foods Review Modern Extraction and Purification Techniques for Obtaining High Purity Food-Grade Bioactive Compounds and Value-Added Co-Products from Citrus Wastes 1, 2, 3, 2, Neelima Mahato * , Mukty Sinha y, Kavita Sharma y , Rakoti Koteswararao y and Moo Hwan Cho 1 1 School of Chemical Engineering, Yeungnam University, Gyeongsan 38541, Korea; [email protected] 2 Department of Medical Devices, National Institute of Pharmaceutical Education and Research, Ahmedabad, Palej, Gandhinagar 382 355, India; [email protected] (M.S.); [email protected] (R.K.) 3 Department of Chemistry, Idaho State University, Pocatello, ID 83209, USA; [email protected] * Correspondence: [email protected]; Tel.: +82-010-2798-8476 These authors contributed equally as the second author. y Received: 23 September 2019; Accepted: 16 October 2019; Published: 23 October 2019 Abstract: Citrus contains a range of highly beneficial bioactive compounds, such as polyphenols, carotenoids, and vitamins that show antimicrobial and antioxidant properties and help in building the body’s immune system. On consumption or processing, approximately 50% of the fruit remains as inedible waste, which includes peels, seeds, pulp, and segment residues. This waste still consists of substantial quantities of bioactive compounds that cause environmental pollution and are harmful to the ecosystem because of their high biological oxygen demand. In recent years, citrus cultivation and the production of processed foods have become a major agricultural industry. In addition to being a substantial source of economy, it is an ideal and sustainable and renewable resource for obtaining bioactive compounds and co-products for food and pharmaceutical industries. In the present article, the various methods of extraction, conventional and modern, as well as separation and isolation of individual bioactive compounds from the extraction mixture and their determination have been reviewed. This article presents both aspects of extraction methods, i.e., on a small laboratory scale and on an industrial mass scale. These methods and techniques have been extensively and critically reviewed with anticipated future perspectives towards the maximum utilization of the citrus waste. Keywords: citrus waste; citrus byproducts; essential oils; waste management; limonene; phenolics; flavonoids; citric acid; environment friendly extraction; phytochemical extraction and purification 1. Introduction Citrus is cultivated as one of the largest fruit crops across the globe and has been known to humans for thousands of years due to its health benefits. The center of origin of citrus species has been discovered to be the southeast foothills of the Himalayas, Meghalaya, eastern areas of Assam, India, Northern Myanmar, and the Western Yunnan province, China, on the basis of genomic, phylogenetic, and biogeographic research on citrus fruits [1,2]. Citrus belongs to the Rutaceae family and comprises 140 genera and 1300 species. The origin, geographical spread, and popular varieties of citrus fruits cultivated across the globe are shown in Figures S1 and S2a–d (Supplementary Information), and the main citrus growing regions in the world map along with their annual productions are shown in Figure1[ 3,4]. The cross between the native varieties and the evolution of popular hybrid variants in citrus fruits and the list of main citrus varieties cultivated widely across the globe has been Foods 2019, 8, 523; doi:10.3390/foods8110523 www.mdpi.com/journal/foods Foods 2019, 8, 523 2 of 81 Foods 2019, 8, 523 2 of 79 presented in Figure S3a–b (Supplementary Information) [5]. The global citrus production in the year global2016 citrus was ~124.3production million in the tons year [3], 2016 with was oranges ~124.3 being million the largesttons [3], among with oranges all citrus being crops. the Thelargest global among allorange citrus crops. production The global of oranges orange for production the year 2018 of/ 2019oranges has beenfor the estimated year 2018/2019 to increase has bybeen ~6.3 estimated million to increaseto 54.3 by million ~6.3 million tons [4 to]. 54.3 The million important tons varieties [4]. The important cultivated varieties commercially cultivated are oranges commercially (61% of are total), oranges (61%mandarin of total), (22% mandarin of total), (22% lime of and total), lemon lime (11% and lemon of total), (11% and of grapefruit total), and (6% grapefruit of total) (6% [3]. of total) [3]. TheThe citrus citrus processing processing industries industries have have been been focusing focusing on on the the production production of juices and essentialessential oilsoils for manyfor manyyears. years. It is estimated It is estimated that 33% that of 33% the of total the totalharves harvestt in the in world the world is used is used for juice for juice production production [6]. [A6 ].high percentageA high percentage of orange of production orange production (70%) is (70%)consumed is consumed for the produc for the productiontion of commercial of commercial derivative derivative products, suchproducts, as