrosis and cancer with its anti- testinal fluid and decreases intestinal motility by stimulating theβ inflammatory as well as antioxidant activities (Park, Ha, Eom, & Choi, adrenergic receptors of the gut (Adeniyi, Omale, Omeje, & Edino, 2013). Kang et al. (2012) investigated the anti-obesity activity of C. 2017). sunki peel extract in high-fat-diet-induced obese mice. The

14 B. Singh, et al. Food Research International 132 (2020) 109114

Table 3 Health benefits of fruit peels of different citrus species.

Peel source Botanical name Beneficial effects References

Lemon C. limon Antidiarrheal activity Adeniyi et al., 2017 Mandarin C. reticulata Anticancer activity Duan et al. (2017) Mandarin orangeCitron C. reticulataC. medica Anticancer activity Kurup et al. (2018) Citrus Citrus spp. Anticancer Activities Wang et al. (2014) Orange C. sinensis Cytotoxic effect against cancer cells Al-Ashaal and El-Sheltawy (2011) Orange C. sinensis Hypocholesterolemic and hypoglycemic effects Ashraf et al. (2017) Orange C. sinensis Hypoglycemic Effects Chau et al. (2003) Orange C. sinensis Improves intestinal health and function Chau et al. (2005) Ponkan C. reticulata Anti-inflammatory activity Ho and Lin (2008) Orange C. sinensis Anti-proliferative activity Manthey and Guthrie (2002) Orange C. sinensis Prevents inflammation and skin cancer Lai et al. (2007), Lai et al. (2011) and Lai, Li, Liu, et al. Prevents inflammation and colorectal cancer (2013) Citrus Citrus spp. Prostate cancer treatment Lai, Li, Miyauchi et al. (2013) Ponkan C. reticulata Anti-neuroinflammatory capacity Ho and Kuo (2014) Ponkan C. reticulate Antiinflammatory properties Huang and Ho (2010) Tonkan C. tankan Orange C. reticulata Antiinflammatory properties Chen et al. (2017) Orange C. reticulata Antiinflammatory properties Duan et al. (2017) Sunki Mandarin C. sunki Anti-inflammatory activity Choi, Ko, et al. (2007) Sweet orange C. sinensis Protective effects against cytotoxicity induced by Chen et al. (2012) oxidative stress Citrus (18 different species) Citrus spp. Protects against disease resulting from excessive NO Choi, Hwang, et al. (2007) production Mandarin orange C. reticulata Hypocholesterolemic and antidiabetic effects Fayek et al. (2017) Mandarin orange C. reticulata Anti-proliferative activity Ferreira et al. (2018) Ortanique Citrus reticulata × Citrus sinensis Hypolipidemic effects Green, Wheatley, Hanchard, et al., 2011 and Green, Wheatley, Mcgrowder, et al. (2011) Tangerine C. reticulata Hypolipidemic effects Kurowska and Manthey (2004) Citrus peel Citrus spp. Antiallergic properties Hagenlocher et al. (2017) Citrus peel Citrus spp. Protective effect in skin cancer Hakim et al. (2000) Sour orange C. aurantium Attenuated liver fibrosis Lim et al. (2016) Yuzu C. junos Protective effect against neurotoxicity Heo et al. (2004) Citron C. medica Antidiabetic Activity Kabra et al. (2012) Sunki Mandarin C. sunki Antiobesity Effects Kang et al. (2012) Shiikuwasa C. depressa Hayata Antiobesity Effects Lee et al. (2011) Yuzu C. junos Antidiabetic Activity Kim et al. (2013) Lemon C. limon Antidiabetic Activity Naim et al. (2012) Orange C. reticulata Antiinflammatory activity Manthey and Bendele (2008) Sweet lemon C. limetta Antiinflammatory activity Maurya, Mohanty, Pal, Chanotiya, and Bawankule (2018) Shiikuwasa C. depressa Hayata Neurotrophic activity Matsuzaki et al. (2008) Grapefruit C. paradisi Inhibits oral carcinogenesis Miller et al. (2007) Satsuma Mandarin C. unshiu Anti-alcoholic fatty liver activity Park et al. (2012) and Park et al. (2013) Sweet orange C. sinensis Antithyroidal activity Parmar and Kar (2008) Sudachi C. sudachi Improves dyslipidemia Tsutsumi et al. (2014) Orange C. reticulata Potent anti-ageing agent of skin Apraj and Pandita (2016) Sweet orange C. sinensis cancer chemoprevention Iwase et al. (2001) Lemon C. limon Cytotoxic effect on cancer cells Jomaa et al. (2012) Satsuma Mandarin C. unshiu Proangiogenic effects Lee et al. (2016) Orange C. sinensis Inhibits dental caries pathogens Shetty et al. (2016) Orange C. sinensis Inhibits proliferation colorectal cancer Silva et al. (2018) Lemon C. limon Anti-urolithic activity Sridharan et al. (2016) supplementation (150 mg/kg/d) of C. sunki peel extract for 70 days Manthey, 2004). Peel extract of ortanique contained six major PMF’s have reduced the body weight (16.9%), increase in weight (40.7%), (tangeretin [twenty nine percent], nobiletin [twenty four percent], adipose tissue weight gain (36%), serum cholesterol (17.6%) and tri- tetramethylscutellarein [twenty three percent], sinensetin [ten per- glyceride (39.3%) amounts in high-fat-diet-induced obese mice. CP cent], hexamethyl-o-quercetagetin [ten percent] and heptamethoxy- exerted anti-obesity effects by elevating the fatty acid β-oxidation and flavone [four percent]) and it is a potential hypolipidemic agent(Green, lipolysis in adipose tissue. The shiikuwasa (C. depressa Hayata) peel Wheatley, Hanchard, et al., 2011). It has shown cholesterol-lowering extract (1.5% w/w) suppressed the white adipose tissue weight, body effects in hypercholesterolemic rats by reducing the activity of hepatic weight gain, sizes of adipocytes, plasma triglyceride and leptin levels in 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMG-CoA) re- high-fat-diet-induced obese mice (Lee et al., 2011). The anti-obesity ductase and hepatic cholesterol levels as well as increasing fecal cho- effects of shiikuwasa (C. depressa Hayata) peel extract reported invol- lesterol output. Hypercholesterolemic rats fed on diets containing 1.5% ving regulating mRNA expressions of lipogenesis-related genes (acti- ortanique peel PMF’s extract for 49 days have shown significant re- vating protein 2, acetyl-CoA-carboxylase 1, stearoyl-CoA desaturase 1, duction in level of serum total cholesterol (45%), LDL cholesterol diacylglycerol acyltransferase 1 and fatty acid transport protein) in the (69%), very low-density lipoprotein cholesterol (30%) and triglyceride white adipose tissue of overweight mice fed on a high fat diet. (24%) and increase in level of serum high-density lipoprotein choles- PMF’s (tangeretin and nobiletin) of CP have serum cholesterol and terol (45%). Green, Wheatley, Mcgrowder, Dilworth, and Asemota triacylglycerol lowering potential and they are beneficial in the treat- (2011) evaluated the effects of ortanique peel PMFs extract onthe ment of hypercholesterolemia and hypertriglyceridemia (Kurowska & organ (liver, kidney, and spleen) morphology of diet-induced

15 B. Singh, et al. Food Research International 132 (2020) 109114 hypercholesterolemic rats and reported beneficial effects on organ mature fruits and it varies among different citrus species due to dif- (ameliorates hypercholesterolemia-associated alterations) structures ferences in flavonoid (nobiletin and tangeretin) contents. The peels of without causing any toxic effect. polymethoxylated flavone (nobiletin)-rich citrus species showed more Orange peel was evaluated for having anti-diabetic effects in in- potent NO production inhibitory activity and they can be used in pro- sulin-resistant diabetic rats, where it was noticed that this peel extract viding protection against health problems resulting from excessive NO was found to be greatly effective (Sathiyabama et al., 2018). Sudachitin production (Choi, Hwang, et al., 2007). Sunki mandarin (C. sunki) is a (5,7,4′-trihydroxy-6,8,3′-trimethoxyflavone), a PMF isolated from su- nobiletin-rich citrus fruit with important anti-inflammatory effects dachi (C. sudachi) peel have anti-diabetic and anti-obesity effects (Choi, Ko, et al., 2007). The fruit peel of Sunki Mandarin at 6–50 µM (Tsutsumi et al., 2014). Sudachitin (5 mg/kg) improved dyslipidemia concentration have exhibited anti-inflammatory role by targeting the

