Recent advances in extraction methodologies for the valorization of mango peel wastes

ABSTRACT Mango is an important tropical edible fruit having economic importance, which is cultivated mainly in India (36.6%). It contains various macronutrients, micronutrients, antioxidants, and other bioactive compounds. It is consumed in fresh form or processed into different products namely pulp, juice, puree, pickle, jam, and nectar. It has been predicted mango processing will increase considerably reaching USD 1.8 billion in 2029.] The by-products generated during the processing of mangoes are peel, pomace, seed, and kernel which constitute 25–40% of fresh fruit. This review article describes the nutritional composition of mango peels and also provides detailed insights into different extraction methodologies for value-added compounds. This review also explores the available literature reports that prove mango peels are an excellent source of carotenoids, pectin, phenolic compounds, and volatile aroma compounds. Mango peels contain significantly higher amounts of minerals compared to pulp. It possesses antioxidant, antimicrobial, cardioprotective, anti-diabetic, and anti-cancer properties. This article emphasizes the advantages of green extraction methodologies like ultrasound or microwave-assisted deep eutectic solvents compared to conventional extraction methods. The sustainable valorization of mango peels generated during processing can be economical as well as environmentally feasible.


Introduction
The mango (Mangifera indica L.) is a famous tropical fruit from the family of Anacardiaceae that is mainly cultivated in India and China.They have a potent aroma, and vibrant peel color and are notable for their alluring scent, delectable flavor, and high nutritional value, which include significant concentrations of vitamin C, and minerals.Various products such as pickles, chutney, powder, and beverages are commercialized from raw mango.Different products like pulp, fresh juice, nectar, smoothies, ice cream, jam, and ready-to-serve beverages from ripened mangoes are also popular. [1- 6]The different mango development stages are the juvenile stage (rapid cellular growth), growth stage (cell enlargement), maturation, ripening stage (increase in respiration and ethylene formation) followed by the senescence stage, and the post-ripening stage (microbial degradation) as shown in Table 1. [7]ango peel, stones, kernel, and pomace after pulp extraction are generally discarded as wastes which constitute 35-55% of the fruit. [8]Mango waste is rich in antioxidants, fiber, phenolic compounds, and carotenoids (Figure 1).Mango peel contains a higher amount of pectin and constitutes around 20-30% of dry weight.Dumping of organic wastes with low nitrogen content (<0.14%) from fruit processing industries in landfills can result in methane emissions.Due to their low nitrogen content, they cannot support decomposition by microorganisms.The valorization of mango wastes generated from fruit processing industries can increase their profitability and reduce pollution.Various value-added functional foods can be prepared by using the compounds extracted from mango by-products.About 41.54 million tonnes of mangoes are produced annually and around 15-25 million tonnes of wastes are being generated per year by the mango processing industries. [9]ifferent search engines like Scopus, PubMed, and Google Scholar were examined by using keywords like "mango peel" and "mango byproducts" to collect the relevant literature data from the years 2001-2023.The present review article critically discussed and summarized the pigment profile, and methods used for the extraction of pectin, phenolic, and aroma compounds from mango peel wastes.

