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Review Biochemical Composition of : A Review

Meenakshi Sharma 1 and Prashant Kaushik 2,3,*

1 Department of Chemistry, Kurukshetra University, Kurukshetra 136119, , ; [email protected] 2 Kikugawa Research Station, Yokohama Ueki, 2265, Kamo, Kikugawa City, Shizuoka 439-0031, Japan 3 Instituto de Conservación y Mejora de la Agrodiversidad Valenciana, Universitat Politècnica de València, 46022 Valencia, Spain * Correspondence: [email protected]; Tel.: +34-722-206-244

Abstract: Eggplant is one of the most important crops known for its nutritive benefits due to the abundance of various bioactive compounds, which include , , minerals, carbo- hydrates, phenolics, and dry matter content. In addition, eggplant has significant pharmaceutical properties that have been recently recognized. Eggplant produces secondary metabolites, including glycoalkaloids, antioxidant compounds, and vitamins, which appear to be the major source of its health benefits. It has been reported that there is a considerable correlation between the regular use of phytochemicals and the defense against diseases. Therefore, researchers must analyze the biochemical composition of to obtain more information about their nutritional quality and health benefits. In this review, an attempt is made to explain the qualitative and quantitative aspects of different biochemicals present in eggplant, in addition to their beneficial health effects.

 Keywords: eggplant; fruits; biochemical; chlorogenic acid; proteins; vitamins; minerals 

Citation: Sharma, M.; Kaushik, P. Biochemical Composition of Eggplant Fruits: A Review. Appl. Sci. 2021, 11, 1. Introduction 7078. https://doi.org/10.3390/ Eggplant (Solanum L.) is an economically important vegetable that is exten- app11157078 sively cultivated in the subtropical and tropical zones of the world [1,2]. Eggplant fruits contain a sufficient amount of nutrition, in addition to other health-promoting bioactive Academic Editors: Pranas Viskelis, compounds [3,4], and contain considerably higher concentrations of phenolic acids, which Dalia Urbonaviciene and are beneficial for human health and development [5,6]. Some anti-nutritional compounds Jonas Viskelis (such as saponins and steroidal glycol- (SGA)) are also found in eggplant fruits, and impart a bitter and have toxic effects on human health [7,8]. Improving the Received: 30 June 2021 bioactive composition of eggplant may ensure vital nourishment [9]. Eggplant displays an Accepted: 28 July 2021 Published: 30 July 2021 excellent accumulation of bioactive metabolites in its fruits, such as phenolic acids, with significant variations in these compounds among the different and their wild

Publisher’s Note: MDPI stays neutral relatives [10,11]. Therefore, eggplant is a valued vegetable crop due to the framework of with regard to jurisdictional claims in its phytochemicals as nutraceuticals, particularly and soluble fiber [12,13]. In published maps and institutional affil- addition, eggplant is a non-climacteric , which results in a short shelf-life despite being iations. harvested in immature phases of development [14,15]. Eggplant is progressively becoming more well-known due to its composition of car- bohydrates, proteins, vitamins, and several other bioactive compounds, such as phenolic acids [16,17]. Eggplant also contains traces of minerals such as copper, , and iron [18]. Due to this biochemical composition, eggplant may possibly be used in the treatment of Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. anemia, atherosclerosis, and fatty degeneration [19–21]. Its antioxidant property potentially This is an open access article reduces the risk of various types of cancer, protects against cardiovascular diseases, and distributed under the terms and prevents acute respiratory infections. [20,22]. Further, eggplant fibers help in digestion by conditions of the Creative Commons removing toxins and harmful materials from the stomach and reducing colon cancer [10,23]. Attribution (CC BY) license (https:// polyphenols present in eggplants can help protect cell membranes and boost the creativecommons.org/licenses/by/ brain’s memory function [10,24]. In addition to cultivated , the wild relatives of 4.0/). eggplant are also useful for quality and yield improvement. The genomic interactions

Appl. Sci. 2021, 11, 7078. https://doi.org/10.3390/app11157078 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 7078 2 of 13

between the cultivated eggplant hybrid and the wild relative result in complicated bio- chemical variations [25,26]. In addition, eggplant wild relatives represent an important source of useful genes and underutilized variations for desired traits, such as disease resistance, drought tolerance, and high content of bioactive compounds, including phenolic acids [27,28]. Phenolic acids have significant antioxidant activity due to their interaction with reactive oxygen and nitrogen species. The reduction of reactive species is the main biological activity of phenolic acids. Because eggplant flesh contains chlorogenic acid, which constitutes up to 90% of the total phenolic acids found in the flesh of the eggplant fruit, breeding eggplant to improve the chlorogenic acid content is imperative [29]. Here, we examine the biochemical potential of eggplant.