(by lowering triglyceride and free fatty acid amounts) and reduced nuclear factor (NF-kB) DNA-binding activity and by suppressing the weight gain in high-fat-diet-induced obese and diabetic mice by en- lipopolysaccharide-induced reactive oxygen species (ROS) generation hancing energy utilization and fatty acid β-oxidation by stimulating (Choi, Ko, et al., 2007). Intraperitoneal injection of orange peel PMF biogenesis of mitochondria in skeletal muscle (Tsutsumi et al., 2014). (3′,4′,3,5,6,7,8-heptamethoxyflavone) at a dose of 100 mg/kg showed Duan et al. (2017) showed the role of PMFs (nobiletin , tangeretin, significant anti-inflammatory effects with dose-dependent reduction of 3,5,6,7,8,3′,4′- heptamethoxyflavone and 5,6,7,3′,4′- pentamethoxy- the tumor necrosis factor-R production in lipopolysaccharide-chal- flavone) extracted from peel of C. reticulata ‘Chachi' in inhibition of lenged mice and fifty six percent inhibition of the carrageenan-induced sterol regulatory element-binding proteins. This inhibition activity can paw edema in rats (Manthey & Bendele, 2008). The antioxidant and be utilized as potential therapeutic agents for the treatment of obesity anti-inflammatory properties of flavonoids (hesperidin, naringin, and type 2 diabetes. The anti-diabetic and hypocholesterolemic effects diosmin, apigenin) reported in CP are responsible for their protection of CP is contributed by the presence of nobiletin. The CP extract con- against aging and common degenerative diseases (Ashraf et al., 2017). taining high level (mandarin peel) of nobiletin have shown the highest The PMF’s present in CP contribute crucially in anti-inflammatory decrease in cholesterol level (48.9–59.3%) compared to CP extract activity (Benavente-Garcia & Castillo, 2008; Huang & Ho, 2010). PMF’s containing low level (sweet orange, white grapefruit, and lime peels) of regulate inducible nitric oxide synthase gene expression in in- nobiletin (Fayek et al., 2017). Nobiletin prevents hepatic triacylgly- flammatory cells and suppress the LPS-stimulated nitric oxide produc- ceride accumulation, stimulates lipolysis in adipocytes, enhances fatty tion (Ho & Lin, 2008). The inhibitory ability of nobiletin on pros- acid beta-oxidation and attenuates dyslipidemia. taglandin E2 (PGE2) and nitric oxide (NO) production correlates with CP may be employed as a source of natural antioxidants in different the anti-inflammatory roles of CP extracts (Huang & Ho, 2010). Among foods and drug preparations. Londoño-Londoño et al. (2010) de- methanol peel extracts of seven citrus fruits, Ponkan and Tonkan con- termined that the flavonoids present in the peel of lime, orange, and tained a high content of nobiletin and they have shown the outstanding tangerine peel significantly inhibited the human LDL oxidation induced inhibitory effect on prostaglandin E2 (PGE 2) and NO production by copper or peroxynitrite by using thiobarbituric acid-reactive sub- (Huang & Ho, 2010). PMF’s containing a higher number of methoxy stances (TBARS) method. Bioactive compounds present in CP provide groups have shown stronger anti-inflammatory activity (inhibition of protection against oxidative stress-induced damage and cytotoxicity. NO production) than those with a fewer number of methoxy groups. CP The water extracts of sweet orange peel contain bioactive compounds contains PMF’s with anti-inflammatory properties that are effective in that showed a protective effect in cytotoxicity of HepG2 cells induced the prevention of mast cell-associated allergic diseases. Citrus peel by tert-butyl-hydroperoxide (Chen et al., 2012). This effect was con- PMF’s (nobiletin and tangeretin) are potential anti-allergic components nected with scavenging reactive oxygen species (ROS), decreasing lipid that have significantly suppressed lipopolysaccharides (LPS) and IgE peroxidation, up-regulating glutathione amounts and antioxidant en- mediated activation of mast cells of the intestine in humans zyme activity. The scavenging of ROS by sweet orange peel may also (Hagenlocher et al., 2017). Nobiletin and 3,5,6,7,8,3′,4′-heptamethox- have a role in regulating the expression of Bcl-2 family proteins, in- yflavone isolated from the peel of C. reticulata strongly inhibited NO creasing mitochondria membrane potential and decreasing the caspase- production in LPS stimulated RAW 264.7 cells due to a high number of 3 activation (Chen et al., 2012). methoxy groups (Duan et al., 2017). The anti-inflammatory activity is Tangerine peel can be used as a dietary supplement for treating highly correlated with the level of PMFs (nobiletin and tangeretin) in several inflammation-related neurodegenerative diseases. The effectve CP extracts. Heat-treatment (100 °C for 120 min) increased the level of anti-neuroinflammatory activity of tangerine peel extract has been as- nobiletin and tangeretin in methanol CP extract and also significantly cribed to the collective effect of three PMF’s (hesperidin, nobiletin, and elevated its antioxidant and anti-inflammatory activities (Ho & Lin, tangeretin) in inhibition of LPS-induced pro-inflammatory cytokine 2008). expression (Ho & Kuo, 2014). The neuron protective effect of nar- CP is an abundant source of phytochemicals which have a protective ingenin from C. junos peel was reported by Heo et al. (2004) against role in human skin cancer. Hakim, Harris, and Ritenbaugh (2000) ob- oxidative cell death induced by Aβ peptide in PC12 nerve cells. The served the significant negative relation between citrus peel consump- anti-amnestic and neuroprotective action of naringenin, a flavanone tion and carcinoma of squamous cells of the skin in a case-control study. from C. junos may be useful in prevention or cure of neurodegenerative Naringin (2.5% solution) isolated from grapefruit peel significantly disease like Alzheimer’s disease (Heo et al., 2004). Nobiletin isolated inhibited the development of oral carcinogenesis in hamsters by low- from CP controls synaptic transmission via the postsynaptic α-amino-3- ering the tumor number and reducing tumor burden by 50 and 70%, hydroxy-5-methyl-D-aspartate (AMPA) receptors and prevents bul- respectively (Miller et al., 2007). Hesperidin is a natural coloring and bectomy and amyloid-β protein-induced memory loss in mice and other chemo-preventive agent in the pharmaceutical industry with a wide related species due to its neurotrophic activity (Matsuzaki et al., 2008). range of therapeutic applications. It displayed high antioxidant activity Flavonoids have a significant role in anti-inflammatory activities of in DPPH assay and significant cytotoxic effect against the selected CP (Benavente-Garcia & Castillo, 2008). Dried CP is a good remedy for human carcinoma (breast, larynx, cervix, and liver) cell lines (Al-Ashaal alleviating coughs and reducing phlegm in inflammation-related re- & El-Sheltawy, 2011). Nobiletin and tangeretin present in CP extracts spiratory tract diseases (Ho & Lin, 2008). The immature and mature have plenty of protective activities and are considered as anticancer and fruits peels of 18 different citrus species were evaluated for anti-in- chemo-preventive agents (Lai, Li, Liu, et al., 2013; Lai, Li, Miyauchi, flammatory properties on the basis on their inhibitory effect onlipo- et al., 2013; Li et al., 2009). Lemon (C. limon) peel extract has shown in polysaccharide-induced nitric oxide (NO) production in RAW 264.7 vitro antitumor activities by decreasing the viability (by over 80%) of cells (Choi, Hwang, et al., 2007). The CP of immature fruits showed colorectal cancer cells (Jomaa, Rahmo, Alnori, & Chatty, 2012). Water significantly higher NO-production inhibitory activities than CPof extracts of C. reticulata (Mandarin orange) and C. medica (round) peel at

