Valorization of Fruit and Vegetable Waste into Sustainable and Value-Added Materials

Abstract

This review aims to streamline the approach to assessing the most used valorization methods for fruit and vegetable waste (FVW) that are eco-friendly, cost-effective, and sustainable within a circular economy framework. Green processing technologies for the extraction of bioactive compounds from FVW, their applications, and the technico-economical assessment of FVW’ biorefinery to support circular economy are highlighted. Important value-added products generated by FVW include bioactive compounds, pectin, protein isolates, such as soy, natural pigments such as anthocyanins, quinones, carotenoids, betalains, and chlorophyll. At this time, the prospects of using FVW have increased in the food supplements, bioactive and edible food packaging, agriculture, energy, and water purification fields. The findings report that proper management of FVW not only minimizes their addition to landfills in the absence of composting, but also promotes the efficient utilization of resources for the development of innovative materials with a wide range of beneficial applications. Implementing the possible solutions described in this paper would not only reduce environmental impact, but also open up new economic opportunities through the valorization of FVW.

1. Introduction

Fruits and vegetables are among the most widely consumed commodities worldwide [1]. In 2001, the most produced fruits worldwide were apples, bananas, watermelons, grapes, oranges, and mangoes, while the most produced vegetables were tomatoes, cucumbers, cabbages, onions, eggplants, and carrots [2]. As outlined in the 2019 research report by the Food and Agricultural Organization (FAO) of the United Nations, approximately 14% of food produced in the world was lost or wasted post-harvest [3]. Moreover, according to the United Nations (UN) Environment Programme’s Food Waste Index Report 2021, 931 million tons are lost annually due to food wastage [4]. Goal 12 of the Sustainable Development Goals (SDGs) incorporated into the 2030 Agenda encompasses the main objectives related to food waste, which involve halving the global amount of wasted food per capita through retail sales and consumption, as well as reducing food loss in the production and distribution processes [4,5,6] to ensure sustainable consumption and production patterns [7]. Fruit and vegetable loss occurs at various stages of the food supply chain, during the production, storage, processing, and distribution stages before consumption [8,9].

Waste from fruits and vegetables makes up approximately 16% of the overall food waste and contributes around 6% to global greenhouse gas emissions [10]. Fruit and vegetable waste is usually discarded or left to spoil at the consumer, retail, or other food outlet levels [1]. In this paper, the expression ’’fruit and vegetable waste’’ (FVW) refers to all parts of fruit and vegetable waste, which occurs through harvesting by the farmer, uneaten food from retail and food services, processing food facilities, preservation, packaging, and end consumers, and which can be further processed and recovered in several ways [11]. By-products in the form of non-edible parts, such as peels, seeds, pomace, and stems generated from industrial processing of fruits and vegetables, are included in FVW.

The beneficial properties of fruit and vegetable waste (FVW), such as biodegradability, cost-effectiveness durability, and lighter weight, underscore their potential for valorization.

Figure 1 illustrates the valorization pathways of the main types of waste, i.e., leaves, peels, pomace, seeds, skin, and roots, resulting from the processing or discarding of FVW prior to reaching the retail phase.

The conversion of fruit waste (such as citrus, banana, apple, grape, and mango) and vegetable waste (including potato, tomato, cabbage, carrot, cauliflower, and broccoli) to adsorbents for water purification has been documented [12].

These raw materials contain ample reserves of valuable molecules, such as phytochemicals, minerals, vitamins, oils, nutrients, and other bioactive compounds recognized for their health advantages [2,11] and food packaging applications [13,14]. These health benefits encompass disease prevention, immune system enhancement, regulation of gastrointestinal transit, and various others [10]. The recovering of fiber and phytochemicals from the peel fractions, pulps, pomace, and seeds provides an integrated perspective for their incorporation into human, animal [15], and fish feed [7,13,16], as well as pharmaceutical and cosmeceutical products [10,17]. The innovative conversion technologies of FVW into high-value-added molecules such as bioactive compounds, pectin, carotenoids, anthocyanins, antioxidants, phenols, flavonoids, proteins, polysaccharides, dietary fibers, or oils are ultrasound-assisted extraction (UAE) [5,18,19], microwave-assisted extraction (MWAE) [20], supercritical fluid extraction (SFE) [5,20], pressurized hot water extraction (PHWE) [20,21,22,23], pressurized liquid extraction (PLE) [5,20,24], supercritical fluid extraction (SFE) [5,20], pulsed electric field-assisted extraction (PEFAE) [5], ohmic heating-assisted extraction (OHAE) [25,26], and enzyme-assisted extraction (EAE) [5].