2. Material and Methods At the time of writing, we are unaware of any comprehensive studies conducted on the biochemical potential of eggplant fruits. Although literature reviews have been conducted on one aspect of eggplant fruit composition, no comprehensive reviews have been published that cover all of the crucial biochemical aspects of eggplant fruit. Therefore, a systematic review of the literature based on the Preferred Reporting Items for Systematic Meta-Analysis (PRISMA) approach was conducted [30]. To locate articles relevant to the study’s objectives, the following keywords were used: eggplant fruits, biochemical, chlorogenic acid eggplant, eggplant fruit proteins, eggplant fruit vitamins and minerals. The findings were retrieved using Google Scholar. There were 197 documents reviewed, with 98 deemed relevant following their examination. Finally, papers published in peer- reviewed journals were chosen, and their results are summarized below.

3. Proteins in Eggplant According to Rodriguez-Jimenez et al. [15], the content found in eggplant is 12.55–12.77% [15]. The USDA database also reports that the protein content for fresh eggplant is 0.98% (12.73% in dry basic). Indian, Thai, Chinese, and eggplants, when dried at the same temperature, produce similar protein contents (12–15%). The estimated concentration of protein from the N content was determined by Raigon et al. [31] with the Kjeldahl method using a Kjeltec 2100 Distillation Unit [31]. Four cultivars—landraces CS16, IVIA371, and H15, and the commercial hybrid Cristal—were assessed for two consecutive years, 2007 and 2008, for their biochemical composition (CS16, IVIA371, and Cristal in 2007, and CS16, H15, and Cristal in 2008). The only remarkable differences observed for the protein content were associated with the factor. In this regard, landrace IVIA371 had considerably higher protein content than landrace CS16 in 2007, whereas H15 exhibited significantly higher protein content than either CS16 or Cristal in 2008. However, no significant impacts of cultivation method (organic or conventional) or cultivar–cultivation method interaction were detected in either of the years. In another study by Raigon et al. [32], protein content varied from 4.1 g/kg (VS22) to 6.2 g/kg (ASIS1) for landrace cultivars and from 4.3 g/kg (Petra F1) to 5.9 g/kg (De Barbentane) for commercial cultivars (Table1)[ 32]. The high nutritional value of eggplant is attributed to the presence of proteins in its fruit. According to Kandoliya et al. [33], when six varieties of eggplant, namely, JBGR-1, GOB-1, GBL-1, GJB-2, GJB-3, and GBH- 2, were investigated for their nutritional composition, the highest level of protein was observed in GOB-1 (1.39%), whereas the lowest was found in GJB-3 (0.66%) [33]. Its intake can significantly contribute to the development of hormones responsible for different body functions, including growth and repair, in addition to the maintenance of the body. Guillermo et al. [18] determined protein (N × 6.25) content in eggplant fruits of American, Chinese, Hindu, Philippine, and Thai cultivars according to AOAC methods. Protein varied from 0.65 g/100 g for the Chinese cultivar to 0.90 g/100 g for the Thai cultivar (Table2). Appl. Sci. 2021, 11, 7078 3 of 13

Table 1. Variation for protein content in g/kg of fresh weight in different eggplant cultivars, adapted from Raigón et al. [32].

Variety Primary Fruit Protein (g/kg) Landraces ANS24 5.2 ANS26 Purple 5.4 ASIS1 6.2 IVIA604 Purple 4.9 VS22 Purple 4.1 VS9 Purple 4.6 Commercial varieties 10–201 F1 Black 4.7 10–501 F1 Black 4.5 Black Beauty Black 4.9 De Barbentane Black 5.9 Mulata F1 Black 4.8 Petra F1 Black 4.3

Table 2. Variations in protein content (g/100 g) in different eggplant cultivars on a fresh weight basis, adapted from Guillermo et al. [18].