16 B. Singh, et al. Food Research International 132 (2020) 109114 a level of 50 μg/ml demonstrated 100% and 73.3% cell death, respec- strong anti-proliferative activity against three different human cancer tively in MTT assays against Dalton's Lymphoma Ascites (DLA) cell lines cell lines (MCF-7, A549, and HepG2) (Duan et al., 2017). PMF’s with a (Kurup et al., 2018). DLA cells treated with C. reticulata peel (25 μg/ml) higher number of methoxy groups have shown strong bioactive po- showed nuclear condensation, irregular membrane blebbing, loss of tential due to their higher hydrophobic effect and penetrating ability in membrane integrity, the formation of apoptotic bodies, cell cycle arrest the target cancer cells. PMF’s and hydroxylated PMF’s can easily in- as well as DNA damage that lead to apoptosis. filtrate into the cells and exhibit their role due to their highcell PMF’s (tangeretin, nobiletin and tetra-O-methylisoscutellarein) membrane permeability and transport ability (Gao et al., 2018). Man- present in tangerine (C. tangerina) peel are the potent inhibitors of darin peel extracts have shown an anti-proliferative and chemopre- tumor cell growth (Chen et al., 1997). 3,5,6,7,8,3′,4′-heptamethoxy- ventive effect against cancer. The hydro-ethanolic extract (500 µg/ml) flavone isolated from sweet orange (C. sinensis L. Osbeck) peel exhibited of mandarin peel have reduced the viability of BT-474 (human breast remarkable anti-tumor-initiating effect (62% reduction) against nitric carcinoma) cells by 60% after 48 h of exposure. The exposure of solid oxide-induced skin carcinogenesis in mouse (Iwase et al., 2001). PMFs phase extraction enriched hydro-ethanolic extract (500 µg/ml) of of orange peel exhibited strong antiproliferative activities against 6 mandarin peel for 48 h have reduced viability of BT-474 by 85% and of types of cancers in humans (lung, prostate, colon, melanoma, and es- Caco-2 (human colon adenocarcinoma) and HepG2 (Human liver he- trogen receptor positive and negative breast cancer) cell lines and they patocellular carcinoma) cell lines by 46.2 and 62.1%, respectively have a potential to be used as anticancer agents for humans (Manthey & (Ferreira et al., 2018). Sinesentin, nobiletin and tangeretin are the main Guthrie, 2002). PMF’s, hydroxylated PMF’s exhibited higher anti- PMF’s responsible for the anticancer properties of orange peel. The anti- proliferative activities than the flavanone aglycons and glycosylation proliferative role of PMF enriched orange peel extract in a (3D) model completely removes the anticancer potential of flavanones (Manthey & of human colorectal cancer (HT29) cell spheroids cultures was reported Guthrie, 2002). The hydroxylated PMF (5-Hydroxy-3,6,7,8,3′,4′-hex- (Silva et al., 2018). The anti-proliferative effects of orange peel extract amethoxyflavone) purified from peels of sweet orange showed astrong involve inhibition of cell proliferation, induction of cell cycle arrest protective effect against epithelial skin cancer. It was an effective an- (G2/M phase), promotion of apoptosis as well as reduction of aldehyde titumor agent capable of preventing skin tumorigenesis by significantly dehydrogenase population on HT29 cell spheroids (Silva et al., 2018). inhibiting mRNA and protein expression of enzymes (cyclooxygenase-2 CP extracts are less toxic to normal cells compared to vincristine (an- and inducible nitric oxide synthase) involved in inflammation, cell ticancer drug) and they can be used as nutraceuticals and cancer pre- proliferation and tumorigenesis (Lai et al., 2007). Monodemethylated ventive agent in food items (Kurup et al., 2018). PMFs (5-hydroxy-3,7,8,3′,4′-pentamethoxyflavone and 5-hydroxy- The bioavailability (portion of the substance that enters human 3,6,7,8,3′,4′ hexamethoxyflavone) from sweet orange (C. sinensis) peel blood circulation) of polyphenols is dependent on the chemical stability were reported to be more potent in growth inhibition of lung cancer of the these compounds, their permeability and behavior biologically H1299, H441, and H460 cells than permethoxylated PMFs (nobiletin i.e. breakdown via cells of the intestines and microbial enzymes in the and 3,5,6,7,8,3′,4′-heptamethoxyflavone). They strongly reduced colon (Ferreira et al., 2017; Singh, Singh, Kaur, & Singh, 2018b). cancer cell growth by decreasing level of oncogenic proteins (inducible Polyphenols undergo a detailed metabolism in human body, which nitric oxide (iNOS), cyclooxygenase-2 (COX-2), myeloid cell leukemia- primarily involves conjugation reactions that are regulated with the 1(Mcl-1) and K-ras), and by inducing apoptosis (Xiao et al., 2009). help of phase II enzymes (in the gut) as well as catabolism by microbial Hydroxylated PMF’s have more effective bioactive potential (anti- enzymes (McKay, Chen, Zampariello, & Blumberg, 2015; Singh et al., cancer as well as anti-inflammatory activities) than their PMF coun- 2018b). Moreover, the matrix (such as tablet or capsule) which is used terparts (Li et al., 2009). Antiproliferative activities of the flavonoid for delivery of these compounds determines their bioavailability and it glycosides (hesperidin and naringin) and PMFs (nobiletin, tangeretin, has been accepted that matrices that have lower processing show better 3,5,6,7,8,3′,4′-heptamethoxyflavone and 5-hydroxy-6,7,8,3′,4′-penta- behavior (Ortuño et al., 2010). Furthermore, it has also been reported methoxyflavone) isolated from Citri Reticulatae Pericarpium were eval- that polyphenols lose most of their pharmacokinetic effects when uti- uated against human hepatoblastoma and human lung carcinoma lized in an isolated way (Dias, Ferreira, & Barreiro, 2015; Ferreira et al., (A549) cell lines (Lai, Li, Liu, et al., 2013). 5-hydroxy-6,7,8,3′,4′-pen- 2017). Last but not the least, even if the in vitro results might show an tamethoxyflavone showed the highest and the flavonoid glycosides excellent bioactivity of a polyphenol, it may have very low bioavail- showed the weaker antiproliferative activity. Treatment with peel ex- ability under in vivo conditions. Therefore, in vivo studies are necessary tract of six citrus fruits (gold lotion) or hydroxylated PMFs from orange to determine the actual effects of polyphenols. peel have suppressed the amount of aberrant crypt foci and tumor formation in colonic tissues of mice by blocking the expression of cy- 7. Bioavailability clooxygenase, inducible nitric oxide synthase, matrix metalloproteinase 9, ornithine decarboxylase, cyclin D1 and vascular endothelial growth Bioavailability of phenolic compounds means the quantity of these factor (Lai et al., 2011; Lai, Li, Liu, et al., 2013). Hydroxylated PMFs health promoting agents that can reach in the bloodstream. When (0.05%) isolated from orange peel extract and gold lotion (peel extract consumed these compounds pass through mouth, reach the stomach containing abundant flavonoids with high content of polymethoxy- and then to the intestine for finding the way through the bloodstream flavones) showed anti-inflammatory, antiangiogenic, anti-proliferative, (Esfanjani, Assadpour, & Jafari, 2018). Notwithstanding of the large and pro-apoptotic activities in an azoxymethane-induced colonic tu- quantity of phenolic compounds in foods, they are not necessarily ab- morigenesis model (Lai et al., 2011) and in a prostate xenograft tumor sorbed in the same fashion. These compounds should be absorbed into model (Lai, Li, Miyauchi et al., 2013). Intraperitoneal injection (1 mg/ the bloodstream in order to show any health effects. The primary di- kg) and oral administration (2 mg/kg) of gold lotion for 5 days/week gestive mechanisms that govern the bioavailability of polyphenolic for 3 weeks have dramatically reduced the tumor weights (by 57 and compounds involve their liberation from food matrix, digestion in sto- 86%, respectively) and volumes (by 78 and 94%, respectively) with no mach and intestine, uptake by the cells as aglycones or conjugated, gross signs of toxicity in human prostate tumor xenograft model (Lai, Li, modifications, transportation in the bloodstream as well as tissues and Miyauchi et al., 2013). finally excretion (Bohn, 2014). Citrus peel PMF’s exhibit antitumorigenic activities by mechanisms The knowledge about the quantity of polyphenols absorbed in hu- such as antiangiogenesis, antigrowth, inhibition of cancer cell mobility, mans is very limited. The consumed phenolic compounds are present in cell cycle arrest, free radical scavenging and apoptosis (Wang et al., the inner cavity of gastrointestinal lumen and their absorption takes 2014). PMF’s (5-demethylnobiletin and 6,7,8,3′,4′-pentamethoxy- place by epithelium of gastrointestinal tract (Santhakumar, Battino, & flavanone) isolated from the peel of C. reticulata ‘Chachi' exhibited Alvarez-Suarez, 2018; Singh et al., 2018b). These are compounds are