Pigments
Mangoes can be classified into green, yellow, and red based on their skin color.The color of fruits is a quality index and it is an important factor that attracts consumers. [10]Chlorophyll is responsible for the green color of mangoes and the reduction in the chlorophyll during ripening can be attributed to the presence of ethylene which stimulates the synthesis of chlorophyllase enzyme. [11][14] Mangoes  INTERNATIONAL JOURNAL OF FOOD PROPERTIES contain lipid-soluble pigment, carotenoids, that are responsible for yellow-orange color.Anthocyanin is a water-soluble pigment that provides a red color to the peel, which can be observed in different mango cultivars. [15,16]The color grade chart for the development of mangoes consists of five ripening stages such as unripe, early ripe, partially ripe, ripe, and over-ripe or decay. [17]This can be divided into pre-climacteric, climacteric, and senescence stages (Figure 2).This can be used for quick identification for classification based on the mango skin color.The first two stages of mangoes are suitable for export and the fourth stage is suitable for mango pulp processing industries to obtain products of superior quality.
Anthocyanins and carotenoids are the pigments mainly responsible for the mango color.The anthocyanin content extracted from Kent mango peels using an ethanol/acetic acid mixture is 39.52 mg/100 g. [18] The hot air-dried green and ripe mango peels retained carotenoid content in the range of 9.69-16.06mg/100 g. [19] Sogi et al. reported lower carotenoid due to the green and red color of the mango peel because the fruits did not turn yellow on ripening. [20]Hot air-drying (67.82 g/kg) is the most suitable method to retain higher carotenoids in Ataulfo mango peels compared to freeze-drying (51.14 g/kg) due to the variation in 13-cis-β-cryptoxanthin content. [21]The total carotenoids in Kasturi, Kent and Haden mango peels obtained by ultrasound-assisted solvent extraction (acetone, dichloromethane, and methanol) and supercritical fluid extraction is in the range of 44.71-65.66µg/g and 43.12-53.87µg/g, respectively. [22]The total carotenoid content in mango peels belonging to different cultivars (Janardhan Pasand and Arka Anmol) was in the range of 0.74-31.18µg/g. [20]The carotenoid concentration in the peels is generally higher than the pulp due to their active role in photosystem assembly as a photoprotector.The greater exposure of peels to sunlight promotes carotenogenesis which in turn increases the concentrations of carotenoids making them the important ). [17]ource for the extraction of pigments. [23,24]Various reports on the extraction of pigments from mango peels are summarized in Table 2.
The intense and uniform color plays an important role in the acceptance of foods because it makes the products more attractive to consumers.The interest of both manufacturers and consumers toward products containing natural ingredients has constantly increased.Natural pigments present in fruits and vegetables can reduce the risk of diseases such as type-1 diabetes, obesity, and coronary diseases. [25]The incorporation of natural pigments in food products can improve the organoleptic, nutritional, and health-promoting properties.However, their application is challenging because the stability of pigments is affected by composition, packaging, and storage environment.The encapsulation and incorporation of pigments in smart packaging are used to overcome stability problems. [26]igments such as carotenoids, anthocyanin, and chlorophyll are added to improve the color of beverages during storage.The nutritional properties of edible oils during storage and cooking can be enhanced by incorporation with natural pigments. [27]The oxidation of flax seed oil can be prevented by the addition of β-carotene while the inclusion of chlorophyll along with gluten in packaging film improved the oxidation index of sesame oil. [28]ber Dietary fiber includes cellulose, hemicellulose, lignin, pectin, and gums. [29]There are two types of fibers called soluble dietary fiber (pectin and gums) and insoluble dietary fiber (cellulose, hemicellulose, and lignin) which are generally present in cell walls and parenchyma cells of agricultural produce. [30]Aziz et al. reported green unripe and ripe Chokanan mango peel flour contains 1.26 and 1.45 g/kg of dietary fiber, respectively. [18]Dietary fiber in the Raspuri and Badami mango peels was in the range of 40.6-72.5%. [31]Kaur et al. reported ultrasound, enzymes (amylase and protease), and a combination method to extract dietary fiber from mango peels. [32]Ultrasound-assisted enzymatic extraction resulted in a higher yield (79%) compared to enzymatic (55%), ultrasound (60%), alkaline (59%), ethanol (44%), and hot water (26%) extraction.The total dietary fiber content in various cultivars like Tommy Atkins, Kent, Palmer, Nam Dokmai, and Ataulfo is 36.3,36.0,28.7, 42.4, and 54.53%, respectively. [33,34]The application of high hydrostatic pressure (55°C) increased the soluble dietary fiber content from 37.4% (control) to 45.7% which is due to the conversion of insoluble dietary fiber to soluble dietary fiber (38.9%-40.5%). [35]Different reports on fiber extraction from mango peels are summarized in Table 3.