Also, these leftover biomass materials offer a feasible alternative feedstock for the generation of diverse biofuels, such as biogas, biodiesel, biohydrogen, and fuel pellets [27].

It has been recorded that the inadequate handling of FVW due to their high perishability leads to the escalation of the climate change crisis with considerable GHG emissions. With the disposal of fruit waste in landfills or through composting, greenhouse gases such as CH₄ and nitrous oxides (N₂O) are released, which have more potent warming effects compared to CO₂ [28]. Also, the biodiversity near farmland is damaged, posing a significant threat to human health.

In a study conducted in the United States on the water and carbon footprint emissions associated with the loss and waste of grains, vegetables, and fruits from 1970 to 2017, it was stated that the level of vegetable food waste was five times higher than that of grains and twice as high as that of fruits [6]. These data led to the conclusion that the focus on mitigating food waste should prioritize vegetables. When considering the overall CO₂ emissions due to food loss and waste, vegetables accounted for 50%, grains for 31%, and fruits for 19% of the total emissions. Furthermore, food waste accounted for 29%, 47%, and 24% of the total wasted water [6].

In this context, the aim of this paper is to explore the reintegration of valuable products obtained from FVW into specific applications, as well as to highlight green extraction technologies for valuable molecules, in line with economic circularity.

2. Sources of FVW

2.1. Industrial Processing Facilities

Every day, food processing industries generate a significant amount of fruit and vegetable waste, including skins, seeds, cores, rinds, pomace, etc. [29]. Engineering processes such as oil extraction from papaya seeds [30], citrus peels [31], juice manufacturing [32,33], and sugar refining are significant contributors to food waste [14]. During industrial operation of fruits and vegetables, about 50% of the waste is discarded [33]. For example, 50–65% of waste is produced during the extraction of bergamot essential oil from the pomace of a hybrid plant of lemon and sour orange [34]. In addition, more than 50% of fresh citrus fruits (including tangerines, lemons, oranges, and limes) are made up of the peel, membranes, and seeds [2].

The waste generated from fruit and vegetable sources contains isoflavones and phenolic compounds, carotenes, minerals, oils, enzymes, alkaloids, and vitamins, all of which possess diverse health-promoting properties [35,36,37].

2.2. Retail Sectors

Unsold and abnormal fruit and vegetables, which have equivalent nutritional value to their visually attractive counterparts, are often discarded because of their less-than-perfect appearance for retail purposes [38,39,40]. In addition, 83.9% of produce vendors report that the vegetable leaves make up the largest portion of their waste [41].

The study conducted by Yenerall and Chen [42] on food retailers from the United States identified online grocery shopping and restaurants as the main sources of FVW.

Employees are essential to the retail sector, as they actively address the causes of food waste and are trained to implement various precautionary measures to prevent and reduce it. From the employees’ perspective, the combination of policy, practice, people, and product themes are the main causes of the generation of FVW in supermarkets [43].

The influence of culture and social economy aspects on consumers’ willingness to buy fruits and vegetables with unconventional shapes was investigated by Campos et al. [40]. The authors demonstrated that elevated cultural levels positively influence the consumer’s willingness to buy them, whereas lower socio-economic aspects of a nation have a negative impact on their willingness to make such purchases.

The best methods to reduce FVW include extra care in fruit and vegetable handling, proper packaging, and facilities such as cold rooms and shelving [44]. In addition, effective supply chain management, information sharing, and collaborative partnerships, as well as consumer education and awareness, are also key strategies.