Fruit Shape Fruit Color Cultivar Protein (g/100 g) Long Purple American-Type 0.67 ± 0.13 Long Purple Chinese-Type 0.65 ± 0.06 Obovate Black Indian-Type 0.75 ± 0.05 Round Black -Type 0.69 ± 0.09 Round Thai-Type 0.90 ± 0.07

4. Vitamins in Eggplant Fraikue [10], in his review, outlined the present in eggplant (raw and cooked), in addition to the variations among them, and reported that nutrients in cooked eggplants showed significant differences when compared to those of raw eggplants [10]. The content in eggplant is summarized in Table3. Das et al. [ 34] demonstrated that there were no significant variations in the biochemical composition of the raw and grilled eggplants; however, the vitamin content was found to be reduced after grilling. Further, Al Nachar [35] observed in an investigation of the fruit calyx of eggplant that the level was 45 mg/100 g [35]; however, the recommended daily intake is 75 mg because it helps in composition, the maintenance of bones, skin, and blood vessels, and numerous other functions, such as the production of collagen and keratin, and the absorption of iron [36]. It was also found that the vitamin B5 level was 7.3 mg/100 g, compared to the daily recommended intake of 5 mg [35]. It has also been suggested that vitamin B5 lowers cholesterol levels and triglyceride in the blood; however, its reduction leads to respiratory infections and increased insulin sensitivity [37]. Appl. Sci. 2021, 11, 7078 4 of 13

Table 3. Vitamins and their content in eggplant (mg per 100 g) [10].

Vitamins Common Name mg/100 g Vitamin A Retinol 0.8 Vitamin B complex - 18–22 Vitamin B1 Thiamine 0.039 Vitamin B2 Riboflavin 0.037–0.11 Vitamin B3 Niacin 0.649 Vitamin B5 Pantothenic acid 0.281 Vitamin B6 Pyridoxine 0.084–0.1 Vitamin B9 Folate 18–22 Vitamin C Ascorbate 1.8–2.2 Tocopherol 0.2–0.3 Menaquinone 2.9–3.5

Further, Shabetya et al. [38] reported that the vitamin C content of eggplant varied from 3.9 to 4.1 mg (per 100 g) in the Belgorod region, and the concentration of vitamin C was found to be dependent on the , and not on the color of the fruit [38]. However, the vitamin C content in eggplant fruits grown in the Crimea foothill zone was slightly higher than that in eggplant grown in the Belgorod region, and ranged from 4.8 to 4.9 mg (per 100 g). In addition, the vitamin C content in eggplant cultivars, as recorded by Guillermo et al. [18], showed values varying from 7.4 mg (100 g−1) in the Thai cultivar to 22 mg (100 g−1) in the Hindu cultivar [18]. This study suggested that eggplant consumption of 100 g per day accounted for 7–22% of the recommended daily intake (RDI) of vitamin C, which lies in the range of 60 to 100 mg.

5. Minerals in Eggplant Eggplants are rich sources of biologically essential minerals, such as Na, K, Ca, Mg, P, Fe, Cu, and Zn, for which contents are similar to those reported in tomatoes and higher than those found in carrots, potatoes, or . Arivalagan et al. [39] carried out a study to evaluate the variability in content (K, Mg, Fe, Cu, and Zn) among five Indian commercial varieties of eggplant (Punjab Sadabahar, Pusa Ankur, Pusa Purple Long, Pusa Upkar, and Pusa Kranti) and to identify mineral-rich [39]. The mean values recorded for K, Mg, Fe, Cu, and Zn in these eggplant cultivars were 193, 9.91, 0.342, 0.0721, and 0.135 mg/100 g, respectively. Davidson and Monulu [40] also conducted a study to determine the mineral composition (Na, K, Ca, P, Fe, Cu, Zn) of raw, boiled, and steamed gboma eggplant ( macrocarpon) Eiervrug . The study found the raw samples contained higher mineral content (naturally-occurring abiogenic substances) than the boiled samples (Table4)[40]. The minerals (sodium, potassium, magnesium, calcium, phosphorous, iron, copper, and zinc) were evaluated according to the official analysis methods defined by the Association of Official Analytical Chemists (AOAC). Further, it was observed that the percentage loss in zinc was the highest (35.29%) when compared to other minerals, and the values of mineral content for both samples were significantly distinct (p ≤ 0.05).

Table 4. Mineral composition of raw, boiled, and steamed eggplant () leaves in mg/100 g [40].