Pectin
Homogalacturonans are the major component of pectin, which is composed of α-(1-4) linear strands of D-galacturonic acid.It also contains substituted galacturonans like D-xylose and D-apiose in xylogalacturonan and apio galacturonan, respectively. [36]A distinct structural type of pectin i.e., rhamnogalacturonan II also occurs less frequently in pectin, hence it is categorized as a substituted galacturonan (Figure 3). [37]Pectin is used in the preparation of food products like jam, jellies, marmalade, dairy products, beverages, confectionery, and sauces. [38,39]This is conventionally extracted from agricultural waste (fruit and vegetable peels) by using an acidic or basic aqueous medium.Industrially, acid extraction is generally performed in which strong mineral/organic acid solutions are heated at high temperatures.These processes are time and energy-consuming and also result in serious environmental problems due to the production of acidic wastewater and equipment corrosion.Hence, there is rising interest in the application of alternative green approaches like ultrasound and microwave (MW) assisted deep eutectic solvents (DES) to facilitate the extraction process as shown in Figure 4. Ultrasound is an eco-friendly process that causes cell wall deterioration in plant tissues and the biomolecules are transferred to the extraction solvent. [40]The advantages over traditional extraction methods are a reduction in solvent consumption and processing time, an increase in repeatability, compounds with higher purity, and lower energy consumption. [41]The MW is a form of electromagnetic radiation that generates heat energy in the sample due to dipole rotation and ionic polarization.The water vapor creates pressure in the cell walls resulting in the dissolution of soluble biomolecules in the extraction medium. [42,43]he acidic water extraction using 0.05 N HCl at 100°C for 1 h resulted in 18% pectin yield from Totapuri mango peels. [44]The lower pH (1.66-2.4) and higher temperature (85-97°C) improved the pectin yield to 30% from the Uba mango peel. [45]The pectin was extracted using hydrochloric acid of different pH (1-3) at different temperatures (60-100°C) for various treatment durations (60-140 min) from mango peel powder. [46]Pectin yield was in the range of 14.6-28.42%and the maximum yield was at pH 1.5 and 90°C for 120 min.Shaibu et al. reported hydrochloric acid extraction from unripe and ripe mango peels at different pH (1.5-3) and temperatures (70-90°C). [47]The pectin yield increased with an increase in temperature and a decrease in pH while the yield was similar for both ripe (22.67%) and unripe peels (21.94%).Hydrochloric acid assisted acid extraction (pH of 1.5) of pectin from seven different mango cultivars such as Rad, Tar Lubnak, Mahachanok, Sampee, Chok Anan, Keaw, Nam Dok Mai is 22.4, 7.1, 3.5, 8.8, 3.3, 1.0 and 0.8%, respectively. [48]The yield of pectin varies between cultivars at the same extraction conditions.The sample-to-solvent ratio and precipitation time during acid extraction also influence the pectin yield.The maximum pectin yield (14.23%) was attained at a sample-to-solvent ratio and precipitation time of 1:20 and 6 h, respectively. [49]The hydrochloric acid-based extraction at 85°C and pH of 1.5 resulted in pectin yield of 24.5 and 22.3% in Ceni and Springfield mango peels, respectively. [50]Koubala et al. reported conventional hydrochloric acid (0.03 M and pH of 1.5) and oxalic acid (0.25% at 85°C for 60 min) based extraction resulted in a pectin yield of 16.1 and 19.41%, respectively. [51]Lai et al. reported a maximum yield of 23.7% pectin from mango peels using 0.1 N HCl at 90°C and pH 2 for 2 h. [52] pectin yield of 21.7% was attained during sulfuric acid-assisted extraction at 100°C for 60 min at pH 2. [53] Rehman et al. reported extraction of pectin at pH 2.5 and 80°C for 120 min resulted in better pectin yield with sulfuric acid (21%) compared to hydrochloric (13.5%) and nitric acid (15.1%). [54]irisakulwat et al. observed a 25-42% pectin yield from Tai mango peels using 1 M H 2 SO 4 with a 60 min extraction time. [55]Sulfuric acid-assisted extraction at various pH (1.66-3.34),temperature (52.5-89.5°C),and time (44-120 min) was reported by Ravikumar and Aklilu. [56]The pectin yield was in the range of 12.1-18.5% while the maximum yield was at 82°C and pH 2 for 105 min.The mango peels of thickness 2 mm heated in a batch reactor containing 1 M H 2 SO 4 in the ratio of 10:1 at 85 and 90°C resulted in the pectin yield of 21 and 35%, respectively.Lattice Boltzmann Method simulation studies of mango peels of different thicknesses (1, 1.5, and 2 mm) were analyzed to understand the effect of peel thickness on pectin yield.The simulation studies proved that the mango peels of lesser thickness required a shorter processing time but resulted in lower pectin yield. [57]The pectin yield from Nam Dokmai mango peel during isopropanol-based direct precipitation (47.7%), fractional alcoholic precipitation (47.6%), and sulfuric acid extraction (50.8%) at pH 1.5, 90°C for 2.5 h was similar. [58]arim et al. [59] reported pectin extraction using citric acid at different durations, temperatures, and pH resulted in 3.1-35.7% of pectin yield.The maximum yield was obtained at the processing condition of pH 2 and 80°C for 5 h.The Hoa Loc, Cat Chu, and Ghep mango cultivars of different maturity stages such as pre-mature, mature, and ripe were used for citric acid-assisted extraction. [60]The ripe stages of all three mango cultivars resulted in higher pectin yield (24.1-31.7%)while the Ghep cultivar contained high pectin.The pectin was sequentially extracted from Kent mango peel residue after anthocyanin extraction using acetic acid (0.1-2%).Pectin yield was high from the anthocyaninextracted residue (120-230 mg/g) compared to conventional acid extraction of mango peel powder (163 mg/g). [61]Organic acids have a lower dissociation constant than mineral acids leading to a lower hydrolyzing capability and depolymerizing effect which in turn results in a higher yield of pectin. [62]ongkaew et al. studied the MW assisted-hydrochloric acid extraction (pH 1.5 and 700 W for 3 min) of pectin from mango peels of different cultivars like Mahachanok (13.67%),Chok Anan (15.07%),Nam Dok Mai (12.76%), and Kaew (7.65%). [63]The yield during MW-assisted hydrochloric acid-based extraction of pectin from mango peel at 413 W and pH of 2.7 was 28.86%. [64]The pectin yield from mango peel powder using acid extraction (2 M HCl) for 2 h was 8.8% and MW-assisted acidic water extraction was 8.6-10.5%. [65]Nam Dokmai mango peel powder subjected to MW-assisted acidic water treatment using 1 M HCl of pH 1.5 at 500 W for 20 min resulted in a 10.33% pectin yield. [66]The MWassisted hydrochloric acid extraction at 600 W and holding time of 8 min resulted in a pectin yield of 13%. [67]Mango peel pectin (cv.Nam Dok Mai) was extracted using MW-assisted acid extraction (2 M HCl) with a yield of 13.9%. [68]Ultrasound-assisted hydrochloric acid extraction of Hilaza mango peels at three different maturity stages and two different ultrasound frequencies (37 and 80 kHz) was studied by Torres-Gallo et al. [69] The overall pectin yield was in the range of 13 to 30% whereas the maximum yield was obtained from mango peels of lower maturity stage when processed at lower frequency.This is attributed to the lower content of pectin in higher maturity stages due to the presence of hydrolytic enzymes (pectin methylesterase and polygalacturonase).During fruit ripening, polygalacturonase and pectin methyl esterase hydrolyze the pectin backbone and solubilize insoluble protopectin into soluble pectin; consequently, more soluble pectin is produced. [70]The MW-assisted acid extraction at 606 W, pH of 1.83, and 5.15 min resulted in higher pectin yield. [71]Ultrasound-assisted citric acid-based extraction of pectin at 20 and 80°C from Tainong No. 1 mango peel was studied by Wang et al. [72] The pectin yield at 80°C (16.70-17.15%)was higher than extraction performed at 20°C (1.55-2.09%).
Subcritical water extraction without the application of acids resulted in an 18.34% pectin yield from the Kesar mango peels. [73]Chen et al. reported that betaine-citric acid and choline chloride-malic acidbased DES showed a better yield of pectin (30-38.72%)compared to hydrochloric acid extraction (13.2%).The application of ultrasound power at a high intensity improved the extraction of low-ester pectins which in turn resulted in lower molecular weight and particle size. [74]Different reports on pectin extraction are provided in Table 4.