2.3. Consumers Households

Usually, consumers do not consume all parts of fruits and vegetables, with waste encompassing seed coats, hulls, husks, peels, seeds, and pomace, leading to the generation of significant amounts of food waste on a global scale. Regarding the overall waste incidence rate, food waste occurs in most households, with an incidence rate of approximately 90.7%. Fruits and vegetables have the highest incidence rate of food waste, at around 87.7% [45]. The literature suggests that routines relating to planning and shopping for fruits and vegetables are the main factors in waste production, in addition to perceived behavioral control [46].

According to the empirical results, consumers should first be educated about food expiration date labels to correct misinterpretations of these labels’ definitions and, furthermore, to avoid the resulting inappropriate discarding behavior [47,48]. Second, the outer packaging labels of pre-packaged food should be improved. Moreover, new labeling technologies should be used. For example, intelligent freshness monitoring labels can show the current quality status of food and the conditions under which it can no longer be consumed [49].

A survey conducted in Pakistan showed that the financial losses from food waste per person per year were USD 12.8, and were greater in wealthy households compared to lower-income households [50].

3. Potential Uses of Some FVW as Sustainable and Value-Added Materials

Conventional methods for disposal of FVW include briquetting, landfills, anaerobic digestion, incineration, and composting [1,51,52,53]. Each technology has disadvantages in terms of environmental pollution, the high cost of acquiring equipment, and potential harm to animals’ food and human health.

Table 1. Potential applications of FVW.