Minerals (mg/100 g) Raw Boiled Steamed Na 16.60 13.80 15.40 K 175.00 164.00 168.00 Ca 31.24 27.69 29.56 P 25.48 21.77 23.54 Fe 1.16 1.06 1.12 Cu 31.06 28.74 29.69 Appl. Sci. 2021, 11, 7078 5 of 13

Significant variations in the concentrations of these macro- and micro-minerals were observed among different varieties, with differences up to 156.3% in the concentration of Ca and 142.2% in the concentration of Fe [32]. It was further reported that, on average, landraces possessed a higher total concentration of macro-minerals (283.8 mg/100 g) than commercial cultivars of eggplant (270.3 mg/100 g) or hybrids between different landraces (257.9 mg/100 g). Moreover, the global mean concentrations of the three micro-nutrients (Fe, Cu, and Zn) were: 0.168 mg/100 g−1 for Fe, 0.062 mg/100 g−1 for Cu, and 0.136 mg/100 g−1 for Zn. According to Guillermo et al. [18], the Hindu cultivar exhibited the highest values for concentrations of K (191.18 mg/100 g), Ca (59.63 mg/100 g), P (33.52 mg/100 g), Mg (28.96 mg/100 g), Zn (0.78 mg/100 g), and Mn (0.44 mg/100 g); whereas the Philippine cul- tivar reported the highest concentrations of Fe (3.13 mg/100 g). No significant differences were found for Cu concentration among the five cultivars (Table5)[18].

Table 5. Mineral content of eggplant cultivars (mg/100 g) on a fresh weight basis [18].

American- Chinese- Philippines- Minerals Indian-Type Thai-Type Type Type Type K 152.15 151.21 191.18 121.06 176.46 Ca 31.36 28.00 59.63 32.80 45.08 P 29.61 21.21 33.52 13.80 30.42 Mg 25.35 15.29 28.96 15.74 20.88 Na 8.49 9.40 11.54 5.76 5.61 Fe 0.86 2.40 1.53 3.13 1.80 Zn 0.51 0.33 0.78 0.26 0.45 Mn 0.41 0.36 0.44 0.30 0.39 Cu 0.15 0.15 0.15 0.13 0.18

6. in Eggplant According to Rodriguez et al. [15], content in eggplant flour was 62–68%, and the main soluble were glucose and fructose [15]. Experiments on different cultivars revealed that available carbohydrates in eggplant contribute to low content in the fruit. San José et al. [41] carried out a study to identify and quantify soluble sugars, total available carbohydrates, and the starch content in scarlet and gboma eggplant varieties; total available carbohydrates varied from 2.89 g/100 g for BBS116 to 8.04 g/100 g for gboma eggplant BBS178; and total soluble sugars varied from 0.21 g/100 g for gboma eggplant RNL371 to 0.55 g/100 g for scarlet eggplant BBS157 (Table6)[ 41]. Moreover, the major soluble sugars found in the scrutinized samples were glucose (57.1%) and fructose (31.0%). This fact was also supported by Boo et al. [42], who concluded that glucose and fructose were present in higher concentrations than sucrose.

Table 6. Carbohydrate content in scarlet and gboma eggplant varieties (g/100 g) [41].

Total Available Total Soluble Sugars Starch Carbohydrates Scarlet eggplant BBS116 2.89 0.43 2.46 BBS157 4.64 0.55 4.09 RNL187 3.28 0.28 3.00 ± 0.22 Gboma eggplant BBS178 8.04 0.36 7.68 BBS196 6.71 0.35 6.36 RNL371 6.22 0.21 6.01 RNL374 4.94 0.34 4.60

The starch content in eggplant is generally higher than that of the total soluble sugars, which is probably because the fruit of eggplant is harvested when physiologically immature Appl. Sci. 2021, 11, 7078 6 of 13

and the hydrolysis of starch is not complete [43]. In another study, San José et al. [44] investigated the biochemical composition of seven eggplant cultivars of occidental varieties with several origins, with the aim to develop eggplant fruits with enhanced quality. The study observed that the fraction of total available carbohydrates varied from 2.99 g/100 g for IVIA371 to 4.19 g/100 g for H-11, with significant differences among the cultivars; total soluble sugars varied from 0.74 g/100 g for BBS-118 to 2.13 g/100 g for H11; and starch content also produced significant differences among the cultivars, and varied from 1.43 g/100 g for Dourga to 2.38 g/100 g for H11 (Table7)[ 44]. The study indicated the main soluble sugars were glucose (47.6%) and fructose (38.6%). Although sucrose and maltose were also detected, their concentrations were relatively low, with 7.6% and 6.9% of the total content of soluble sugars, respectively. With the exception of Dourga, the content in starch for the other six cultivars was found to be higher than the total soluble sugar content. Therefore, the ratio of total soluble sugars/starch was in all cases lower than 1 (1.50 for Dourga) (Table7)[44].