Application of fiber
Mango peel flour can be used as functional flour for the development of various food products.It acts as an excellent source of dietary fiber and various other bioactive compounds.The mango peel powder is a rich source of fiber that can be used for the preparation of bakery and extruded products that in turn can enhance the shelf life and nutritive value.The sponge cake was prepared by partial substitution of wheat flour with mango peel flour at different concentrations (5-30%).The prepared cake retained higher dietary fiber with low fat, calorie, and glycemic index compared with the control.The cake prepared with 10% mango peel flour had better sensorial characteristics. [75]Jellies prepared from acid-extracted pectin (1%) possess better sensorial characteristics. [45]The high degree of esterification, degree of acetylation, protein, and ferulic acid content of pectin promote the emulsifying properties.Pectin is used in the preparation of jams, jellies, and low-calorie foods as a sugar substitute.Pectin acts as a fat substitute to develop products such as low-fat mayonnaise, dairy products, ice cream, and meat products of superior nutritional quality.The textural profile and sensorial attributes of Chinese sausage which contain 5% pectin as a fat substitute are similar to that of control. [67]Pectin is used in the preparation of films that are suitable for food packaging systems and drug delivery system applications. [65]It is also used in tissue engineering, gene therapy, and to reduce blood cholesterol levels.

Phenolic compounds
Phenolic compounds are secondary metabolites present in plants that possess a common structure comprised of aromatic ring substituted with one or many hydroxyl substituents.These compounds can be divided into simple phenolic and polyphenolic compounds.Simple phenolic compounds are divided into various groups such as simple phenolics, phenolic acids, hydroxybenzoic acid, hydroxycinnamic acid, and coumarin. [76]On the other hand, the classes of polyphenolic compounds are flavonoids and non-flavonoids. [77]Phenolic compounds are conventionally extracted using Soxhlet, maceration, and hydrodistillation.Soxhlet technique and maceration involve the addition of a high  [78] The major phenolic compounds in the mango are mangiferin, gallic acid, and catechins. [15]Factors such as soil type, weather, and temperature affect the biosynthetic pathway of phenolic compounds thereby altering the phenolic composition. [79,80]mamahesh et al. reported that Sindhura mango peels had higher total phenolic content (87 mg GAE/g) than Alphonso, Malgua, Rumani, and Banisha. [81]Sogi et al. reported drying of mango peel (Tommy Atkins) using freeze drying, hot air, vacuum, and infrared radiation. [19]Phenolic compounds extracted using methanol ranged from 2032-3185 mg GAE/100 g.The content of phenolic compounds in the Ataulfo mango peel is 18.67 mg GAE/100 g whereas gallic acid, mangiferin, and catechin are 106.27,355.39, and 98.65 µg/g, respectively. [33]The phenolic compounds in mango peels of Ataulfo and Tommy Atkins cultivars are 68.13 and 42.4 mg GAE/g, respectively while the antioxidant capacity is 12.76 and 9.29 μmol Trolox equivalent (TE)/g, respectively. [82]The unripe Ataulfo peel flour contains 16.51 mg/g of antioxidant capacity. [83]ajor phenolic compounds present in Keitt, Osteen, and Sensación mango peels are gallic acid, p-hydroxybenzoic acid, and ellagic acid. [84]The total phenolic content of Bambangan peel powder is 98.3 mg GAE/g. [85]he phenolic compounds in Raspuri and Badami mango peels are in the range of 55-110 mg/g of dry peel powder. [86]Espada and Tommy Atkins mango peels were used for phenolic compounds extraction using different solvents like pure water and ethanol of different concentrations (50-100%) by Palmeira et al. [87] The application of 70% ethanol resulted in a higher extraction yield (33.7%) compared to pure water and ethanol of other concentrations.Rojas et al. reported that Ataulfo mango peel contains phenolic compounds of 72.61 mg/g by conventional ethanol extraction. [88]The ripe peels contain more phenolic (8.1-29.5 mg/g) and flavonoid (0.101-0.392 mg/g) content than the raw peels in Raspuri and Badami cultivars. [30]The major phenolic compounds of mango peels are gallic, protocatechuic, and syringic acids while flavonoids are kaempferol and quercetin.The content of phenolic compounds in mango peel dietary fiber is 8.12, 29.52, 10.45, and 28.10 mg gallic acid equivalent (GAE)/g in Raspuri raw, Raspuri ripe, Badami raw, and Badami ripe mango peel, respectively.Ajila et al. reported phenolic compounds like gallic acid, syringic acid, and mangiferin are present in both raw and ripe mango peels. [89]The total phenolic content of various cultivars such as Luzon, Narcissus, Royal, Big Tainong, Keitt, Australian mango, Thai mango, Small Tainong, and Egg mango was in the range of 462.2-4071 mg GAE/100 g.The phenolic content was lower in the Lvsong cultivar while the highest was in the peels of the Xiao Tainang cultivar. [90]The application of aqueous solvents such as methanol, ethanol, and acetone (1:1) resulted in the recovery of higher antioxidant capacity (8-28 g TE/100 g). [91]eitt mango peel contains high phenolic content mainly due to the high amount of galloyl glucose, 5-galloylquinic acid, digalloylquinic acid, hexagalloyl glucose, and macluring galloyl glucoside. [92]allic acid, hydroxycinnamic acid, caffeic acid, and p-hydroxybenzoic acid are the major phenolic compounds in freeze-dried Ataulfo mango peels. [93]The freeze-dried and hot air-dried (60°C) mango peels contain similar levels of phenolic compounds. [22]The ultrasound-assisted extraction of phenolic compounds from mango peel powder (Chok Anan cultivar) after 15, 30, and 45 min is 972, 939, and 885 mg/100 g, respectively.The reduction is due to the destruction of phenolic compounds by ultrasound with an increase in extraction time. [94]The total phenolic content and antioxidant capacity in the peel of Pica mango are 72.01 and 32.49 mg GAE/100 g, respectively. [95]The major phenolic compounds are mangiferin, xanthone, and valoneic acid dilactone.The alkali hydrolyzed fraction of Ataulfo mango peel residue contains higher phenolic compounds than the acid hydrolyzed fraction. [96]The phenolic content and antioxidant capacity of Kent mango peels are 128.9mg/g GAE and 0.13 μg GAE, respectively. [61]he phenolic compounds obtained by ultrasound-MW assisted extraction (solid-liquid ratio of 1:5, 50% ethanol, and 10 min) from Ataulfo mango peels is 54.15 mg/g. [97]The application of ultrasoundassisted aqueous ethanol (80%) extraction resulted in a recovery of higher phenolic compounds (67.6 mg GAE/g) from Chaunsa mango peels. [98]The higher amount of phenolics was extracted using ultrasoundassisted extraction at 54°C for 10 min from the Criollo mango peel. [99]The ultrasound-assisted extraction using 46% ethanol at 60% amplitude for 6.5 min resulted in higher phenolic compounds recovery. [100]The antioxidant capacity of MW-assisted ethanolic extract from peels of Keitt mango was 1.5-6 times higher than the conventional solvent extraction. [91]The antioxidant capacity in the mango peel extract obtained by MW-assisted DES and conventional 70% ethanol-based extraction is 82.64% and 80.13%, respectively. [101]These studies indicated application of pure organic solvents resulted in poor recovery of polyphenolic compounds while the aqueous mixture of organic solvents enhanced the recovery of polyphenolic compounds due to the improved solvation.Even though ethanol and methanol have similar polarities, ethanol resulted in lesser recovery of phenolics due to the increased length of ethyl radical leading to poor solvation of antioxidant molecules.
β-alanine and choline chloride-based DES was effective in extracting higher concentrations of total phenolics (3 times) from criollo mango peels compared to ethanol or methanol-based conventional extraction.The extraction efficiency was influenced by the extraction solvent and the application of ultrasound and agitation. [102]Phenolic content and antioxidant capacity are 62.5 mg GAE/g and 81.6 µmol TE/100 mL, respectively in aqueous mango peel extract obtained by sequential ultrafiltration and nanofiltration. [103]The application of a pulsed electric field sequentially with aqueous extraction at 50°C and pH 6 for 3 h resulted in enhanced recovery of phenolic compounds (+400%). [104]Freezedried mango peel retained a higher amount of phenolic compounds while vacuum-drying retained a lower amount.Freeze-dried mango peel powder retained a higher antioxidant capacity (197 µmol TE/g) followed by the infrared, cabinet, and vacuum-dried samples.Subcritical water extraction (30.62 mg GAE/g) resulted in a higher yield of phenolic compounds from Phalun cultivar mango peels compared with ethanol-based Soxhlet extraction (21.76 mg GAE/g). [105]Different reports on the extraction of phenolic compounds are provided in Table 5.