	Designation Purpose	Process	Features	Ref.
Food and Food Packaging
Citrus peels	Pectocellulosic active food packaging films	Casting	Improved mechanical properties, easy recycling, rapid biodegradation, water stability, water vapor barrier properties, and antioxidant activity	[54]
Stem, peel, and rachis parts of the banana plant	Edible coatings of strawberry fruits	Dip-coating	Extension of the shelf-life by 6 days at 22 °C and 16 days at 4 °C	[55]
Carrot, leek, celery, and cabbage waste	Coloring flavoring agents, natural preservatives for reformulated processed foods	Hot air drying or freeze-drying	Stable powders with highly reduced water activity levels	[16]
Apple pomace	Pectin and phenolic compounds, pullulan, and single-cell protein		Extraction yields of pectin and phenolic were 28.37% and 3.42%, respectively; high antioxidant capacity demonstrated by DPPH, 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS) and ferric ion reducing antioxidant potential (FRAP) tests	[56]
Orange peel, apple pomace, beet pomace, ginseng residue, and corn steep liquor	Carbon sources for Euglena gracilis cultivation for beta-glucan production	Extraction for 40 min at a temperature of 121 °C and a pressure of 1.3 atm	15 ± 1.1 g/L of glucose, sucrose, and fructose	[57]
Agriculture
Plasticized PLA loaded with spinach stems (SS), tomato pomace (TP), and cocoa shells (CS) up to 30 wt% of each vegetable residue	Biodegradable agricultural mulch films	Twin-screws extruder; compression molding	The material has low water solubility (1–15 wt%) and moisture content (1–3 wt%), with a water vapor permeability up to 3 × 10−10 g s−1 m−1 Pa−1, comparable to that of Mater-Bi	[58]
Vegetable wastes include rotten peels, stems of spinach, and scrapped portions of FVW	Organic fertilizer	Aerobic solid-state fermentation	Solid organic fertilizer leads to effective growth of Solanum lycopersicum and Capsicum annum crops	[59]
Tomato, Chinese cabbage (Brassica rapa L. ssp. pekinensis), kale (Brassica oleracea L. var. acephala DC.), apple (Malus pumila Mill.), banana (Musa nana Lour.), and lettuce (Lactuca sativa Linn.) waste	Liquid organic fertilizer	Hydrothermal treatment (temperature of 165 °C and retention time of 45 min) and biological treatments	Hydrolyzed liquid met the requirements of regulating fertilizing products	[60]
Cauliflower and cabbage wastes; banana peels	Biochar for soil amendment	Fifty grams of each feedstock are pyrolyzed at different temperatures (300, 400, 500 and 600 °C)	The biochar yield decreased as the pyrolysis temperature increased; pH of 7.62 ± 0.03; cation exchange capacity (CEC) in the range of 37 to 60 cmolc kg−1	[61]
Energy
Fruit and vegetable peels	Hydrogen and biochar	Fermentation/pyrolysis	H2 production for vegetable waste such as spinach, pea, and tomato of 1.61 ± 0.36, 6.93 ± 0.78, 5.34 ± 0.30 mmol, respectively, and fruit peels like banana and orange of 7.48 ± 0.53 and 1.03 ± 0.16 mmol, respectively; maximal chemical oxygen demand (COD) removal efficiency of 56.2 ± 4.6% for a mixture of pea, banana, tomato, and orange peels	[62]
Mixture of shredded FVW	Biogas	Three-stage hybrid AD system comprises a first-stage semi-anaerobic suspended-growth hydrolytic reactor, a second-stage strictly anaerobic down-flow hybrid acidogenic reactor, and a third-stage up-flow hybrid methanogenic reactor	A maximum CH4 yield of 484 mL/g CODremoved and a bio-H2 yield of 54.5 mL/g chemical oxygen demand (CODremoved) from the methanogenic stage; a maximum biogas yield of 781.8 mL/g CODremoved	[63]
FVW	Biohydrogen	Lactate-driven dark fermentation	H2 yield of 49.5 NmL/g volatile solids (VSfed) at a remarkable rate of 976.4 NmL H2/L-h was achieved at pH 7; 5% total solids (TS) and 1800 mg initial cell biomass concentrations (VSS)/L	[64]
	Methane	Anaerobic digestion process	258–367 NmL/gm volatile solids (VS) for fruit waste; 274–407 NmL/gm VS for vegetable waste	[65]
Water Purification
Fruit waste-derived activated carbon, (FWAC) and its binary metal oxide hybrid nanocomposite (Fe-Ni/FWAC)	Methylene blue removal	Carbonization in a muffle furnace at 400 °C for 2 h; precipitation and calcination	The adsorption capacities of the FWAC were 204.082, 217.391, and 312.5 mg/g, while those of the Fe-Ni/FWAC were found to be 256.41, 303.03, and 370.37 mg/g at 303, 313, and 323 K, respectively	[66]
Nutmeg fruit rind waste	Removal of hexavalent chromium (Cr(VI)) from real textile wastewater	Drying at 70 °C for 24 h; baking at 103 °C; powdering and sieving as the particle size was less than 250 µm	The adsorption capacity 9.35 mg/g at 0.1 g L−1 dosage sorbent and 10 mg L−1 concentration of sorbate	[67]
Banana peel waste incorporated into sodium alginate	Heavy metal removal from synthetic solutions	Drying in an oven at 140 °C for 4 h and two-stage pyrolysis in a calcination furnace; microbead formation in 1% sodium alginate	100% removal of Cr, Cu, Pb, and Zn	[68]
Walnut shell biowaste	Removal of paracetamol and methylene blue	Hydrothermal carbonization	Removal efficiencies of 99.3% for methylene blue and 60% for paracetamol	[69]

illustrates different solutions for reducing the amount of FVW by using innovative applications.

3.1. Value-Added Products

Figure 2 depicts some industrial applications of polyphenols, pectin, and carotenoids extracted from FVW.

3.1.1. Bioactive Compounds

Bioactive compounds include phenolics, carotenoids, flavoring (anthocyanins), chlorophylls, and betalains [70], with potential applications for people’s health, functional food, cosmetics, and pharmaceuticals [17,23,70,71]. Dhalaria et al. [72] evidenced the roles of antioxidants, nutraceuticals, and phenolic acids extracted from the waste of many fruits for their health benefits. Bioactive compounds isolated from FVW have been reported to be suitable for food emulsion stability, textural qualities, and food preservation due to their antimicrobial properties [36].

Bioactive compounds extracted from apple and carrot by-products have been effectively employed in the nutrition of post-weaning piglets. This use resulted in decreased oxidant and inflammatory processes, along with positive modulation of the microbiota. These compounds served as valuable alternatives to pharmaceutical ZnO and in-feed antibiotics [73]. Similar, citrus pulp and grape pomace were demonstrated as novel, cost-effective, and natural sources of cellulose, polyunsaturated fatty acids, and phytochemicals which are beneficial for the ruminant livestock industry [74]. These components possess nutritional value along with anthelmintic, antioxidant, and antimicrobial properties.

