Fruit and Vegetable Peels: Utilization of High Value Horticultural Waste in Novel Industrial Applications

Fruits and vegetables are the highly used food products amongst the horticultural crops. These items are consumed uncooked, nominally cooked or fully cooked, according to their nature and cooking process. With the change in diet habits and rising population, the production, as well as the processing of horticultural crops, has exponentially improved to meet its increasing demand. A large amount of peel waste is generated from fruit and vegetable-based industries and household kitchen and has led to a big nutritional and economic loss and environmental problems. Processing of fruits and vegetables alone generates a significant waste, which amounts to 25–30% of the total product. Most common wastes include pomace, peels, rind and seeds, which are highly rich in valuable bioactive compounds such as carotenoids, enzymes, polyphenols, oils, vitamins and many other compounds. These bioactive compounds show their application in various industries such as food to develop edible films, food industries for probiotics and other industries for valuable products. The utilization of these low-cost waste horticultural wastes for producing the value-added product is a novel step in its sustainable utilization. The present review intends to summarize the different types of waste originating from fruits as well as vegetables peels and highlight their potential in developing edible films, probiotics, nanoparticles, carbon dots, microbial media, biochar and biosorbents.


Introduction
In the European Union, approximately 89 million tons of food waste is generated, and this value is expected to increase by 40-fold in coming years. The Food and Agriculture Organization (FAO) estimated that around 40% of the food produced in India is wasted [1,2]. Moreover, the Food Corporation of India reported this loss ranging between 10 to 15 percent of the total production. The Ministry of Food Processing Industries (MFPI) India estimated fruit and vegetable losses to be 12 and 21 million tons, respectively, amounting to an approximate value of about 4.4 billion USD, with a total food value loss and waste produce of 10.6 billion USD [2]. A broader term is "fruit and vegetable waste" (FVW), which refers to indigestible parts that are thrown away at different stages such as collection, handling, shipping and processing [3]. In accordance with the above definition, FVW can be considered as fruit and vegetable loss instead of waste. FVW can be produced at different steps from farm to consumer, involving both pre-and post-consumer stages of the food supply chain [4].
There are high amounts of phytochemical constituents present in FVW and studied for dietary fibers, phenolic compounds and other bioactive compounds extraction [5]. Studies revealed that essential nutrients and phytochemicals are abundantly found in the peels, seeds and other constituent of vegetables and fruits in common use [6]. For instance, the skin of avocados, grapes, lemons, seeds of jackfruits and mangoes contains 15% higher phenolic concentrations as compared to fruit pulp [7,8]. The FVWs can be utilized to extract as well as obtain bioactive compounds that can be used in cosmetics, food, textile and pharmaceutical industries as shown in Figure 1. Some of the FVWs waste originates from horticultural supplies, which is not considered important at present. Their proper use will not only solve the environmental issues, but will act as a sustainable approach to improve health via enriched food containing health-enhancing substances [9]. The current review summarizes the scientific inventions and reviews the recent advances in the exploitation of waste fruits and vegetables peel as a valuable commodity of the future.
Molecules 2020, 25,2812 2 of 20 Corporation of India reported this loss ranging between 10 to 15 percent of the total production. The Ministry of Food Processing Industries (MFPI) India estimated fruit and vegetable losses to be 12 and 21 million tons, respectively, amounting to an approximate value of about 4.4 billion USD, with a total food value loss and waste produce of 10.6 billion USD [2]. A broader term is "fruit and vegetable waste" (FVW), which refers to indigestible parts that are thrown away at different stages such as collection, handling, shipping and processing [3]. In accordance with the above definition, FVW can be considered as fruit and vegetable loss instead of waste. FVW can be produced at different steps from farm to consumer, involving both pre-and post-consumer stages of the food supply chain [4].
There are high amounts of phytochemical constituents present in FVW and studied for dietary fibers, phenolic compounds and other bioactive compounds extraction [5]. Studies revealed that essential nutrients and phytochemicals are abundantly found in the peels, seeds and other constituent of vegetables and fruits in common use [6]. For instance, the skin of avocados, grapes, lemons, seeds of jackfruits and mangoes contains 15% higher phenolic concentrations as compared to fruit pulp [7,8]. The FVWs can be utilized to extract as well as obtain bioactive compounds that can be used in cosmetics, food, textile and pharmaceutical industries as shown in Figure 1. Some of the FVWs waste originates from horticultural supplies, which is not considered important at present. Their proper use will not only solve the environmental issues, but will act as a sustainable approach to improve health via enriched food containing health-enhancing substances [9]. The current review summarizes the scientific inventions and reviews the recent advances in the exploitation of waste fruits and vegetables peel as a valuable commodity of the future.