Applications of phenolic compounds
Phenolic compounds can provide health benefits due to their antioxidant activity. [106]The other important characteristic related to phenolic compounds is antimicrobial activity.They can retard the microbial growth by which putrefaction in fruits and vegetables can be prevented.Phenolic compounds are more suitable for food preservation due to their antimicrobial activity. [107]The shelf life of food products can be extended by fortification with phenolic compounds by converting them into functional foods or by incorporating them into packaging material.Phenolic compounds can play an important role in active food packaging by improving the characteristics of packaging material and extending the shelf life of foods.
In the recent past, consumers have preferred foods containing natural food ingredients due to safety concerns.Phenolic compounds started to replace chemical additives in food and are considered to be safe. [108]The major application method is by direct addition of these compounds into the products.Researchers are looking for alternate solutions to avoid the undesirable inactivation of phenolic compounds during food processing.The direct spraying, coating, and dipping methods are considered valid options before food product packaging.Numerous studies on the incorporation of phenolic compounds into various perishable foods showed that they can be effectively used for the preservation of foods and are potential alternatives to synthetic preservatives. [109]Phenolic compounds are considered bio-preservatives because of the safe shelf life extension of food products by inhibiting or delaying the oxidation of food ingredients and the proliferation of microorganisms.Mango peel phenolic extract can act as a color enhancer in food products.The addition of phenolic compounds extracted from mango peel improved the color of the model solution containing strawberries and red radishes because of the intermolecular copigmentation. [110]