The mango seed kernel contains phenolic compounds that have shown antifungal properties against postharvest pathogens, which pose a major risk to the global food supply chain [75].

Natural pigments (colorants) extracted from vegetable waste are acknowledged for their numerous advantages, such as enhancing consumer acceptance; boosting the nutritional content of processed foods; and decreasing the use of synthetic dyes that can cause various side effects such as food allergies, respiratory, and cardiovascular problems [76]. Also, natural pigments incorporated into food products not only enhance the color of foods, but also increase their antioxidant capacity. This process helps to chelate free radicals from the body, providing health benefits when included in the diet. Additionally, it improves the aesthetic appeal for consumers, increases profitability for manufacturers, and benefits the environment by reducing industry biomass waste [77]. A variety of FVWs have been used for extraction of natural pigments [77,78,79]. For instance, apple peel, eggplant peel, banana peel, and grape skin have been exploited for the extraction of anthocyanins, which have shown high antioxidant capacity and are useful in health care.

Lombardelli et al. [80] investigated the recovery of betalains (in amount of 200 g) from unsold red beets using the enzyme-assisted extraction (EAE) method, which operates at a temperature of 25 °C and for a duration of 240 min.

An integrated method for the simultaneous extraction of betalains and chlorophylls pigments from red beet stems and leaves using reversible aqueous biphasic systems (ABSs) containing ionic liquids (ILs) was conducted by Rosa et al. [81]. The highest extraction yields achieved were 6.67 wt% for betalains and 1.82 wt% for chlorophylls (per unit weight of biomass).

A review by Phan et al. [82] highlighted the prevalence of natural dyes like anthocyanins, quinones, and carotenoids, extracted in crude form from FVW, and their use in small-scale applications. Carotenoids are primarily stored in the peels of edible and non-edible parts of vegetables and fruits such as tomatoes, oranges, grapes, and carrots, which are often discarded [5]. Carotenoids are used as natural colorants or for combating vitamin A deficiencies in children and pregnant woman [83].

Powders derived from vegetable waste could also serve as coloring and flavoring agents or as natural preservatives in the food industry, or could be used to reformulate processed foods to enhance their nutritional profiles [84].

Waste and by-products of plants (fruits, exotic fruits, vegetables and seeds) have been valorized as bioactive compounds [85] or incorporated in natural-derived matrices to obtain green composites with performant properties [86,87].

3.1.2. Biopolymers

FVW is among the most promising prospects in the next generation of raw materials for obtaining biopolymers because not only are they naturally biodegradable and produced using renewable resources, but they can also be edible, meeting nutritional needs for the expanding population. Examples of biopolymers extracted from FVW include pectin; carbohydrates such as starch, sugars, and dietary fibers; nanocellulose; and proteins. Biopolymers from FVW offer sustainable alternatives to synthetic materials, contributing to waste valorization and the development of new food ingredients [88,89], as well as eco-friendly products.

Pectin is a valuable polysaccharide extracted from the cell walls of many fruits, especially citrus peels and apple pomace, which possess interesting features such as retaining moisture, oil, and aroma barriers and diminishing the respiration rate and food oxidation in food packaging [90,91]. Other FVW investigated for pectin extraction are watermelon rind [92], banana and papaya peels [93], and beetroot and red cabbage powder, which are obtained as by-products [94]. Future prospects include the application of pectin in the pharmaceutical and paper industries as a foam or a plasticizer [95].

The prebiotics promoting the growth of useful bacteria in the colon have been identified as novel applications of compounds of fructooligosaccharide (FOS), a carbohydrate extracted from FVW [96]. In a same line, a review paper by Freire et al. [97] showed that Limosilactobacillus (L.) fermentum strains isolated from by-products of fruit processing have shown probiotic properties that promote host health benefits.