Fruit and Vegetable Peel Based Edible Coatings/Films
Edible coatings are made up of thin layers applied on the surface of the food to make its shelf life longer, maintain the properties, characteristics and functionality of foods at minimum cost [17]. This application can increase its functionality by extending the shelf life, preventing the microbial spoilage and acting as a carrier matrix for antimicrobial agents [18,19]. Coating can be considered as an effective method of preservation during transport of fruits and vegetables easily affected by microorganisms, insects, pre-and post-harvesting conditions [20]. Coatings also help in developing a modified atmosphere to induce varied alterations in minimally processed and fresh foodstuff in various areas such as sensory quality, antioxidant properties, color, firmness, ethylene production, microbial growth inhibition and organic compounds under anaerobic processes [21]. Recently, essential oils (EOs) and their main components have received substantial attention due to the

Fruit and Vegetable Peel Based Edible Coatings/Films
Edible coatings are made up of thin layers applied on the surface of the food to make its shelf life longer, maintain the properties, characteristics and functionality of foods at minimum cost [17]. This application can increase its functionality by extending the shelf life, preventing the microbial spoilage and acting as a carrier matrix for antimicrobial agents [18,19]. Coating can be considered as an effective method of preservation during transport of fruits and vegetables easily affected by microorganisms, insects, pre-and post-harvesting conditions [20]. Coatings also help in developing a modified atmosphere to induce varied alterations in minimally processed and fresh foodstuff in various areas such as sensory quality, antioxidant properties, color, firmness, ethylene production, microbial growth inhibition and organic compounds under anaerobic processes [21]. Recently, essential oils (EOs) and their main components have received substantial attention due to the presence of effective antimicrobial properties in them. The main EO component of lemongrass citral (3,7-dimethyl-2,6-octadienal) has been reported for its antimicrobial activity against a variety of foodborne pathogens and is also explored as an antimicrobial agent in edible coatings [22]. The EOs are commonly considered as safe, as they impart maximum effects with the minimum change in the organoleptic properties of the food [23]. Of late, in the development of edible coatings applications of this emerging technology included a variety of nanosystems, consisting of nanoemulsions, polymeric nanoparticles and nanocomposites to release antioxidants, and showing antibacterial activities on the surface of the food. Because of the many phenolic substances with excellent antioxidant capacity, fruits and vegetable peel are considered as suitable materials for inclusion into films and coatings.
Fish gelatin is believed to be a valuable biopolymer source for fabricating biofilms because of its biodegradable nature and high myofibrillar protein content [24]. Additionally, because of the variations in the sequence of amino acid, fruit and vegetable peel-based films show less water permeability in contrast to mammalian gelatin-based film. Enriching pomegranate peel powder in gelatin films considerably increased their water vapor permeability (WVP) as incomplete dissolution of pomegranate peel in the film matrix resulted in more heterogeneous microstructure [25]. Both hydrophobic and hydrophilic components present in the peel of pomegranate balances the hygroscopic properties and do not alter the moisture content of the films (Table 1). The peel of potato contains adequate quantities of cellulose, fermentable sugars, hemicellulose, and starch [33]. The films with low concentration peel of potato after comparison with high concentrations peel in the biopolymer film resulted in a higher WVP due to bigger pore size of film matrix despite its denser structure [33]. Potato peel biopolymer film-coating proved helpful for designing biodegradable food packaging with more-value commercial use. Generally, fish gelatin/polyethylene bilayer films solubility was lowered by enriching with different fruit peels [34].