Sequential extraction of phenolic compounds and pectin
Plants secrete secondary metabolites like phenolic compounds, which are essential for their growth.These phenolic compounds are extensively studied because of their antioxidant, cardioprotective, anticancer, and anti-microbial properties.The extracted phenolic compounds can substitute the use of synthetic antioxidants in various food products. [111]The sequential extraction of phenolics followed by pectin extraction is an efficient way to utilize mango peel completely because the remaining residue obtained during phenolics extraction can be reused for pectin extraction. [112,113]Banerjee et al. reported hydrothermal treatment without acid resulted in the pectin yield of 15.7 and 27.2% in Calypso and Totapuri mango peels, respectively.Gallic acid (525.1-1405.6 mg/L) and mangiferin (7.8-162.2mg/L) were recovered from the liquid concentrate obtained during pectin extraction. [114]ojas et al. extracted phenolics using water at 60°C for 30 min, and 121°C for 10 and 20 min in a ratio of 1:40. [115]Pectin was precipitated using isopropanol with a yield of 3-16%.The yield of phenolic compounds was higher at 121°C for 20 min and the content ranged from 137 to 509 mg GAE/g.Phenolics content in the extract obtained by ethanol-based extraction from mango peels was 11.66 mg/ g dry peels.The sequential MW-assisted extraction resulted in a pectin yield of 3.9-10.4%.The yield of pectin was 5.7, 14.9, and 26.6% for ultrasound-assisted water, HCl, and lemon juice-based extraction, respectively.Ultrasound-assisted sequential extraction of phenolics by aqueous ethanol and pectin by nitric acid (1.0 M, pH 2) assisted extraction was reported by Guandalini et al. [113] A higher phenolic yield (67%) was obtained when extracted using 50% ethanol without ultrasound.Pectin yield during ultrasoundassisted acid extraction for 10 min, control (only acid extraction without ultrasound), and conventional-acid extraction for 30 min were 8.6, 5.6, and 5.4%, respectively.Banerjee et al. reported that ethanol-assisted Soxhlet extraction of phenolic compounds resulted in a yield of 15.2%.The pectin yield during ultrasound-assisted water, hydrochloric acid, and lemon juice was 6.2, 25.2, and 27.3%, respectively. [116]Microwave-assisted extraction followed by liquid chromatography resulted in a 3.9-10.4%yield of pectin and phenolic content of 11.66 mg/g of dry peels. [117]The various reports on simultaneous phenolics and pectin extraction are provided in Table 6.

Aroma compounds
Aroma is an important decisive characteristic that plays a vital role in selecting a good quality product.The aroma compounds in mangoes are present in two different forms such as free volatile form and nonvolatile precursors. [118]The volatile aroma compounds are classified into seven different groups such as monoterpenes, sesquiterpenes, alcohols, aldehydes, ketones, C13-nor isoprenoids, and furanic compounds.One of the interesting findings is both the pulp and the mango by-products contain similar aroma-active compounds.Monoterpenes are considered to be the primary volatile and abundant odor-active component. [119]The monoterpene content in the peel (80.15 µg/g) was severalfold higher than the mango pulp (6.37 µg/g) and seed (3.02 µg/g).The dominant monoterpene in both edible and non-edible portions is 3-carene.Mango peel contains a higher amount of aldehydes (octanal, decanal, and nonanal) compared to other fractions.The odor-active volatile compounds in mango peels have the highest aromatic impact with green, fruity, floral, and resinous odor-active volatile compounds.The volatile oil content extracted using the Soxhlet apparatus from mango peel contains α-pinene, terpinolene, myrcene, and β-pinene. [120]Glycosidically-bound aroma volatile compounds like γ-terpenyl-β-D-glucopyranosides and derivatives are present in mangoes.The enzymatic and chemical reactions during the ripening stages will result in the release of aromatic compounds (aglycones) from the glycosidic compounds.The further processing of mangoes in industries can also stimulate the formation of aroma compounds. [121]The concentration and type of flavor compounds are dependent on the origin while the major and minor volatile compounds play a key role in imparting characteristic aroma.Mango peel samples contain higher volatile aroma compounds compared to mango pulp. [122]Lalel et al. also reported Kensington Pride mango peel contains higher aroma volatile compounds than mango pulp at different stages of maturity. [123]erpenes are the major compounds responsible for the aroma present in the aglycones extract of ripened mangoes.Mango peel contains odor-active compounds in high quantity compared to mango pulp while 3-Carene is the dominant compound.The other compounds such as decanal, 1-octen-3-one, nonanal, limonene, β-damascenone, and 2-nonenal are the other odor-active compounds in mango peel.The sensorial features of mango peel are fresh herbaceous, fruity, floral, and resinous. [124]ango fruit pulp was homogenized with water and volatile compounds were extracted using simultaneous distillation-extraction. [125] The volatile compounds were present in the range of 18-123 mg/kg of fresh fruit and major terpenes are δ-3-carene, limonene, terpinolene, and α-phellandrene.Tamura et al. characterized volatile constituents of the peel and pulp of Green Thai mango using the Likens-Nickerson apparatus in terms of odor thresholds.γ-Terpinene, (E)-ocimene, (E)-2-hexenal, hexanal, and (Z)-3-hexen-1-ol are found to be the major components. [126]The major compound responsible for the sweet and caramel-like aroma in Haden, White Alfonso, Praya Sowoy, Royal Special, and Malindi mangoes is 4-hydroxy-2,5-dimethyl-3(2 H)-furanone. [127]The major volatile compounds of mango peel and pulp extracted using a Likens-Nickerson-based simultaneous distillation-extraction apparatus with dichloromethane as the extraction solvent are terpinolene and caryophyllene. [128]