Many fruits and vegetables contain starch, a polysaccharide carbohydrate that can serve as a primary ingredient for bioplastic production [98]. Starch can be extracted from various wastes of apple, kiwifruit, annatto, pineapple, mango, tamarind, litchi, longan, loquat, jackfruit, avocado, and banana through various methods, such as grinding, washing, and enzymatic treatment [99].

Nanocellulose is a natural biopolymer that can be synthesized from several FVWs, such as nut biomass, kiwi pruning residues, pineapple leaf and peel, and carrot root and pomace [100]. Diverse applications of nanocellulose have been disclosed, including its use in various functional composite materials, aerogels, and drug-delivery systems. Abdullah et al. [55] extracted cellulose from the rachis of banana fruit (Musa sapientum L. cv. Mali-Ong) and converted it into carboxymethyl cellulose (CMC), resulting in its successful use as an innovative edible coating to retard the aging and extend the shelf life of strawberry fruit.

Proteins extracted from waste plants such as corn, wheat, soybean, peanuts, cottonseed, and rice remain suitable and acceptable for individuals adhering to a vegetarian diet [101].

Resent research on the use of FVW has described the isolation of unconventional vegetable protein (UVP) from leaves, roots, and stems using the enzyme-assisted extraction (EAE) method [102]. The principle involves the breakdown of cellulose, hemicellulose, and pectin components in plant cell walls by the enzyme or a complex enzyme mixture, thereby impairing cell wall integrity and aiding in the release of proteins from the plant matrix. At present, the use of EAE techniques in UVP remains in the experimental phase rather than the commercial expansion stage because of its high expenses. Furthermore, variable outcomes and intricate purification processes also pose challenges to industrial production [102].

Onion peels, orange peels, and pea pods have been investigated for the production of single-cell protein (SCP) using a specific strain of Lactobacillus sp. as an alternative to conventional protein sources [89]. A maximum SCP yield of 8.6 g was achieved. The nutritional properties revealed a protein content of 48.34 ± 0.05%, a fat content of 2.44 ± 0.2%, and a fiber content of 2.97 ± 0.16%.

The amphiphilic nature of soy protein enables the formation of a stable network through denaturation. According to de Oliveira et al. [103], soy protein can interact with carboxymethylcellulose (CMC), enhancing the mechanical strength and oxygen permeability of edible films.

3.2. Food and Food Packaging

According to the UN’s projection, the global population is expected to reach about 9.8 billion people by the year 2050, suggesting that food production will need to increase by around 50–60% to meet the growing demand [104]. Promising opportunities for reducing food waste, promoting sustainability, and enhancing food quality and safety can include the development of edible food packaging and oil.

Utilizing vegetable waste as a growth substrate for cultivating fresh earthworms with the aim of producing dried meal for both feed and food purposes can be considered a practical and sustainable solution [105]. In comparison to the conventional meal used as feed, earthworm meal exhibits a greater environmental impact, which can be attributed to its higher economic value and the increased electrical consumption during the freeze-drying process [105].

FVW from markets has become a rearing substrate for Hermetia illucens (BSF black soldier fly) [106]. Meal insects can be used as fish feed to increase the sustainability of aquaculture. Recently, Yu et al. [57] used orange peel and apple pomace as substrates for microbial growth.

Producing functional powders from a variety of vegetable wastes, including carrot, leek, celery, and cabbage sourced from both fresh products and ready-to-eat lines of the cooperative to be utilized as functional food ingredients, will contribute to a circular economy [16].

It has been demonstrated that the natural antioxidants derived from FVW can serve as edible oil alternatives to artificial antioxidants, which may have detrimental impacts on human health [107].

Pomelo peel-derived carbon dots synthesized through a hydrothermal method were introduced, up to 3 wt%, into a biopolymeric matrix based on fish scale gelatin/alginate dialdehyde [108]. This composite film showed significant antioxidant and antimicrobial effects.

An extension of the shelf-life of strawberries by 24 days was reported when a coating with pectin, pullulan, and phenolic compounds extracted from apple pomace was made [56]. By incorporating phenolic compounds, tocopherols, and carotenoids, which help to protect cells from oxidative damage and inflammation, into edible oil, manufacturers can enhance the stability and shelf life of food products, promoting sustainability and reduces environmental impact by utilizing FVW that would otherwise be discarded.