Table 7. Carbohydrate content in the seven eggplant cultivars (g/100 g) [44].

Total Available Cultivar Total Soluble Sugars Starch Carbohydrates BBS118 3.05 0.74 2.31 CS16 3.82 1.48 2.34 Dourga 3.55 2.13 1.43 H11 4.19 1.81 2.38 IVIA371 2.99 1.30 1.69 LF3-24 3.55 1.49 2.06 Listada Clemente 3.62 1.24 2.38

7. Phenolics in Eggplant Phenolics in eggplant have been identified as major bioactive compounds responsi- ble for their antioxidant effects. As reported by Guillermo et al. [18], the levels of total soluble phenolics varied from 1350 mg of chlorogenic acid equivalents per 100 g (mg CAE/100 g) of dry sample for the Chinese cultivar to 2049 mg CAE/100 g for the Thai cultivar (Table8 )[18]. In another study, Okmen et al. [45] examined different Turkish egg- plant cultivars based on their shape: Topan (wider than long), Uzun (longer), Beyli (longer than wide), and Domates ( shaped), and found that the total phenolic content varied from 615 mg/kg in MM738 to 1389 mg/kg in Eskisehir Tombul (Table8), and the mean phenolic content was 992 ± 46 (SE) mg/kg for all of the cultivars [45]. Ninfali et al. [46] examined total phenolic content in Black Beauty and Violetta Lunga eggplant variants, and the amounts reported were 57.4 and 64.8 mg of caffeic acid equivalents per 100 g (mg CafAE/100 g) FW, respectively [46]. Further, significant differences for total phenolic content among cultivars were observed and varied from 41.0 mg/100 g for Dourga to 81.7 mg/100 g for BBS118 [44].

Table 8. Variations in total phenolic content in different eggplant cultivars [18,45].

Cultivar Total Phenolic Content (mg/100 g) American-Type 1512.5 Chinese-Type 1350.0 g Indian-Type 1750.0 Philippines-Type 1562.7 Thai-Type 2049.8 Turkish Eskisehir Tombul 1388.9 MM738 614.8 Appl. Sci. 2021, 11, 7078 7 of 13

Raigón et al. [32] observed that the phenolic content varied from 344.6 mg/kg for Mulata F1 to 570.8 mg/kg for 10–501 F1 for commercial cultivars, and from 422.1 mg/kg for SUDS5 to 607.0 mg/kg for IVIA604 for landraces; commercial varieties had significantly lower values than the mean value of the landraces, except for variety 10–501 F1, which had a comparatively higher phenolic content (570.8 mg/kg) (Table9)[ 32]. Hanson et al. [47] studied S. melongena and S. aethiopicum varieties from different countries for two years and reported that the mean values for total phenolic content were 0.64 g/100 g chlorogenic acid equivalent for Year 1 on a dry weight basis, and 0.97 g/100 g chlorogenic acid equivalent for Year 2 [47]. The increase in total phenolic content in Year 2 (compared to Year 1) was attributed to the impact of environmental factors such as , water, and temperature, which potentially alters the phenolic content in fruits and . Further, the values of total phenolic content varied from 740 mg ChlAE/100 g for S00690 (Bangladesh) to 1430 mg ChlAE/100 g for S00355 (India). Kaur et al. [48] concluded that the total phenolic content in eggplant genotypes, including cultivated genotypes and associated wild species, displayed a wide variation, ranging from 22.62 to 234.46 mg GAE/100 g fw (244.28 to 2990.64 mg GAE/100 g dw) [48].

Table 9. Variations in phenolic content in mg/kg in different eggplant cultivars [32].