Applications of aroma compounds
The volatile aroma compounds extracted from mango peel by various conventional and modern extraction methods can be used in various food, pharmaceutical, and cosmetic industries.Essential oils and their components are classified as Generally Recognized as Safe (GRAS) substances by the US FDA (Food and Drug Administration 2016) [129] and used as natural flavoring agents in the food industry. [130]The dried mango peel powder provides a natural aroma and acts as a flavor additive in yogurt. [131]Mango peel volatile oil is used to prepare healthy instant-flavor drinks. [120]A significant difference in terms of sensorial attributes such as color, sweetness, aroma, flavor, and overall acceptability between commercial flavor drinks and mango peel flavor drinks was not observed.The essential oil can be used as a preservative due to its antibacterial and antifungal activities.The essential oil obtained from Zebdeya, Hindi and Cobaneya mango peels showed cytotoxicity against breast, colon, and liver carcinoma cell lines.It also showed anti-breast cancer activity comparable to the antitumor drug doxorubicin.The Zebdeya and Cobaneya mango peel essential oils serve as immunostimulants because of the low macrophage migration and high phagocytic indices. [132]

Conclusion
Mango processing leads to the generation of by-products which results in waste disposal problems.The direct dumping of organic wastes is harmful to the environment because it can result in methane emissions and also an economic loss to industries.Mango peels are an important source of phenolic compounds, carotenoids, anthocyanins, dietary fiber, and aroma compounds which can be recovered by using extraction technologies.However, the extraction of these compounds with minimal loss poses a great challenge for researchers and scientists.To overcome the drawbacks associated with conventional methods, the results of the application of alternate green technologies like ultrasound or MW-assisted DES are encouraging.Dietary fiber along with the associated phenolic compounds with superior antioxidant capacity from mango peels can be used as a functional food ingredient resulting in the development of a low-cost nutritional supplement.Currently, reports on the incorporation of extracted biomolecules from mango peels into food products are limited.Hence, future research can be focused on the application of biotechnological approaches to improve the yield of value-added compounds and the development of new food products enriched with the extracted bioactive compounds.

Figure 1 .
Figure 1.Value-added compounds present in mango peel.

Table 1 .
Stages of mango fruit development (Adapted from Dar et al.).7

Table 2 .
Pigment extraction from mango peel by using different techniques.
Carotenoids in Kasturi, Kent and Haden mango peels obtained by ultrasound-assisted solvent extraction (acetone, dichloromethane, and methanol) and supercritical fluid extraction are in the range of 44.71-65.66µg/g and 43.12-53.87µg/g, respectively 22 6 Hexane Carotenoids in mango peels (Janardhan Pasand and Arka Anmol) were in the range of 0.74-31.18µg/g 20

Table 3 .
Fiber extraction from mango peel by using different techniques.

Table 4 .
Pectin extraction from mango peel by using different techniques.volume of organic solvents like methanol, ethyl acetate, acetone, and hexane which have health hazards.Hydrodistillation consumes high energy and also requires longer processing time.These conventional techniques have certain limitations such as lower extraction yield and efficiency.The various security risks like solvent toxicity, chances of toxic residues in the final products, and poor yield necessitated the development of alternate green extraction techniques.

Table 5 .
Extraction of phenolics from mango peel.

Table 6 .
Sequential extraction of pectin and phenolics from mango peel.