3.3. Agriculture

FVW subjected to the drying process as well as milling of particles with sizes less than 50 μm was used to achieve biofertilizers through melting technology [58]. Compared with traditional fertilizers, such biofertilizers offer numerous benefits, such as cost-effectiveness, greater nutrient availability, improved soil fertility, protection of crops from soil-borne pathogens, viable agricultural production, increased tolerance to biotic and abiotic stresses, stimulation of phytohormone production, better soil health, and significantly reduced environmental contamination [109].

The fabricated biocomposites based on PLA and 20 wt% spinach stems (SS) designed for mulch film fabrication reached nearly 40 wt% biodegradation after 6 months, and they provided micro- and macronutrients such as P, K, B, Ca, Cu, Fe, Mn, Mg, S, and Zn for plant growth [58].

Biochar derived from FVW has been used as a soil amendment, which assures the proper pH and CEC for the growing plant. This is another innovative application of FVW [61].

3.4. Energy

Improper landfill disposal of biodegradable FVW contributes substantially to climate change and leachate. This happens because the waste releases various greenhouse gases and particles into the atmosphere [28]. FVWs are valuable feedstocks for producing renewable hydrogen through fermentation [62] and dark fermentation (DF) processes [64], as well as biomethane [65,110], offering a potentially efficient method for generating renewable energy and, thus, decreasing the use of non-renewable energy sources.

A major challenge in sustainable bioenergy production and supply is feedstock availability. It has been proven that the anaerobic digestion (AD) of FVW alone is not adequate, as it can cause harmful levels of acidification due to the rapid generation and buildup of volatile fatty acids, which negatively affect methane production [111]. Instead, using FVW as a co-substrate in the aerobic digestion of municipal sewage sludge can positively impact the energetic self-sustainability of wastewater treatment facilities [111]. This approach increases the biodegradability of municipal sewage sludge and leverages the unused capacity of current infrastructure to improve both the amount and quality of the generated biogas.

It has been found that 20% and 30% of sludge contents can be considered as a source of microorganisms, and 80% and 75% of mixed FVW collected from the University of Texas at Arlington (UTA) Compost Center were introduced into a digestor for degradation in the absence of oxygen via enzymatic and bacterial activities. The digestor operated at 37 °C for about five months [110]. KOH was added to increase the pH value. The rate of methane production was 14.77 L of methane per kg of food waste.

3.5. Water Purification

Water purification of industrial and pharmaceutical contaminants [66,69,112,113] and soil remediation of heavy metals [114] demonstrate that the FVW consists of promising materials for these environmental applications. The binding capacities of biosorbents based on FVW depend on various factors, including their source, chemical structure, type of metal, modification technology used, and the process conditions applied to enhance their functionality [114]. It has been observed that the modified fibers extracted from vegetable waste lead to better results than unmodified fibers. Various treatments, such as physical, enzymatic, bacterial, and chemical modifications, can enhance fiber porosity and surface area, thus increasing the number of adsorption sites, favoring the retention of heavy metals on biosorbent surfaces [114]. The greatest challenge posed by biosorbents after chemical neutralization is their disposal method in landfills, which brings a potential risk of secondary contamination to the soil, groundwater, surface water, and air due to the leaching process.

3.6. Other Applications

Patil et al. [115] introduced 5% fruit vegetable waste powder into a composition based on acrylic polymer resin, glycerol, and polyvinyl acetate for the purpose of fabricating a semi-synthetic leather, which exhibited an abrasion resistance of 20,000 abrasion cycles.

An innovative application of fresh kitchen waste, such as vegetable and fruit peels, in the production of eco-enzymes was revealed by Benny et al. [116]. The fermentation of a mixture of sugar, citrus peels, and water in a ratio of 1:3:10 was carried out for three months at a temperature of 27 °C. This eco-enzyme offers multiple potential applications, including cleaning and disinfection for floors, utensils, gardening, wastewater treatment, and plant growth.