17 B. Singh, et al. Food Research International 132 (2020) 109114 initially made bioaccessible prior to absorption in small intestine. study the effect of mechanical processing on in vitro digestibility. The Therefore, while getting absorbed, phenolic compounds are linked to results showed that the biological characteristics were more retained in the small intestine, and afterwards later in liver, where conjugation reac- larger particles (500–710 μm) as compared to smaller ones (125–180 μm) tions occur (such as glucuronidation, methylation and sulfation) redu- which suggested release of functional components depends on the pro- cing the amounts of aglycones in the blood (Heleno, Martins, Queiroz, cessing conditions used (Cai, Qin, Ketnawa, & Ogawa, 2020). In a recent & Ferreira, 2015). Conjugation reactions are required for detoxification study, the oral delivery effectiveness CP extract, in form of nanoemulsion, and enhancing hydrophilicity of phenolic compounds so that they can that contained PMF’s was tested in two in vitro digestion models, pH-stat be easily excreted in the urine. Cardona, Andrés-Lacueva, Tulipani, lipolysis model and TNO gastro-intestinal model. The study showed that Tinahones, and Queipo-Ortuño (2013) reported that just around five to PMF’s were better bioaccessible in nanoemulsion as compared to pure oil ten percent of the total consumed polyphenols are absorbed in the small and nobiletin had better accessibility than tangeretin (Lu et al., 2020). intestine. Zanotti et al. (2015) reported that phenolic acids got max- As phenolic compounds are rapidly metabolized by humans as xe- imum absorption in small intestine as compared to flavonols which are nobiotic compounds, a lot of research is still necessary to completely absorbed in very less amounts. Gut microbiota acts on unmodified put the therapeutic uses elucidated by in vitro assays into proven in polyphenols (90–95%) in large intestine and cause splitting of glyco- vivo activity. In addition, phenolic compounds are generally weakly sidic linkages and heterocyclic backbones (Singh et al., 2018b). absorbed in the bloodstream and are metabolized easily but designing As aforementioned, the major flavanones in CP are hesperidin and of polyphenol analogs or modifying naturally present original ones naringenin, while major flavones are tangeretin and nobiletin (PMFs). (having high bioavailability) is a matter of ongoing investigation Most studies regarding bioavailability in CP has been done on these (Nielsen et al., 2006). polyphenols. It has been observed that flavonoids have less oral bioavailability owing to large conjugation of their free hydroxyl groups 8. Conclusion and future prospects (Manach & Donovan, 2004). Ameer, Weintraub, Johnson, Yost, and Rouseff (1996) also reported low bioavailability (around 25%) of nar- CP is promising source of phytochemicals (such as phenolic com- ingin and hesperidin and suggested that after absorption, CP flavanones pounds) that may be employed in foods for the reduction or treating may undergo glucuronidation prior to urinary excretion. However, in diseases. Pharmaceutical and food applications (specifically as bioac- case of PMFs, these have a benzo-γ-pyrone skeleton containing a car- tive compounds and dietary fiber source) are attractive ways ofCP bonyl group at the carbon 3rd position and methoxy groups in various valorization as it is a good source of antioxidants. A multidisciplinary positions on the benzo- γ-pyrone skeleton. Moreover, the exclusive effort is required for effective valorization of CP waste. If thevalor- character in the structure of PMFs is the polymethylation (increasing ization is to be done at a large scale in industries, then these industries oral bioavailability) of polyhydroxylated flavonoids that results in en- must be linked to the agricultural sector. The isolation procedures for hanced metabolic stability as well as membrane transport in intestine phenolic compounds from this waste should not only be efficient (such and liver (Evans, Sharma, & Guthrie, 2012). as ultrasound or microwave assisted ones) but also not harmful to the Manach and Donovan (2004) reported that flavanone absorption environment. Moreover, CP can be also directly incorporated which is took place in large intestine owing to their binding to a rutinose/ an economical way for effective utilization but it should be free from neohesperidose moiety in foods. Moreover, they documented that no- microbial toxins and harmful pesticides. In addition, the variation in biletin had higher bioavailability as well as efficacy in comparison with phenolic compounds amongst different citrus fruits can be utilized to tangeretin. Onoue et al. (2011) documented that as nobiletin was obtain a phenolic fingerprint, which is essentially a chemotaxonomic poorly soluble and bioavailable in the bloodstream, higher doses were marker. Mostly chromatographic analysis has been done for separation required for eliciting its response in the central nervous system. They as well as identification of phenolics but using spectroscopic analysis suggest that an amorphous, nanosized amorphous solid dispersion (using NMR and IR) is also an area that needs to be investigated (where could be a viable option for enhancing the bioavailability and central sample preparation is easy). nervous system delivery of nobiletin. Datla, Christidou, Widmer, Phytochemicals of CP (phenolics and flavonoids) exhibit a wide Rooprai, and Dexter (2001) showed that in rats, tangeretin was able to range of biological activities that are attracting the attention of scien- cross the blood-brain barrier. Therefore, it can be used as an effective tists for benefitting humans. Therefore, it might be used as a source of neuroprotective agent, further supporting the role of CP phenolic functional substances and preservatives in the development of newer compounds. In a recent report, tangeretin was shown to have an oral food products that are safe and have health enhancing activities. bioavailability of around 27% in rats (Hung et al., 2018). Their study However, the bioactivities of CP phytochemicals are mainly reported by provided important knowledge not only related with the absorption but in vitro studies as well as animal models, so more clinical trials are also distribution and excretion of tangeretin. These authors reported needed to determine the potential and safety of these compounds. Even that maximum concentration of this polyphenol in organs (such as by the latest advances in this aspect, the actual efficacy of polyphenols kidney, lung, liver, spleen and heart) occurred at four or eight hours is uncertain. In the matrices, there is synergistic or antagonistic reac- after ingesting it in diet. Further, maximum amounts of tangeretin were tions among these compounds which make the results from in vitro and found at four hours of ingestion, while in parts of large intestine it in vivo studies variable. Furthermore, shelf life as well as clinical effects reached the maximum concentrations at twelve hours. of phenolic compounds in CP need to properly assessed for their final In case of hesperidin, Nielsen et al. (2006) demonstrated that removing incorporation in food products. Beyond shadow of doubt, more human rhamnose group to produce hesperetin-7-glucoside improved its bioa- studies need to be performed where sample size is big such as large vailability in human subjects. This improvement was mainly due to change populations, in order to determine the true effectiveness and optimal in the absorption site of this polyphenol from large intestine to the small dose of CP phenolic compounds. We expect more research on under- intestine. Hesperidin also had a more therapeutic effect than naringenin standing the mechanism of action and therapeutic potential of CP regardless of its lesser bioavailability and no added effect was observed by phytochemicals. the combination these two flavanones in comparison to the supplementation with hesperidin only (Habauzit et al., 2011). Siddique, Firdous, Durrani, Khan, and Saeed (2016) in an in vitro study, proved that he- Declaration of Competing Interest speridin boosted the bioavailability of micronutrients (in particular Ca) in chicken egg shell samples. This effect of hesperidin can be translated in The authors declare that they have no known competing financial humans reflecting its role in preventing bone loss due to calcium defi- interests or personal relationships that could have appeared to influ- ciency. Pulverized CP tissue of different particle sizes was prepared to ence the work reported in this paper.