Variety Primary Fruit Color Phenolics (mg/kg) Landraces ANS24 Purple 597.4 ANS26 Purple 539.8 IVIA371 Black 485.8 IVIA604 Purple 607 MUS8 Purple 566.8 SUDS5 Purple 422.1 Commercial varieties 10–201 F1 Black 435.3 10–501 F1 Black 570.8 Black Beauty Black 408.6 De Barbentane Black 468.8 Mulata F1 Black 344.6 Petra F1 Black 433.5

8. Phenolic Acids in Eggplant Eggplant fruits contain major phenolic compounds that are highly beneficial for human health due to their known biological activities, and can be potentially used in treat- ments of several metabolic and cardiovascular diseases [49]. Whitaker and Stommel [50] determined the presence of 14 hydroxycinnamic acids conjugates in seven commercial eggplant cultivars, namely, Black Magic, Classic, Epic, Ghostbuster, Elondo, Orient Express, and Pirouette, and divided the phenolic acid into five groups: chlorogenic acid iso- mers; isochlorogenic acid isomers; amide conjugates; unidentified caffeic acids conjugates; and acetylated chlorogenic acid isomers [50]. Chlorogenic acid isomers constitute the major class of conjugates of hydroxycinnamic acid (77.6–94.9% of the total conjugates), followed by amide conjugates (3.4–20.4%). The remainder of the three groups, i.e., isochlorogenic acid isomers, unidentified caffeic acid conjugates, and acetylated chlorogenic acid iso- mers, covered only 0.1–2.8% of total hydroxycinnamic acid conjugates in the fruit of these cultivars [50]. Plazas et al. [51] evaluated 18 accessions of Spanish landrace eggplant variants with different sizes, shapes, and ; the chlorogenic acid content presented a mean value Appl. Sci. 2021, 11, 7078 8 of 13

of 3.55 g/kg and varied from 2.47 g/kg for V21 to 6.27 g/kg for V17 [51]. Chlorogenic acid, determined by the Folin–Ciocalteu method, represented an average of 21.1% of the total phenolic content; although significant differences in the chlorogenic acid content were observed among different accessions, it ranged between 13.6% (B32) and 36.2% (V19) of total phenolic content. Zaro et al. [52] examined different development stages of the eggplant fruit in Monarca and Perla Negra cultivars. In all stages, hydroxycinnamic acids represented 75–80% of total phenolic content in both cultivars [52]. Further, chlorogenic acid was found to be higher in the initial stages of eggplant development (2000 mg/kg) than in fruit harvested at later stages (900 and 600 mg/kg in Perla Negra and Monarca, respectively). Luthria [53] indicated in a study that a UV spectral scan method can be used as a simple, high-throughput screening method at an affordable cost for the prediction of the antioxidant potential and phenolic acid content of the pulp extracts of eggplants [53]. This method can also evaluate the impact of the environment or soil type on the antioxidant potential and phenolic acid content of eggplant.

9. in Eggplant rich foods are known to be highly effective against various health prob- lems, such as diabetes, neuronal disorders, cardio-vascular disorders, and cancer [54]. Moreover, in vivo supplements rich in anthocyanins have been shown to exhibit effects such as prevention and treatment of cancer, and may benefit cardiovascular health [55,56]. Consumption of purple eggplant containing a high concentration of the compound nasunin prevents peroxidation of lipids and accumulation of reactive oxygen species (ROS) [57]. In addition, anthocyanin found in eggplant peel appears to be vital in preventing obe- sity by reducing the levels of serum triglyceride and cholesterol, and increasing HDL (high-density lipoprotein) [58]. Anthocyanins present in eggplant are highly beneficial due to their anti-allergic, antioxidant, anti-inflammatory, anti-mutagenic, anti-microbial, and anti-viral activities, and vision improvement effect [59]. Nisha et al. [60] conducted a study on different varieties of Indian eggplant for the evaluation of anthocyanin content, which varied from 0.048 mg/100 g for “long green” to 0.756 mg/100 g for “purple color small size” as Cya-3-glu equivalents FW [60]. Boulekbache-Makhlouf et al. [61] analyzed an eggplant peel (byproduct) for bioactive compounds with different solvents, and found 82.83, 62.92, and 51.56 mg/100 g as Cya-3-glu equivalents DW in methanol, acetone, and ethanol, respectively. Guillermo et al. [18], while evaluating the anthocyanin content of commercial vari- eties of eggplant, reported that values ranged from 3.9 mg/100 g for the Thai cultivar to 161.1 mg/100 g for the Philippine cultivar [18]. Dranca and Oroian [62] extracted total monomeric anthocyanin from the peels of eggplant utilizing ultrasonic treatment with two solvents, namely, propanol and methanol. Results from the response surface methodology (RSM) suggested methanol was a suitable solvent for anthocyanin extraction, with a concen- tration of 54.4%, and the optimal conditions found were an ultrasonic frequency of 37 kHz, a temperature of 55.1 ◦C, and an extraction time of 44.85 min, which led to the recovery of 2410.71 mg of total monomeric anthocyanin. Scalzo et al. [63] used Tunisina, Buia, and L305 genotypes to evaluate their anthocyanin content and the data obtained indicated the contents of 41.3, 155.3, and 96.3 mg/100 g in Tunisina, Buia, and L305, respectively [63]. In addition, Wu et al. [64] conducted a study to evaluate anthocyanin content in common foods consumed in the United States, based on their precise composition, and observed that eggplants contained a total anthocyanin content of 85.7 mg/100 g of fresh weight.