fresh such juice, as dehydrated fresh juice, citrus dehydrated products citrus or marmalades, products or marmalades,jams, and flavoring jams, and agents flavoring for beverages agents [7]. Approximatelyfor beverages 50%–60% [7]. Approximately of the fruit 50–60% parts remaining of the fruit after parts processing remaining are after converted processing to arecitrus converted wastes (peels, to seeds,citrus and wastes membrane (peels, residue) seeds, and [8]. membrane Accordingly, residue) the built-up [8]. Accordingly, amount of the semisolid built-up amountand solid of citrus semisolid waste is alarminglyand solid huge. citrus wasteAnnually, is alarmingly the citrus huge. waste Annually, created theby citrusprocessing waste createdindustries by processingis estimated industries to be over is 60 millionestimated tons toworldwide be over 60 million[9]. The tons bioactive worldwide molecules [9]. The ob bioactivetained from molecules citrus obtainedwaste have from been citrus reported waste to exhibithave antimicrobial, been reported antiallergic, to exhibit antimicrobial, anticancer, and antiallergic, antidiabetic anticancer, properties and and antidiabetic hence have properties been promoted and as dietaryhence supplements have been promoted for nutrition as dietary and supplements health. The forvarious nutrition health and benefits health. Theand various applications health of benefits bioactive moleculesand applications in neutraceutical/pharmaceutical/therapeutic of bioactive molecules in neutraceutical applications/pharmaceutical and/therapeutic food preservation applications have and been extensivelyfood preservation reviewed have in our been previous extensively publications reviewed [10–14]. in our previous publications [10–14]. Figure 1. Climate sustainability and the annual production of citrus fruits in different geographical Figure 1. Climate sustainability and the annual production of citrus fruits in different geographical regions regions across the globe [3,4,15]. across the globe [3,4,15]. 1.1. Anatomy and Waste Composition 1.1. Anatomy and Waste Composition A typical citrus waste is composed of semisolid residue composed of endocarp residual membranes, vesicles,A typical pith citrus residue, waste and, is tocomposed a large extent, of semisolid albedo andresidue exocarp composed or flavedo. of endocarp A typical residual composition membranes, of vesicles,citrus wastespith residue, acquired and, from to a juice-producing large extent, albedo industries and exocarp is presented or flavedo. in Table A1. ty Thepical overall composition composition of citrus wastesalso dependsacquired onfrom the juice-producin citrus species,g variety, industries and theis presented harvesting in season. Table 1. The The compositions overall composition of typical also dependscitrus wasteon the (peel citrus and species, rag) and variety, dried and citrus the pulp harves areting shown season. in Figures The compositions2 and3. of typical citrus waste (peel and rag) and dried citrus pulp are shown in Figures 2 and 3. Table 1. The composition of citrus wastes acquired from juice-producing industries; Units are expressed in (g/100 g). Foods 2019, 8, 523 3 of 81 Table 1. The composition of citrus wastes acquired from juice-producing industries; Units are expressed Foods 2019in, (g 8,/ 100523 g). 3 of 79 CompositionComposition Citrus ByproductByproduct PeelPeel Pulp Pulp MoistureMoisture content content 8.58.5 ± 0.200.20a a 75.3 10.2075.3c ± 10.2085.7 c 0.00 d 85.7 ± 0.00 d ± ± ± Crude proteinCrude protein 12.5 ± 0.870.87b b 10.2 3.7010.2c ± 3.70 c8.6 0.00 d 8.6 ± 0.00 d ± a ± c ± e Crude fiber 75.7 2.10 a 57.0 10.0 c7.3 0.80 e Crude fiber 75.7 ±± 2.10 ± 57.0 ± 10.0 ± 7.3 ± 0.80 Crude fat 0.5 0.02 b 2.2 6.10 c 4.9 0.00 f Crude fat 0.5 ±± 0.02 b ± 2.2 ± 6.10 c ± 4.9 ± 0.00 f Total ash 8.1 0.41 b 3.3 0.50 c 6.5 0.00 d Total ash 8.1 ±± 0.41 b ± 3.3 ± 0.50 c ± 6.5 ± 0.00 d Reference: a [16]; b [9]; c [17]; d [18]; e [19]; f [20]. Reference: a [16]; b [9]; c [17]; d [18]; e [19]; f [20]. FigureFigure 2. 2.TheThe main main citrus citrus varieties varieties and and its its physical physical composit compositionion in interms terms of ofedible edible juice juice and and inedible inedible waste part,waste which part, consists which consists of peel of (flavedo peel (flavedo and albedo), and albedo), rags rags (pith, (pith, pulp pulp residue, residue, and and segment segment membrane), membrane), and seedsand seeds[10,13,21] [10,13. ,21]. Foods 2019, 8, 523 4 of 81 Foods 2019, 8, 523 4 of 79 FigureFigure 3. 3.(a ()a The) The composition composition of of typical typical citrus citrus waste waste (peel (peel and and rag); rag); (b) ( theb) the composition composition of dried of dried citrus citrus pulp pulp[9,22–27] [9,22–27. ]. The waste is rich in sugars, fibers, organic acids, amino acids and proteins, minerals, essential oils The waste is rich in sugars, fibers, organic acids, amino acids and proteins, minerals, essential oils (mainly d-limonene), lipids, and large amounts of polyphenolic compounds and vitamins [28].