18 B. Singh, et al. Food Research International 132 (2020) 109114

Acknowledgements peels. International Food Research Journal, 16(2), 203–213. Chau, C. F., Huang, Y. L., & Lee, M. H. (2003). In vitro hypoglycemic effects of different insoluble fiber-rich fractions prepared from the peel of Citrus sinensis L. cv. Liucheng. Authors are thankful to Science and Engineering Research Board, Journal of Agricultural and Food Chemistry, 51(22), 6623–6626. University Grants Commission, New Delhi and CSIR Project No. Chau, C. F., Sheu, F., Huang, Y. L., & Su, L. H. (2005). Improvement in intestinal function 38(1419)/16/EMR-II for providing financial assistance. and health by the peel fibre derived from Citrus sinensis L cv Liucheng. Journal of the Science of Food and Agriculture, 85(7), 1211–1216. Chavan, P., Singh, A. K., & Kaur, G. (2018). Recent progress in the utilization of industrial Appendix A. Supplementary material waste and by-products of citrus fruits: A review. Journal of Food Process Engineering, 41(8), e12895. Supplementary data to this article can be found online at https:// Cheigh, C. I., Chung, E. Y., & Chung, M. S. (2012). Enhanced extraction of flavanones hesperidin and narirutin from Citrus unshiu peel using subcritical water. Journal of doi.org/10.1016/j.foodres.2020.109114. Food Engineering, 110(3), 472–477. Chen, J., Montanari, A. M., & Widmer, W. W. (1997). Two new polymethoxylated fla- References vones, a class of compounds with potential anticancer activity, isolated from cold pressed dancy tangerine peel oil solids. Journal of Agricultural and Food Chemistry, 45(2), 364–368. Adeniyi, O. S., Omale, J., Omeje, S. C., & Edino, V. O. (2017). Antidiarrheal activity of Chen, M. L., Yang, D. J., & Liu, S. C. (2011). Effects of drying temperature on the fla- hexane extract of Citrus limon peel in an experimental animal model. Journal of vonoid, phenolic acid and antioxidative capacities of the methanol extract of citrus Integrative Medicine, 15(2), 158–164. fruit (Citrus sinensis (L.) Osbeck) peels. International Journal of Food Science & Al-Ashaal, H. A., & El-Sheltawy, S. T. (2011). Antioxidant capacity of hesperidin from Technology, 46(6), 1179–1185. citrus peel using electron spin resonance and cytotoxic activity against human car- Chen, X. M., Tait, A. R., & Kitts, D. D. (2017). Flavonoid composition of orange peel and cinoma cell lines. Pharmaceutical biology, 49(3), 276–282. its association with antioxidant and anti-inflammatory activities. Food Chemistry, 218, Al-Saman, M. A., Abdella, A., Mazrou, K. E., Tayel, A. A., & Irmak, S. (2019). 15–21. Antimicrobial and antioxidant activities of different extracts of the peel of kumquat Chen, Z. T., Chu, H. L., Chyau, C. C., Chu, C. C., & Duh, P. D. (2012). Protective effects of (Citrus japonica Thunb). Journal of Food Measurement and Characterization, 13(4), sweet orange (Citrus sinensis) peel and their bioactive compounds on oxidative stress. 3221–3229. Food Chemistry, 135(4), 2119–2127. Ameer, B., Weintraub, R. A., Johnson, J. V., Yost, R. A., & Rouseff, R. L. (1996). Flavanone Cheong, M. W., Chong, Z. S., Liu, S. Q., Zhou, W., Curran, P., & Yu, B. (2012). absorption after naringin, hesperidin, and citrus administration. Clinical Characterisation of calamansi (Citrus microcarpa). Part I: Volatiles, aromatic profiles Pharmacology & Therapeutics, 60(1), 34–40. and phenolic acids in the peel. Food Chemistry, 134(2), 686–695. Anagnostopoulou, M. A., Kefalas, P., Kokkalou, E., Assimopoulou, A. N., & Papageorgiou, Cho, H. E., Ahn, S. Y., Kim, S. C., Woo, M. H., Hong, J. T., & Moon, D. C. (2014). V. P. (2005). Analysis of antioxidant compounds in sweet orange peel by HPLC–diode Determination of flavonoid glycosides, polymethoxyflavones, and coumarins in array detection–electrospray ionization mass spectrometry. Biomedical herbal drugs of citrus and poncirus fruits by high performance liquid chromato- Chromatography, 19(2), 138–148. graphy–electrospray ionization/tandem mass spectrometry. Analytical Letters, 47(8), Anagnostopoulou, M. A., Kefalas, P., Papageorgiou, V. P., Assimopoulou, A. N., & Boskou, 1299–1323. D. (2006). Radical scavenging activity of various extracts and fractions of sweet or- Choi, M. Y., Chai, C., Park, J. H., Lim, J., Lee, J., & Kwon, S. W. (2011). Effects of storage ange peel (Citrus sinensis). Food Chemistry, 94(1), 19–25. period and heat treatment on phenolic compound composition in dried Citrus peels Apraj, V. D., & Pandita, N. S. (2016). Evaluation of skin anti-aging potential of Citrus (Chenpi) and discrimination of Chenpi with different storage periods through tar- reticulata blanco peel. Pharmacognosy Research, 8(3), 160. geted metabolomic study using HPLC-DAD analysis. Journal of Pharmaceutical and Ashraf, H., Butt, M. S., Iqbal, M. J., & Suleria, H. A. R. (2017). Citrus peel extract and Biomedical Analysis, 54(4), 638–645. powder attenuate hypercholesterolemia and hyperglycemia using rodent experi- Choi, S. Y., Hwang, J. H., Ko, H. C., Park, J. G., & Kim, S. J. (2007). Nobiletin from citrus mental modeling. Asian Pacific Journal of Tropical Biomedicine, 7(10), 870–880. fruit peel inhibits the DNA-binding activity of NF-κB and ROS production in LPS- Baldi, A., Rosen, R. T., Fukuda, E. K., & Ho, C. T. (1995). Identification of nonvolatile activated RAW 264.7 cells. Journal of Ethnopharmacology, 113(1), 149–155. components in lemon peel by high-performance liquid chromatography with con- Choi, S. Y., Ko, H. C., Ko, S. Y., Hwang, J. H., Park, J. G., Kang, S. H., ... Kim, S. J. (2007). firmation by mass spectrometry and diode-array detection. Journal of Chromatography Correlation between flavonoid content and the NO production inhibitory activity of A, 718(1), 89–97. peel extracts from various citrus fruits. Biological and Pharmaceutical Bulletin, 30(4), Ballistreri, G., Fabroni, S., Romeo, F. V., Timpanaro, N., Amenta, M., & Rapisarda, P. 772–778. (2019). Anthocyanins and other polyphenols in citrus genus: Biosynthesis, chemical Costa, R., Albergamo, A., Arrigo, S., Gentile, F., & Dugo, G. (2019). Solid-phase micro- profile, and biological activity. Polyphenols in plants (pp. 191–215). Academic Press. extraction-gas chromatography and ultra-high performance liquid chromatography Banerjee, J., Singh, R., Vijayaraghavan, R., MacFarlane, D., Patti, A. F., & Arora, A. applied to the characterization of lemon wax, a waste product from citrus industry. (2017). Bioactives from fruit processing wastes: Green approaches to valuable che- Journal of Chromatography A, 1603, 262–268. micals. Food chemistry, 225, 10–22. Craft, B. D., Kerrihard, A. L., Amarowicz, R., & Pegg, R. B. (2012). Phenol-based anti- Benavente-Garcia, O., & Castillo, J. (2008). Update on uses and properties of citrus fla- oxidants and the in vitro methods used for their assessment. Comprehensive Reviews in vonoids: New findings in anticancer, cardiovascular, and anti-inflammatory activity. Food Science and Food Safety, 11(2), 148–173. Journal of Agricultural and Food Chemistry, 56(15), 6185–6205. Dahmoune, F., Boulekbache, L., Moussi, K., Aoun, O., Spigno, G., & Madani, K. (2013). Bocco, A., Cuvelier, M. E., Richard, H., & Berset, C. (1998). Antioxidant activity and Valorization of Citrus limon residues for the recovery of antioxidants: Evaluation and phenolic composition of citrus peel and seed extracts. Journal of Agricultural and Food optimization of microwave and ultrasound application to solvent extraction. Chemistry, 46(6), 2123–2129. Industrial Crops and Products, 50, 77–87. Bohn, T. (2014). Dietary factors affecting polyphenol bioavailability. Nutrition Reviews, Datla, K. P., Christidou, M., Widmer, W. W., Rooprai, H. K., & Dexter, D. T. (2001). Tissue 72(7), 429–452. distribution and neuroprotective effects of citrus flavonoid tangeretin in a rat model Boysen, R. I., & Hearn, M. T. W. (2010). High performance liquid chromatographic se- of Parkinson's disease. Neuroreport, 12(17), 3871–3875. paration methods. Comprehensive Natural Products II: Chemistry and Biology (pp. 5– de Moraes Barros, H. R., de Castro Ferreira, T. A. P., & Genovese, M. I. (2012). 49). Elsevier. Antioxidant capacity and mineral content of pulp and peel from commercial cultivars Bustamante, J., van Stempvoort, S., García-Gallarreta, M., Houghton, J. A., Briers, H. K., of citrus from Brazil. Food Chemistry, 134(4), 1892–1898. Budarin, V. L., ... Clark, J. H. (2016). Microwave-assisted hydro-distillation of es- Di Rauso Simeone, G., Di Matteo, A., Rao, M. A., & Di Vaio, C. (2020). Variations of peel sential oils from wet citrus peel waste. Journal of Cleaner Production, 137, 598–605. essential oils during fruit ripening in four lemon (Citrus limon (L.) Burm. F.) cultivars. Buyukkurt, O. K., Guclu, G., Kelebek, H., & Selli, S. (2019). Characterization of phenolic Journal of the Science of Food and Agriculture, 100(1), 193–200. compounds in sweet lime (Citrus limetta) peel and freshly squeezed juices by LC-DAD- Dias, M. I., Ferreira, I. C., & Barreiro, M. F. (2015). Microencapsulation of bioactives for ESI-MS/MS and their antioxidant activity. Journal of Food Measurement and food applications. Food & Function, 6(4), 1035–1052. Characterization, 13(4), 3242–3249. Duan, L., Dou, L. L., Yu, K. Y., Guo, L., Bai-Zhong, C., Li, P., & Liu, E. H. (2017). Cai, Y., Qin, W., Ketnawa, S., & Ogawa, Y. (2020). Impact of particle size of pulverized Polymethoxyflavones in peel of Citrus reticulata ‘Chachi’and their biological activities. citrus peel tissue on changes in antioxidant properties of digested fluids during si- Food Chemistry, 234, 254–261. mulated in vitro digestion. Food Science and Human Wellness. https://doi.org/10. El-aal, H. A., & Halaweish, F. T. (2010). Food preservative activity of phenolic compounds 1016/j.fshw.2019.12.008 (in press). in orange peel extracts (Citrus sinensis L.). Lucrări Ştiinţifice, 53, 233–240. Cardona, F., Andrés-Lacueva, C., Tulipani, S., Tinahones, F. J., & Queipo-Ortuño, M. I. Esfanjani, A. F., Assadpour, E., & Jafari, S. M. (2018). Improving the bioavailability of (2013). Benefits of polyphenols on gut microbiota and implications in human health. phenolic compounds by loading them within lipid-based nanocarriers. Trends in Food The Journal of Nutritional Biochemistry, 24(8), 1415–1422. Science & Technology, 76, 56–66. Casquete, R., Castro, S. M., Martín, A., Ruiz-Moyano, S., Saraiva, J. A., Córdoba, M. G., & Evans, M., Sharma, P., & Guthrie, N. (2012). Bioavailability of citrus polymethoxylated Teixeira, P. (2015). Evaluation of the effect of high pressure on total phenolic con- flavones and their biological role in metabolic syndrome and hyperlipidemia. tent, antioxidant and antimicrobial activity of citrus peels. Innovative Food Science & Readings in advanced pharmacokinetics-Theory, methods and applications. Intech, Emerging Technologies, 31, 37–44. 267–284. Castro-Muñoz, R., Yáñez-Fernández, J., & Fíla, V. (2016). Phenolic compounds recovered Fayek, N. M., El-Shazly, A. H., Abdel-Monem, A. R., Moussa, M. Y., Abd-Elwahab, S. M., & from agro-food by-products using membrane technologies: An overview. Food El-Tanbouly, N. D. (2017). Comparative study of the hypocholesterolemic, anti- Chemistry, 213, 753–762. diabetic effects of four agro-waste Citrus peels cultivars and their HPLC standardi- Chan, S. W., Lee, C. Y., Yap, C. F., Mustapha, W. A. W., & Ho, C. W. (2009). Optimisation zation. Revista Brasileira de Farmacognosia, 27(4), 488–494. of extraction conditions for phenolic compounds from limau purut (Citrus hystrix) Ferreira, I. C., Martins, N., & Barros, L. (2017). Phenolic compounds and its