10. Flavonoids in Eggplant Ninfali et al. [46] scrutinized total flavonoid content of eggplant extract from Black Beauty and Violetta Lunga varieties and reported 28.4 and 25.7 mg/100 g of catechin equivalents (mg CatE/100 g) FW, respectively, using the AlCl3 method [46]. In addition, Boulekbache-Makhlouf et al. [61], after evaluation of eggplant peel (byproduct) with different solvents, found the highest level of flavonoid content in acetonic extract (18.52 mg Appl. Sci. 2021, 11, 7078 9 of 13

QE/100 g DE), whereas methanolic and ethanolic extracts contained similar concentrations (16.26 and 16.13 mg QE/100 g DE, respectively) [61]. Further, Bor et al. [65] performed water extraction of phytochemicals in some commonly consumed vegetables and noted total flavonoid content of 1733 mg QueE/100 g FW in eggplant. Nayanathara et al. [66] carried out a study to analyze five genotypes of eggplant (violet nadan, long green, small round green, violet suphol, and violet with white stripes), and noted the flavonoid content varied from 22.62 mg/gm in long green to 102.01 mg/gm in violet suphol genotypes. Kaur et al. [48] investigated the total flavonoid content in different genotypes of Indian eggplant using the AlCl3 method and reported that the corresponding values varied from 3.23 to 25.96 mg QE/100 g on a fresh weight basis (34.61 to 324.17 mg QE/100 g on a dry weight basis) [48]. Akanitapichat et al. [67] reported that, for flavonoid content obtained from methanolic extracts of five varieties of Thailand eggplant, the values varied from 1991.29 mg CE/100 g for SM3 to 3954.20 mg CE/100 g for the SM1 variety. Piao et al. [68] also inspected the flavonoid content in leaves and fruits of eggplant and observed four distinctive flavonoid aglycones (apigenin, isorhamnetin, kaempferol, and quercetin) in all of the samples of eggplant leaves; however, two (apigenin and isorhamnetin) were not observed in eggplant fruits. The study revealed that the flavonoid content in the leaves was much higher (average value of 15.6 µg/mg) than that in the fruits (average value 0.9 µg/mg), which suggested that eggplant leaves are a potential source of naturally occurring antioxidants due to the presence of high flavonoid concentrations. In addition, Plazas et al. [69] analyzed the responses to water stress in four eggplant varieties, namely, MEL3, MEL4, MEL5, and MEL6, to provide information about the mechanisms that are initiated under conditions of drought in this vegetable crop. Results showed that the total flavonoid content increased significantly in MEL3 and MEL4 under stress, but not in MEL5 or MEL6. That is, flavonoid content in the leaves of the four varieties analyzed in this study were similar to those in the control ; however, the differences increased under stress.

11. Dry Matter Content of Eggplant Arivalagan et al. [39] selected the genotypes that represented diversity in their mor- phological characteristics, primarily the fruit color, shape, and size. They observed that the dry matter content in commercial varieties varied from 6.25 g/100 g for Punjab Sadabahar to 7.45 g/100 g of fresh weight for Pusa Ankur (Table 10)[39]. Further, Raigon et al. [31] reported that the dry matter content in 2007 varied from 8.39 g/100 g for IVIA371 to 9.06 g/100 g for CS16, whereas in 2008, it varied from 8.14 g/100 g for CS16 to 8.56 g/100 g for H15 [31]. However, no significant impact of cultivation method or cultivar × cultiva- tion method interaction was detected in either of the two years. In addition, in another study, Raigón et al. [32] observed that for commercial varieties the variation in dry matter content ranged from 54.9 g/kg for De Barbentane to 61.1 g/kg for Mulata F1, whereas for landraces, the corresponding variations ranged from 46.9 g/kg for IVIA25 to 67.0 g/kg for VS9 (Table 11)[32].