Fruit and Vegetable Peel Fortified Probiotics
Over the past two centuries, fruits have been used as a remedy for dry cough, severe thirst and sore throat in medicine. In the last few years, the demand for novel functional foods has increased and probiotics are commonly consumed all over the world and considered as one of the main functional food products [35]. Additionally, the fruits and peels are of great value and high source of bioactive compounds. In pomegranate, citrus, mango and Opuntia ficus-indica (barbary fig) peel, the functional ingredients present in abundance are antioxidants, fiber and oligosaccharides (as prebiotics) [36][37][38][39]. Probiotics and dietary fiber both have been reported to reduce the incidence of colon cancer and relieve constipation [40]. Additionally, some dietetic fibers obtained from fruits have shown considerable effect on the viability of these bacteria and are recommended as an ingredient in probiotic dairy foods [11]. Various attempts have been made to increase the biological activities of probiotics, including supplementation with fruit peel.
Probiotic yogurt prepared with pineapple peel powder improved the anticancer, antioxidant and antibacterial activities against Escherichia coli, but no significant effect was observed on Staphylococcus aureus [41]. The addition of apple, banana and passion fruit peel powder in probiotic yogurt improved the rheological properties and enhanced the growth of Lactobacillus casei, Bifidobacterium animalis subsp. lactis, Lactobacillus acidophilus and Lactobacillus paracasei [11]. The effect of milk supplementation with mango peels on the kefir microorganism's growth rates and antioxidant properties were also estimated in fermented products [42]. Composite fruit peel powder (orange, passion fruit and pineapple) was used in different proportions i.e., 1%, 0.5% and 0.7% (w/v), respectively, to develop fat and sugar-free probiotic set yogurt [43]. Increase in firmness and consumer acceptability, decrease in syneresis and high lactic acid bacteria counts were observed in yogurt incorporated with 0.5% peel mixtures.

Fruit and Vegetable Peel-Derived Metallic Nanoparticles
Beneficial bioactive molecules such as alkaloids, amino acids, enzymes, phenolics, proteins, polysaccharides, tannins, saponins, vitamins and terpenoids and other compounds present in fruits and vegetable waste generally act as reducing agents in metal nanoparticles (NPs) synthesis [44,45]. Some biomolecules play the role of modeling agents directing particle growth in a specific direction, while other biomolecules function as capping agents, preventing nanoparticle from getting agglomerated [46,47]. Nanoparticles biosynthesized by using FVW have also emerged as a reliable, sustainable and eco-friendly technology with lower risk to human health and environmental as compared with chemicals and toxic solvents based conventional manufacturing protocol [48]. A significant interest in use of NPs was reported due to their distinctive physicochemical properties and applications in various fields of biomedicine and pharmaceuticals. The biogenic NPs are synthesized by following the bottom-up approach, in which atoms as well as compounds act as the building block and self-assembled themselves to form the final product [49,50].
Various noble metal/metal oxide NPs have been synthesized from fruits and vegetables peel extracts as shown in Table 2.  Gold (Au) NPs have been synthesized using dried onion peels (OP) aqueous extract, which reduces Au 3+ to OP-AuNPs by forming a colloidal solution [64]. The phytoconstituents, mainly the cysteine derivatives, found in the onion bulb, might be responsible for the synthesis of OP-AuNPs. Zinc oxide (ZnO) nanoparticles were synthesized with domestic waste potato peel after 24 h of incubation using starch present in the potato peel to reduce the metal ion [63]. Banana, pomegranate, lemon and orange peel extracts have also been found to possess the ability to reduce silver (Ag + ) ions in aqueous solutions to synthesize silver (Ag) nanoparticles [54][55][56][57]. Likewise, ZnO-NPs have been synthesized using tomato, grapefruit, lemon and orange peel [61].