19 B. Singh, et al. Food Research International 132 (2020) 109114

bioavailability: In vitro bioactive compounds or health promoters? Advances in food Hou, H. S., Bonku, E. M., Zhai, R., Zeng, R., Hou, Y. L., Yang, Z. H., & Quan, C. (2019). and nutrition research, Vol. 82, 1–44. Extraction of essential oil from Citrus reticulate Blanco peel and its antibacterial ac- Ferreira, S. S., Silva, A. M., & Nunes, F. M. (2018). Citrus reticulata Blanco peels as a tivity against Cutibacterium acnes (formerly Propionibacterium acnes). Heliyon, 5(12), source of antioxidant and anti-proliferative phenolic compounds. Industrial Crops and e02947. Products, 111, 141–148. Huang, Y. S., & Ho, S. C. (2010). Polymethoxy flavones are responsible for the anti- Food and Agriculture Organization (2017). Citrus fruit fresh and processed - Statistical inflammatory activity of citrus fruit peel. Food Chemistry, 119(3), 868–873. Bulletin 2016. Rome: Food and Agriculture Organization of the United Nations. Hung, W. L., Chang, W. S., Lu, W. C., Wei, G. J., Wang, Y., Ho, C. T., & Hwang, L. S. http://www.fao.org/3/a-i8092e.pdf. (2018). Pharmacokinetics, bioavailability, tissue distribution and excretion of tan- Gao, Z., Gao, W., Zeng, S. L., Li, P., & Liu, E. H. (2018). Chemical structures, bioactivities geretin in rat. Journal of Food and Drug Analysis, 26(2), 849–857. and molecular mechanisms of citrus polymethoxyflavones. Journal of Functional Inoue, T., Tsubaki, S., Ogawa, K., Onishi, K., & Azuma, J. I. (2010). Isolation of hesperidin Foods, 40, 498–509. from peels of thinned Citrus unshiu fruits by microwave-assisted extraction. Food Ghanem, N., Mihoubi, D., Kechaou, N., & Mihoubi, N. B. (2012). Microwave dehydration Chemistry, 123(2), 542–547. of three citrus peel cultivars: Effect on water and oil retention capacities, color, Iwase, Y., Takemura, Y., Ju-ichi, M., Yano, M., Ito, C., Furukawa, H., ... Nishino, H. shrinkage and total phenols content. Industrial Crops and Products, 40, 167–177. (2001). Cancer chemopreventive activity of 3, 5, 6, 7, 8, 3′, 4′-heptamethoxyflavone Ghasemi, K., Ghasemi, Y., & Ebrahimzadeh, M. A. (2009). Antioxidant activity, phenol from the peel of citrus plants. Cancer Letters, 163(1), 7–9. and flavonoid contents of 13 citrus species peels and tissues. Pakistan Journal of Jeong, S. M., Kim, S. Y., Kim, D. R., Jo, S. C., Nam, K. C., Ahn, D. U., & Lee, S. C. (2004). Pharmaceutical Sciences, 22(3), 277–281. Effect of heat treatment on the antioxidant activity of extracts from citrus peels. Gómez-Mejía, E., Rosales-Conrado, N., León-González, M. E., & Madrid, Y. (2019). Citrus Journal of Agricultural and Food Chemistry, 52(11), 3389–3393. peels waste as a source of value-added compounds: Extraction and quantification of Jomaa, S., Rahmo, A., Alnori, A. S., & Chatty, M. E. (2012). The cytotoxic effect of es- bioactive polyphenols. Food Chemistry, 295, 289–299. sential oil of Syrian Citrus limon peel on human colorectal carcinoma cell line González-Mas, M. C., Rambla, J. L., López-Gresa, M. P., Blázquez, M. A., & Granell, A. (Lim1863). Middle East Journal of Cancer, 3(1), 15–21. (2019). Volatile compounds in Citrus essential oils: A comprehensive review. Frontiers Kabra, A. O., Bairagi, G. B., & Wanare, R. S. (2012). Antidiabetic activity of ethanol in Plant Science, 10, 12. extract of Citrus medica L. peels in streptozotocin induced diabetic rats. Journal of Gorinstein, S., Martín-Belloso, O., Park, Y. S., Haruenkit, R., Lojek, A., Ĉıž,́ M., ... Pharmacy Research, 5(3), 1287–1289. Trakhtenberg, S. (2001). Comparison of some biochemical characteristics of different Kaderides, K., Mourtzinos, I., & Goula, A. M. (2020). Stability of pomegranate peel citrus fruits. Food Chemistry, 74(3), 309–315. polyphenols encapsulated in orange juice industry by-product and their incorporation Green, C. O., Wheatley, A. O., Hanchard, B., Gibson, T. N., McGrowder, D. A., Dilworth, L. in cookies. Food Chemistry, 310, 125849. L., & Asemota, H. N. (2011). Histopathological alterations in organ structures of Kanaze, F. I., Termentzi, A., Gabrieli, C., Niopas, I., Georgarakis, M., & Kokkalou, E. hypercholesterolemic rats fed Ortanique peel polymethoxylated flavones. Basic and (2009). The phytochemical analysis and antioxidant activity assessment of orange Applied Pathology, 4(3), 71–77. peel (Citrus sinensis) cultivated in Greece-Crete indicates a new commercial source of Green, C. O., Wheatley, A. O., Mcgrowder, D. A., Dilworth, L. L., & Asemota, H. N. (2011). hesperidin. Biomedical Chromatography, 23(3), 239–249. Hypolipidemic effects of ortanique peel polymethoxylated flavones in rats with diet- Kang, H. J., Chawla, S. P., Jo, C., Kwon, J. H., & Byun, M. W. (2006). Studies on the induced hypercholesterolemia. Journal of Food Biochemistry, 35(5), 1555–1560. development of functional powder from citrus peel. Bioresource Technology, 97(4), Green, C. O., Wheatley, A. O., Osagie, A. U., Morrison, E. Y. S. A., & Asemota, H. N. 614–620. (2007). Determination of polymethoxylated flavones in peels of selected Jamaican Kang, S. I., Shin, H. S., Kim, H. M., Hong, Y. S., Yoon, S. A., Kang, S. W., ... Kim, S. J. and Mexican citrus (Citrus spp.) cultivars by high-performance liquid chromato- (2012). Immature Citrus sunki peel extract exhibits antiobesity effects by β-oxidation graphy. Biomedical Chromatography, 21(1), 48–54. and lipolysis in high-fat diet-induced obese mice. Biological and Pharmaceutical Guimarães, R., Barros, L., Barreira, J. C., Sousa, M. J., Carvalho, A. M., & Ferreira, I. C. Bulletin, 35(2), 223–230. (2010). Targeting excessive free radicals with peels and juices of citrus fruits: Khan, M. K., Abert-Vian, M., Fabiano-Tixier, A. S., Dangles, O., & Chemat, F. (2010). Grapefruit, lemon, lime and orange. Food and Chemical Toxicology, 48(1), 99–106. Ultrasound-assisted extraction of polyphenols (flavanone glycosides) from orange Habauzit, V., Sacco, S. M., Gil-Izquierdo, A., Trzeciakiewicz, A., Morand, C., Barron, D., ... (Citrus sinensis L.) peel. Food Chemistry, 119(2), 851–858. Horcajada, M. N. (2011). Differential effects of two citrus flavanones on bone quality Kim, H. G., Kim, G. S., Lee, J. H., Park, S., Jeong, W. Y., Kim, Y. H., ... Lee, S. J. (2011). in senescent male rats in relation to their bioavailability and metabolism. Bone, 49(5), Determination of the change of flavonoid components as the defence materials of 1108–1116. Citrus unshiu Marc. fruit peel against Penicillium digitatum by liquid chromatography Hagenlocher, Y., Feilhauer, K., Schäffer, M., Bischoff, S. C., & Lorentz, A. (2017). Citrus coupled with tandem mass spectrometry. Food Chemistry, 128(1), 49–54. peel polymethoxyflavones nobiletin and tangeretin suppress LPS-and IgE-mediated Kim, S. H., Hur, H. J., Yang, H. J., Kim, H. J., Kim, M. J., Park, J. H., ... Hwang, J. T. activation of human intestinal mast cells. European Journal of Nutrition, 56(4), (2013). Citrus junos Tanaka Peel Extract Exerts Antidiabetic Effects via AMPK and 1609–1620. PPAR-both In Vitro and In Vivo in Mice Fed a High-Fat Diet. Evidence-based Hakim, I. A., Harris, R. B., & Ritenbaugh, C. (2000). Citrus peel use is associated with Complementary and Alternative Medicine, 2013. reduced risk of squamous cell carcinoma of the skin. Nutrition and Cancer, 37(2), Kurowska, E. M., & Manthey, J. A. (2004). Hypolipidemic effects and absorption of citrus 161–168. polymethoxylated flavones in hamsters with diet-induced hypercholesterolemia. Han, S., Kim, H. M., Lee, J. M., Mok, S. Y., & Lee, S. (2010). Isolation and identification of Journal of Agricultural and Food Chemistry, 52(10), 2879–2886. polymethoxyflavones from the hybrid Citrus, Hallabong. Journal of Agricultural and Kurup, S. R., Nair, A. S., & Baby, S. (2018). Citrus peels prevent cancer. Phytomedicine, 50, Food Chemistry, 58(17), 9488–9491. 231–237. Hayat, K., Hussain, S., Abbas, S., Farooq, U., Ding, B., Xia, S., ... Xia, W. (2009). Optimized Lagha-Benamrouche, S., & Madani, K. (2013). Phenolic contents and antioxidant activity microwave-assisted extraction of phenolic acids from citrus mandarin peels and of orange varieties (Citrus sinensis L. and Citrus aurantium L.) cultivated in Algeria: evaluation of antioxidant activity in vitro. Separation and Purification Technology, Peels and leaves. Industrial Crops and Products, 50, 723–730. 70(1), 63–70. Lai, C. S., Li, S., Chai, C. Y., Lo, C. Y., Ho, C. T., Wang, Y. J., & Pan, M. H. (2007). Hayat, K., Zhang, X., Chen, H., Xia, S., Jia, C., & Zhong, F. (2010). Liberation and se- Inhibitory effect of citrus 5-hydroxy-3, 6, 7, 8, 3′, 4′-hexamethoxyflavone on12-O- paration of phenolic compounds from citrus mandarin peels by microwave heating tetradecanoylphorbol 13-acetate-induced skin inflammation and tumor promotion in and its effect on antioxidant activity. Separation and Purification Technology, 73(3), mice. Carcinogenesis, 28(12), 2581–2588. 371–376. Lai, C. S., Li, S., Liu, C. B., Miyauchi, Y., Suzawa, M., Ho, C. T., & Pan, M. H. (2013). He, D., Shan, Y., Wu, Y., Liu, G., Chen, B., & Yao, S. (2011). Simultaneous determination Effective suppression of azoxymethane-induced aberrant crypt foci formation inmice of flavanones, hydroxycinnamic acids and alkaloids in citrus fruits by HPLC- with citrus peel flavonoids. Molecular Nutrition & Food Research, 57(3), 551–555. DAD–ESI/MS. Food Chemistry, 127(2), 880–885. Lai, C. S., Li, S., Miyauchi, Y., Suzawa, M., Ho, C. T., & Pan, M. H. (2013). Potent anti- Hegazy, A. E., & Ibrahium, M. I. (2012). Antioxidant activities of orange peel extracts. cancer effects of citrus peel flavonoids in human prostate xenograft tumors. Food & World Applied Sciences Journal, 18(5), 684–688. Function, 4(6), 944–949. Heleno, S. A., Martins, A., Queiroz, M. J. R., & Ferreira, I. C. (2015). Bioactivity of Lai, C. S., Tsai, M. L., Cheng, A. C., Li, S., Lo, C. Y., Wang, Y., ... Pan, M. H. (2011). phenolic acids: Metabolites versus parent compounds: A review. Food Chemistry, 173, Chemoprevention of colonic tumorigenesis by dietary hydroxylated polymethoxy- 501–513. flavones in azoxymethane-treated mice. Molecular Nutrition & Food Research, 55(2), Heo, H. J., Kim, D. O., Shin, S. C., Kim, M. J., Kim, B. G., & Shin, D. H. (2004). Effect of 278–290. antioxidant flavanone, naringenin, from Citrus junos on neuroprotection. Journal of Lee, J., Yang, D. S., Han, S. I., Yun, J. H., Kim, I. W., Kim, S. J., & Kim, J. H. (2016). Agricultural and Food Chemistry, 52(6), 1520–1525. Aqueous extraction of Citrus unshiu peel induces proangiogenic effects through the Hirata, T., Fujii, M., Akita, K., Yanaka, N., Ogawa, K., Kuroyanagi, M., & Hongo, D. FAK and ERK1/2 signaling pathway in human umbilical vein endothelial cells. (2009). Identification and physiological evaluation of the components from Citrus Journal of Medicinal Food, 19(6), 569–577. fruits as potential drugs for anti-corpulence and anticancer. Bioorganic & Medicinal Lee, Y. S., Cha, B. Y., Saito, K., Choi, S. S., Wang, X. X., Choi, B. K., ... Woo, J. T. (2011). Chemistry, 17(1), 25–28. Effects of a Citrus depressa Hayata (shiikuwasa) extract on obesity in high-fat diet- Ho, S. C., & Kuo, C. T. (2014). Hesperidin, nobiletin, and tangeretin are collectively re- induced obese mice. Phytomedicine, 18(8–9), 648–654. sponsible for the anti-neuroinflammatory capacity of tangerine peel (Citri reticulatae Li, C., Gu, H., Dou, H., & Zhou, L. (2007). Identification of flavanones from peel of Citrus pericarpium). Food and Chemical Toxicology, 71, 176–182. changshan-huyou YB Chang, by HPLC–MS and NMR. European Food Research and Ho, S. C., & Lin, C. C. (2008). Investigation of heat treating conditions for enhancing the Technology, 225(5–6), 777–782. anti-inflammatory activity of citrus fruit (Citrus reticulata) peels. Journal of Li, G. J., Wu, H. J., Wang, Y., Hung, W. L., & Rouseff, R. L. (2019). Determination of citrus Agricultural and Food Chemistry, 56(17), 7976–7982. juice coumarins, furanocoumarins and methoxylated flavones using solid phase ex- Hosni, K., Zahed, N., Chrif, R., Abid, I., Medfei, W., Kallel, M., ... Sebei, H. (2010). traction and HPLC with photodiode array and fluorescence detection. Food Chemistry, Composition of peel essential oils from four selected Tunisian Citrus species: Evidence 271, 29–38. for the genotypic influence. Food Chemistry, 123(4), 1098–1104. Li, S., Lo, C. Y., & Ho, C. T. (2006). Hydroxylated polymethoxyflavones and methylated