Table 10. Variations in dry matter content in Indian eggplant cultivars in g/100 g of fresh weight adapted from Arivalagan et al. [39].

Variety Dry Matter (g/100 g of Fresh Weight) Punjab Sadabahar 6.25 Pusa Purple Long 6.68 Pusa Ankur 7.45 Pusa Kranti 6.66 Pusa Upkar 7.42 Appl. Sci. 2021, 11, 7078 10 of 13

Table 11. Variation in dry matter content in g/kg for different eggplant cultivars [32].

Variety Primary Fruit Color Dry Matter (g/kg) Landraces ANS24 Purple 58.5 ANS26 Purple 60.9 ASIS1 Black 46.9 IVIA604 Purple 47.8 VS22 Purple 50.4 VS9 Purple 67 Commercial varieties 10–201 F1 Black 58 10–501 F1 Black 59.1 Black Beauty Black 55 De Barbentane Black 54.9 Mulata F1 Black 61.1 Petra F1 Black 57.4

According to Shabetya et al. [38], the dry matter content depended on the subspecies and the fruit color; in the West Asian sub-species, it averaged around 7.9–8.0%, whereas in the East Asian sub-species, it was slightly higher and varied from 8.2% to 8.4% [38]. Moreover, it was observed that the higher air temperatures and drought conditions during the growing season lead to the generation of higher concentrations of dry matter. Further, the species S. tomentosum and S. elaeagnifolium exhibited the highest concentration of dry matter, which was about 29% [26]. Yadav et al. [11] concluded that the dry matter content in the peel of different Indian eggplant cultivars varied from 7.14% in Pusa Upkar to 9.8% in Arka Shirish [11]. Scalzo et al. [61] revealed that the fresh weight of the eggplant decreased after grilling due to water loss from the eggplant tissues, in comparison to the raw eggplant, and increased after boiling due to water intake in all of the samples (with the exception of the Tunisina sample) [63]. As a result, the dry matter content increased in grilled fruits and decreased in boiled fruits, as compared to the raw fruits. The dry matter content recorded in the three eggplant (raw) cultivars was 7.8%, 9.1%, and 8.0% for Tunisina, Buia, and L 305, respectively [63].

12. Conclusions and Future Directions The phytochemical content and morphological features of eggplant fruits are highly diverse. Additionally, due to its short growing season, broad popularity, and tolerance to a range of climates and settings, it is an excellent crop for improving its biochemical composition, which will have a far-reaching effect on the human population. However, more research and analysis are needed on the bioactive compounds obtained from eggplant in order to explain their probable mechanisms of action. Therefore, powerful techniques should be employed for assessing the biochemical and nutritional composition of eggplant. It is worth mentioning that many of the bioactive compounds have not been discovered or effectively characterized yet. In this regard, analytical tools and techniques, such as metabolomics and metabolic profiling, have been significantly developed for the efficient and less time-consuming discovery and characterization of numerous bioactive compounds. Exploration of eggplants for their bioactive compounds can help in the development of pharmaceutical products, in addition to other agricultural products. Thus, metabolomics may provide information for breeding programs aimed at crop quality improvement. In the family, numerous studies on metabolites have been carried out in and tomato. In spite of the fact that eggplant represents a vital source of nutraceuticals Appl. Sci. 2021, 11, 7078 11 of 13

and pharmaceuticals, limited research or studies have been conducted to investigate the bioactive compounds in eggplant, beyond the phenolic contents and their antioxidant capacity. Moreover, certain primary metabolites, including carbohydrates and amino acids, have been mostly overlooked. Therefore, eggplant cultivars with significant quantities of the above-mentioned bioactive compounds should be recognized, and more effort should be made to comprehend the nutritional and pharmaceutical importance of eggplant. New breeding technologies and strategies may be developed for this vegetable crop.

Author Contributions: P.K. conceived of and designed the project; P.K. supervised the study; M.S. and P.K. wrote the paper; P.K. checked and corrected the final draft. Both authors have read and agreed to the published version of the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors are thankful to the anonymous reviewers for their careful reading. Conflicts of Interest: The authors declare no conflict of interest.

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