Fruit and Vegetable Peel Derived Carbon Dots
Carbon dots (CDs) are a very small (<10 nm) photoluminescent material synthesized by two approaches, i.e., top-down and bottom-up synthetic routes [67,68]. In the top-down synthetic route, a large carbon structure breaks down by involving acid assisted chemical oxidation, electro-oxidation or laser ablation in the synthesis process [69]. However, this approach requires a complex and extreme synthetic condition, which is considered as one of the disadvantages of this method. On the other hand, the bottom-up approach using plants and their byproducts without using chemicals was found advantageous over the top-down approach. Food waste is a matter of serious concern worldwide, and this waste needs to be given proper attention. The use of food as waste offer economic benefits and is one of the most interesting starting materials as carbon sources for the synthesis of CDs [67]. Presence of functional components such as carotenoids, dietary fiber, gallic acid, polyphenols, flavonoids and mangiferin in mango peel and pineapple peel makes these suitable for the development of CDs [70,71]. The increasing number of publications shows that the fruit and vegetable by-products are source of antibacterial, antioxidant and nutritional dietary fiber [72]. Additionally, because of their biocompatibility, innocuousness, low toxicity, low cost and photostability properties, they are suitable starting materials for CDs [73,74]. The appropriate properties, i.e., pyrolyzation at high temperature and oxygenolysis with concentrated acid, carbonization, oxidation, polymerization and nucleation are needed during peels treatment to synthesize CDs [14]. CDs have shown a promising potential for applications in biomedical to energy storage devices, determination of pathogens, environmental studies, detection of heavy metals and additives in the food and water purification processes (Table 3) [75,76].  NA-not applicable.

Fruit and Vegetable Peel Based Microbiological Media
In microbiological studies, microorganisms are grown by adding suitable culture media with favorable environment under laboratory conditions [89]. In most of the cases, commercially available media such as Cetrimide agar, Nutrient agar and MacConkey agar are used, but these are generally considered high cost media [89]. The growth and isolation of organisms have been reported using different substrates and media [90]. Some fruits and vegetables, such as cabbage, carrot, gooseberry, tomato, pumpkin etc., have been used as a substitute for nutrient agar to culture both bacteria and fungi, [91]. Other reports have used black gram, cowpea and green gram as starch and protein substitutes to ease the microbial media cost [92]. Fruits and vegetable bio-waste contain simple and complex sugars that are metabolized by microorganisms and have received much attention for their use in animal feed, bio-ethanol and biogas production [93,94].
Different type of agriculture waste is now used for the production of low-cost growth media for microorganisms (Table 4) [95,96]. Watermelon Citrullus lanatus Watermelon peel waste extract (500 g/L) and dextrose (20 g/L) Evaluation of fungal growth such as Rhizopus oryzae, Lichtheimia corymbifera, Aspergillus niger, Penicillium Expansium and Fusarium oxysporum [99] Orange

Citrus sinensis; Solanum tuberosum; Moringa oleifera
Peel powder of orange (0.20 g/100 mL), potato (0.25 g/100 mL), drum stick (1 g/100 mL) and agar (2.5%) Growth analysis of Trichoderma sp., Aspergillus sp. [103] Agar of Dragon fruit peel (DFP) was also used as a microbial growth media [92]. Grapefruit, banana and melon peels contain a high amount of carbohydrates, which act as a good substrate for the production of amylase [10]. Banana peel was found to be an economically low and effective medium for the growth of fungi [98]. The watermelon peel extract is found rich in macronutrients such as lipids, reducing sugars and total proteins [99]. The study reported that watermelon peel waste (WPW) was best for the growth of Aspergillus niger, Fusarium oxysporum, Lichtheimia corymbifera, Penicillium expansium and Rhizopus oryzae, respectively. The study also revealed that formulated watermelon peel waste dextrose agar (WPWDA) (watermelon peel waste dextrose agar) medium was found as an alternative way for some commonly used media such as Czapek's Dox agar (CzDA) and Potato dextrose agar (PDA); in addition, this medium was very cheap and eco-friendly. Orange peel waste (OPW) was used as a liquid medium in producing biodiesel used oleaginous yeasts and found that and Cryptococcus laurentii UCD 68-201 and Rhodosporidium toruloides NRRL 1091 strains yielded 31.9% and 36.9% of biodiesel, respectively [100]. Pea peel waste was used as a growth medium at 30 • C with Trichoderma reesei to produce cellulase enzyme [102].