20 B. Singh, et al. Food Research International 132 (2020) 109114

flavonoids in sweet orange (Citrus sinensis) peel. Journal of Agricultural and Food Comparison of microwave, ultrasound and accelerated-assisted solvent extraction for Chemistry, 54(12), 4176–4185. recovery of polyphenols from Citrus sinensis peels. Food Chemistry, 187, 507–516. Li, S., Pan, M. H., Lo, C. Y., Tan, D., Wang, Y., Shahidi, F., & Ho, C. T. (2009). Chemistry Negro, V., Mancini, G., Ruggeri, B., & Fino, D. (2016). Citrus waste as feedstock for bio- and health effects of polymethoxyflavones and hydroxylated polymethoxyflavones. based products recovery: Review on limonene case study and energy valorization. Journal of Functional Foods, 1(1), 2–12. Bioresource Technology, 214, 806–815. Li, W., Wang, Z., Wang, Y. P., Jiang, C., Liu, Q., Sun, Y. S., & Zheng, Y. N. (2012). Nielsen, I. L. F., Chee, W. S., Poulsen, L., Offord-Cavin, E., Rasmussen, S. E., Frederiksen, Pressurised liquid extraction combining LC–DAD–ESI/MS analysis as an alternative H., ... Williamson, G. (2006). Bioavailability is improved by enzymatic modification method to extract three major flavones in Citrus reticulata ‘Chachi’(Guangchenpi). of the citrus flavonoid hesperidin in humans: A randomized, double-blind, crossover Food Chemistry, 130(4), 1044–1049. trial. The Journal of Nutrition, 136(2), 404–408. Lim, S. W., Lee, D. R., Choi, B. K., Kim, H. S., Yang, S. H., Suh, J. W., & Kim, K. S. (2016). Onoue, S., Uchida, A., Takahashi, H., Seto, Y., Kawabata, Y., Ogawa, K., ... Yamada, S. Protective effects of a polymethoxy flavonoids-rich Citrus aurantium peel extract on (2011). Development of high-energy amorphous solid dispersion of nanosized nobi- liver fibrosis induced by bile duct ligation in mice. Asian Pacific journal of Tropical letin, a citrus polymethoxylated flavone, with improved oral bioavailability. Journal Medicine, 9(12), 1158–1164. of Pharmaceutical Sciences, 100(9), 3793–3801. Liu, E. H., Zhao, P., Duan, L., Zheng, G. D., Guo, L., Yang, H., & Li, P. (2013). Ortuño, J., Covas, M. I., Farre, M., Pujadas, M., Fito, M., Khymenets, O., ... de la Torre, R. Simultaneous determination of six bioactive flavonoids in Citri Reticulatae (2010). Matrix effects on the bioavailability of resveratrol in humans. Food Chemistry, Pericarpium by rapid resolution liquid chromatography coupled with triple quadru- 120(4), 1123–1130. pole electrospray tandem mass spectrometry. Food Chemistry, 141(4), 3977–3983. Ozturk, B., Parkinson, C., & Gonzalez-Miquel, M. (2018). Extraction of polyphenolic Liu, L., Xu, X., Cheng, D., Yao, X., & Pan, S. (2012). Preparative separation of poly- antioxidants from orange peel waste using deep eutectic solvents. Separation and methoxylated flavones from Ponkan (Citrus reticulata Blanco cv. Ponkan) peel by Purification Technology, 206, 1–13. high-speed countercurrent chromatography and their antifungal activities against Park, H. Y., Ha, S. K., Eom, H., & Choi, I. (2013). Narirutin fraction from citrus peels Aspergillus niger. European Food Research and Technology, 235(4), 631–635. attenuates alcoholic liver disease in mice. Food and Chemical Toxicology, 55, 637–644. Londoño-Londoño, J., de Lima, V. R., Lara, O., Gil, A., Pasa, T. B. C., Arango, G. J., & Park, H. Y., Park, Y., Lee, Y., Noh, S. K., Sung, E. G., & Choi, I. (2012). Effect of oral Pineda, J. R. R. (2010). Clean recovery of antioxidant flavonoids from citrus peel: administration of water-soluble extract from citrus peel (Citrus unshiu) on suppressing Optimizing an aqueous ultrasound-assisted extraction method. Food Chemistry, alcohol-induced fatty liver in rats. Food Chemistry, 130(3), 598–604. 119(1), 81–87. Parmar, H. S., & Kar, A. (2008). Antiperoxidative, antithyroidal, antihyperglycemic and Lu, X., Zhang, H., Zheng, T., Liu, Q., Zhu, J., & Huang, Q. (2020). Evaluation of oral cardioprotective role of Citrus sinensis peel extract in male mice. Phytotherapy bioaccessibility of aged citrus peel extract encapsulated in different lipid based sys- Research, 22(6), 791–795. tems: A comparison study using different in vitro digestion models. Journal of Peng, M., Liu, J., Liu, Z., Fu, B., Hu, Y., Zhou, M., ... Xu, N. (2018). Effect of citrus peel on Agricultural and Food Chemistry, 68(1), 97–105. phenolic compounds, organic acids and antioxidant activity of soy sauce. LWT-Food Lu, Y., Zhang, C., Bucheli, P., & Wei, D. (2006). Citrus flavonoids in fruit and traditional Science and Technolofy, 90, 627–635. Chinese medicinal food ingredients in China. Plant Foods for Human Nutrition, 61(2), Rafiq, S., Kaul, R., Sofi, S. A., Bashir, N., Nazir, F., & Nayik, G. A. (2018). Citruspeelasa 55–63. source of functional ingredient: A review. Journal of the Saudi Society of Agricultural Ma, Y. Q., Chen, J. C., Liu, D. H., & Ye, X. Q. (2008). Effect of ultrasonic treatment on the Sciences, 17(4), 351–358. total phenolic and antioxidant activity of extracts from citrus peel. Journal of Food Ramful, D., Bahorun, T., Bourdon, E., Tarnus, E., & Aruoma, O. I. (2010). Bioactive Science, 73(8), T115–T120. phenolics and antioxidant propensity of flavedo extracts of Mauritian citrus fruits: Ma, Y. Q., Chen, J. C., Liu, D. H., & Ye, X. Q. (2009). Simultaneous extraction of phenolic Potential prophylactic ingredients for functional foods application. Toxicology, compounds of citrus peel extracts: Effect of ultrasound. Ultrasonics Sonochemistry, 278(1), 75–87. 16(1), 57–62. Ramírez-Pelayo, C., Martínez-Quiñones, J., Gil, J., & Durango, D. (2019). Coumarins from Ma, Y. Q., Ye, X. Q., Fang, Z. X., Chen, J. C., Xu, G. H., & Liu, D. H. (2008). Phenolic the peel of citrus grown in Colombia: Composition, elicitation and antifungal activity. compounds and antioxidant activity of extracts from ultrasonic treatment of Satsuma Heliyon, 5(6), e01937. mandarin (Citrus unshiu Marc.) peels. Journal of Agricultural and Food Chemistry, Safdar, M. N., Kausar, T., Jabbar, S., Mumtaz, A., Ahad, K., & Saddozai, A. A. (2017). 56(14), 5682–5690. Extraction and quantification of polyphenols from kinnow (Citrus reticulate L.) peel Mahato, N., Sharma, K., Koteswararao, R., Sinha, M., Baral, E., & Cho, M. H. (2019). using ultrasound and maceration techniques. Journal of Food and Drug Analysis, 25(3), Citrus essential oils: Extraction, authentication and application in food preservation. 488–500. Critical Reviews in Food Science and Nutrition, 59(4), 611–625. Santhakumar, A. B., Battino, M., & Alvarez-Suarez, J. M. (2018). Dietary polyphenols: Mahato, N., Sharma, K., Sinha, M., & Cho, M. H. (2018). Citrus waste derived nutra-/ Structures, bioavailability and protective effects against atherosclerosis. Food and pharmaceuticals for health benefits: Current trends and future perspectives. Journal Chemical Toxicology, 113, 49–65. of Functional Foods, 40, 307–316. Satari, B., & Karimi, K. (2018). Citrus processing wastes: Environmental impacts, recent Manach, C., & Donovan, J. L. (2004). Pharmacokinetics and metabolism of dietary fla- advances, and future perspectives in total valorization. Resources, Conservation and vonoids in humans. Free Radical Research, 38(8), 771–786. Recycling, 129, 153–167. Mandalari, G., Bennett, R. N., Bisignano, G., Saija, A., Dugo, G., Lo Curto, R. B., ... Sathiyabama, R. G., Gandhi, G. R., Denadai, M., Sridharan, G., Jothi, G., Sasikumar, P., ... Waldron, K. W. (2006). Characterization of flavonoids and pectins from bergamot Ramos, A. G. B. (2018). Evidence of insulin-dependent signalling mechanisms pro- (Citrus bergamia Risso) peel, a major byproduct of essential oil extraction. Journal of duced by Citrus sinensis (L.) Osbeck fruit peel in an insulin resistant diabetic animal Agricultural and Food Chemistry, 54(1), 197–203. model. Food and chemical toxicology, 116, 86–99. Manthey, J. A., & Bendele, P. (2008). Anti-inflammatory activity of an orange peel Sawalha, S. M., Arráez-Román, D., Segura-Carretero, A., & Fernández-Gutiérrez, A. polymethoxylated flavone, 3′, 4′, 3, 5, 6, 7, 8-heptamethoxyflavone, in the ratcar- (2009). Quantification of main phenolic compounds in sweet and bitter orange peel rageenan/paw edema and mouse lipopolysaccharide-challenge assays. Journal of using CE–MS/MS. Food Chemistry, 116(2), 567–574. Agricultural and Food Chemistry, 56(20), 9399–9403. Scoma, A., Bertin, L., Zanaroli, G., Fraraccio, S., & Fava, F. (2011). A physicochem- Manthey, J. A., & Grohmann, K. (2001). Phenols in citrus peel byproducts. Concentrations ical–biotechnological approach for an integrated valorization of olive mill waste- of hydroxycinnamates and polymethoxylated flavones in citrus peel molasses. Journal water. Bioresource Technology, 102(22), 10273–10279. of Agricultural and Food Chemistry, 49(7), 3268–3273. Sharma, K., Mahato, N., & Lee, Y. R. (2019). Extraction, characterization and biological Manthey, J. A., & Guthrie, N. (2002). Antiproliferative activities of citrus flavonoids activity of citrus flavonoids. Reviews in Chemical Engineering, 35(2), 265–284. against six human cancer cell lines. Journal of Agricultural and Food Chemistry, 50(21), Shetty, S. B., Mahin-Syed-Ismail, P., Shaji Varghese, B. T. G., Kandathil-Thajuraj, P., Baby, 5837–5843. D., Haleem, S., ... Devang-Divakar, D. (2016). Antimicrobial effects of Citrus sinensis Marzouk, B. (2013). Characterization of bioactive compounds in Tunisian bitter orange peel extracts against dental caries bacteria: An in vitro study. Journal of Clinical and (Citrus aurantium L.) peel and juice and determination of their antioxidant activities. Experimental Dentistry, 8(1), e71. BioMed Research International, 2013. Siddique, S., Firdous, S., Durrani, A. I., Khan, S. J., & Saeed, A. (2016). Hesperidin, a Matsuzaki, K., Miyazaki, K., Sakai, S., Yawo, H., Nakata, N., Moriguchi, S., ... Yamakuni, citrus flavonoid, increases the bioavailability of micronutrients of Gallus domesticus T. (2008). Nobiletin, a citrus flavonoid with neurotrophic action, augments protein (chicken) eggshell: In vitro study. Chemical Speciation & Bioavailability, 28(1–4), kinase A-mediated phosphorylation of the AMPA receptor subunit, GluR1, and the 88–94. postsynaptic receptor response to glutamate in murine hippocampus. European Silva, I., Estrada, M. F., Pereira, C. V., da Silva, A. B., Bronze, M. R., Alves, P. M., ... Serra, Journal of Pharmacology, 578(2–3), 194–200. A. T. (2018). Polymethoxylated flavones from orange peels inhibit cell proliferation Maurya, A. K., Mohanty, S., Pal, A., Chanotiya, C. S., & Bawankule, D. U. (2018). The in a 3D cell model of human colorectal cancer. Nutrition and Cancer, 70(2), 257–266. essential oil from Citrus limetta Risso peels alleviates skin inflammation: In-vitro and Singh, B., Singh, J. P., Kaur, A., & Singh, N. (2017). Phenolic composition and antioxidant in-vivo study. Journal of Ethnopharmacology, 212, 86–94. potential of grain legume seeds: A review. Food Research International, 101, 1–16. McKay, D. L., Chen, C. Y. O., Zampariello, C. A., & Blumberg, J. B. (2015). Flavonoids and Singh, B., Singh, J. P., Kaur, A., & Singh, N. (2018a). Insights into the phenolic com- phenolic acids from cranberry juice are bioavailable and bioactive in healthy older pounds present in jambolan (Syzygium cumini) along with their health-promoting adults. Food Chemistry, 168, 233–240. effects. International Journal of Food Science & Technology, 53(11), 2431–2447. Miller, E. G., Peacock, J. J., Bourland, T. C., Taylor, S. E., Wright, J. M., Patil, B. S., & Singh, B., Singh, J. P., Kaur, A., & Singh, N. (2018b). Phenolic compounds as beneficial Miller, E. G. (2007). Inhibition of oral carcinogenesis by citrus flavonoids. Nutrition phytochemicals in pomegranate (Punica granatum L.) peel: A review. Food Chemistry, and Cancer, 60(1), 69–74. 261, 75–86. Naim, M., Amjad, F. M., Sultana, S., Islam, S. N., Hossain, M. A., Begum, R., ... Amran, M. Singh, J. P., Kaur, A., Singh, N., Nim, L., Shevkani, K., Kaur, H., & Arora, D. S. (2016). In S. (2012). Comparative study of antidiabetic activity of hexane-extract of lemon peel vitro antioxidant and antimicrobial properties of jambolan (Syzygium cumini) fruit (Limon citrus) and glimepiride in alloxan-induced diabetic rats. Bangladesh polyphenols. LWT-Food Science and Technology, 65, 1025–1030. Pharmaceutical Journal, 15(2), 131–134. Soquetta, M. B., Tonato, D., Quadros, M. M., Boeira, C. P., Cichoski, A. J., de Marsillac Nayak, B., Dahmoune, F., Moussi, K., Remini, H., Dairi, S., Aoun, O., & Khodir, M. (2015). Terra, L., & Kuhn, R. C. (2019). Ultrasound extraction of bioactive compounds from