Fruit and Vegetable Peel Derived Biochar
Biochar is a stable carbon-rich solid generated by pyrolysis as a result of the thermochemical decomposition of organic feedstock material at high temperatures under oxygen-free conditions [104]. Different types of food waste have been used for the production of biochar and its yield and physicochemical properties have been reported in detail [105,106]. Biochar is generally used to remove different types of pollutant containing heavy metals from contaminated water bodies [107][108][109]. It also serves as an intermediate for producing bioethanol from biological waste collected from food processing industries as well as different agricultural plant residues like husk, bran etc. [110,111]. Different studies have shown the production of biochar from different types of fruits and vegetables peel wastes (Table 5). Maximum removal efficiency was found to be 95% with 120 mg/L of initial Cr(VI) concentration with 3 g/L of biochar dose [121] Potato Solanum tuberosum Pyrolysis at 500 • C for 5 min Hydrogen sulfide (H 2 S) was achieved 53 mg/g at 500 • C, under space velocity (8000 L min -1 kg -1 ) [122] Potato peel waste (PPW) was used to produce biochar by fast pyrolysis using the fluidized bed system to remove H 2 S [122]. Biochar produced from pineapple peel showed that H-bonding interacts with oxytetracycline (OTC) for its sorption. However, thermodynamic parameters showed that the OTC sorption onto the biochar was endothermic and is a spontaneous process [115]. Biochar derived from pineapple, sweet lime and pomelo peel was developed to remove hexavalent chromium from aqueous solution [114,117,121]. In another report, biochar was prepared from rambutan and pomegranate peel for the removal of copper (II) ions from aqueous and soil system, respectively [118,119]. Biochar derived from pomelo and litchi peels were used to remove congo red, methyl orange and malachite green from wastewater [113,118].

Fruit and Vegetable Peel Derived Biosorbents
Biosorption can be explained as a mechanism when a sorbate (i.e., an atom, ion or compound) reacts with the biomass or biomaterial (stated as biosorbent), which causes the acclimatization of sorbate ions over the surface of biosorbents, which subsequently reduces the sorbate concentration in the solution [123]. This mechanism has attained significant attention due to its ability to immobilize the heavy metal contaminated from the water (especially contaminated with the discharge of electroplating and mining industries) or metal processing industries. Numerous biosorbents have been developed with the help of different biomasses such as algae, fungi (e.g., Mucorrouxii), yeasts and bacteria (e.g., Bacillus thuringiensis) [124,125]. The natural biomass complex compendium signifies the contribution of different processes that describe the mechanism of how biosorbents works in eliminating the various contaminants; however, these processes are still being explored. Several functional groups are attached to these biosorbents to attract and sequester the contaminants, which relies on the type of biosorbent and functional groups (amine, amides carboxyl, hydroxyl, carbonyl, sulfhydryl, phenolic, sulfonate and phosphate groups) attached to it [126,127].
Sponge gourd peel was considered as an inexpensive natural biosorbent to remove malachite green (MG), a cationic dye, in batch mode [140]. Banana peel was used as an efficient biosorbent for removing rhodamine-B, a cationic water-soluble dye of basic nature [138]. The contact time selected for the adsorption of rhodamine-B on banana peel powder was 60 min. In another study, a bisorbent was developed by using natural banana peel (NBP), methylated banana peel (MBP) and banana peel activated carbon (BPAC), respectively, and used in the treatment of palm oil mill effluent (POME) [136].

Conclusions
Development of the sustainable solution for managing fruit and vegetable waste has become extremely important in present scenario. Therefore, it demands the development of solution that could utilize the full potential of these waste material and support in attaining the social, environmental and economic benefits from these wastes. Furthermore, utilization of the fruit as well as vegetable waste especially peels in developing value-added products such as edible films, probiotics, nanoparticles, carbon dots, biochar and biosorbents will be an eco-friendly and sustainable way to create novel business opportunities and also functionalizing this waste for a useful purpose. Most of these interventions are in its infancy stage and lacks the technological advances and findings. Hence, there is high need to develop consortia of researcher and industrialist to improve the economic potential of these valuable horticultural wastes with a support of initial investment. Moreover, it will aid in promoting the usage of horticultural waste for synthesizing value-added commodities.