21 B. Singh, et al. Food Research International 132 (2020) 109114

Citrus reticulata peel using electrolyzed water. Journal of Food Processing and Wang, L., Wang, J., Fang, L., Zheng, Z., Zhi, D., Wang, S., ... Zhao, H. (2014). Anticancer Preservation, 43(12), e14236. activities of citrus peel polymethoxyflavones related to angiogenesis and others. Sridharan, B., Mehra, Y., Ganesh, R. N., & Viswanathan, P. (2016). Regulation of urinary BioMed Research International, 2014. crystal inhibiting proteins and inflammatory genes by lemon peel extract and for- Wang, Y. C., Chuang, Y. C., & Hsu, H. W. (2008). The flavonoid, carotenoid and pectin mulated citrus bioflavonoids on ethylene glycol induced urolithic rats. Food and content in peels of citrus cultivated in Taiwan. Food Chemistry, 106(1), 277–284. Chemical Toxicology, 94, 75–84. Xiao, H., Yang, C. S., Li, S., Jin, H., Ho, C. T., & Patel, T. (2009). Monodemethylated Thériault, M., Caillet, S., Kermasha, S., & Lacroix, M. (2006). Antioxidant, antiradical and polymethoxyflavones from sweet orange (Citrus sinensis) peel inhibit growth of antimutagenic activities of phenolic compounds present in maple products. Food human lung cancer cells by apoptosis. Molecular Nutrition & Food Research, 53(3), Chemistry, 98(3), 490–501. 398–406. Trabelsi, D., Aydi, A., Zibetti, A. W., Della Porta, G., Scognamiglio, M., Cricchio, V., ... Xing, T. T., Zhao, X. J., Zhang, Y. D., & Li, Y. F. (2017). Fast separation and sensitive Mainar, A. M. (2016). Supercritical extraction from Citrus aurantium amara peels quantitation of polymethoxylated flavonoids in the peels of citrus using UPLC-Q-TOF- using CO2 with ethanol as co-solvent. The Journal of Supercritical Fluids, 117, 33–39. MS. Journal of Agricultural and Food Chemistry, 65(12), 2615–2627. Tsutsumi, R., Yoshida, T., Nii, Y., Okahisa, N., Iwata, S., Tsukayama, M., ... Shuto, E. Zanotti, I., Dall'Asta, M., Mena, P., Mele, L., Bruni, R., Ray, S., & Del Rio, D. (2015). (2014). Sudachitin, a polymethoxylated flavone, improves glucose and lipid meta- Atheroprotective effects of (poly) phenols: A focus on cell cholesterol metabolism. bolism by increasing mitochondrial biogenesis in skeletal muscle. Nutrition & Food & Function, 6(1), 13–31. Metabolism, 11(1), 32. Zhang, L., Geng, Y., Zhu, H., Mu, Y., Yu, J., Li, J., & Wang, X. (2017). Preparative se- Wang, D., Wang, J., Huang, X., Tu, Y., & Ni, K. (2007). Identification of polymethoxylated paration of six coumarins from the pummelo (Citrus maxima (Burm.) Merr. Cv. flavones from green tangerine peel (Pericarpium Citri Reticulatae Viride) by chroma- Shatian Yu) peel by high-speed countercurrent chromatography. Journal of Liquid tographic and spectroscopic techniques. Journal of Pharmaceutical and Biomedical Chromatography & Related Technologies, 40(19), 991–996. Analysis, 44(1), 63–69.