4. Extraction Methods

Conventional methods for extracting bioactive compounds, pectin, carotenoids, anthocyanins, antioxidants, phenols, flavonoids, proteins, polysaccharides, dietary fibers, or oils from FVW, such as Soxhlet extraction, maceration, and steam distillation, usually involve the use of hydroethanolic mixtures as solvents and are time-consuming [36,117]. Usually, the conventional extraction systems of valuable compounds from FVW involve the use of organic solvents and heating for prolonged periods of time, promoting the degradation of compounds of interest. This highlights the necessity of developing innovative techniques for retaining high-value-added molecules from FVW, such as ultrasound-assisted extraction (UAE), microwave-assisted extraction (MWAE), supercritical fluid extraction (SFE), pressurized hot water extraction (PHWE), pressurized liquid extraction (PLE), supercritical fluid extraction (SFE), pulsed electric field-assisted extraction (PEFAE), ohmic heating-assisted extraction (OHAE), and enzyme-assisted extraction (EAE) [5,18,19,20,21,25,118,119].

Table 2. Green methods applied for extraction of high-value compounds from FVW.

Extraction Method	Advantages	Barriers	Ref.
Ultrasound-assisted extraction (UAE)	- Safety risk - Simple operation - Lower solvent and energy consumption - Economic and environmental benefits - Removal of toxic and persistent pollutants from wastewater - Short processing time - Enhanced yield	- Large amounts of solvent, long time for extraction, and power - Inefficient for heat-liable and oxidation-sensitive bioactive compounds - Sudden temperature changes caused by the rapid cavitation phenomenon, which could produce free radicals, causing degradation issues	[5,18,19]
		A pretreatment step including blanching, drying, and milling is necessary	[118]
		A defatting step before drying and milling is necessary Direct contact of ultrasonic horns with the fruit and vegetables waste should be avoided during extraction
	Prevents the thermal degradation of heat-sensible bioactive components due to low-temperature extraction	Industrial application needs to be scaled up	[119]
Microwave-assisted extraction (MWAE)	- Rapid heating - Decreased thermal gradient - Enhanced extraction yield with low solvent volume	- Decrease in the selected bioactive molecules in the peel when the microwave power exceeds a certain threshold - Increase in technical difficulty - Initial installation charge - Safety protocols owing to the application of electromagnetic waves	[20]
Pressurized hot water extraction (PHWE)	- Energy demand to heat liquid water (25 °C to 250 °C or even 374 °C, 5 MPa) is almost three times less than that needed to vaporize the water to create steam - Non-toxic, safe, and cost-effective - Minimizes the environmental impact associated with the use of water in chemical processes - Water stands out as the safest and most cost-effective solvent option, with the ability to modify its properties through temperature variation	- Low purity of bioactive compounds - Risk of clogging inside the tubing during the extraction	[20,21,22,23]
Pressurized liquid extraction (PLE)	- High recovery (above 90%) of mainly flavonoids and anthocyanin - High purity of bioactive compounds - Can save time and reduce organic solvent	- Large initial investment charge - The content of analytes in the tested sample is limited to 10 g	[5,20,24]
Supercritical fluid extraction (SFE)	- It is scalable, cost-effective, and achieves recovery of used solvent and reduced solvent consumption - Low temperature and low pressure are necessary when the supercritical CO2 is used	The presence of humidity, coupled with contaminants, in laboratory-grade CO2 could impede the extraction process	[5,20]
Pulsed electric field-assisted extraction (PEFAE)	- High temperatures are not involved in the process - Low energy consumption - High efficiency - Low process costs	There is a need to adapt to each type of sample, as the process parameters are dependent on the electrical conductivity and texture of the raw material	[5]
Ohmic heating-assisted extraction (OHAE)	- Highly energy-efficient technology, which requires less processing time and allows for precise control of the temperature - It can reduce solvent use and the need for chemicals, shorten processing times, minimize waste generation, and reduce energy consumption	The selection of the most appropriate food waste streams is an essential step	[25,26]
Enzyme-assisted extraction (EAE)	- A higher-yielding method with lower energy expenditure - Environmentally friendly - Enhances the purity of final products	A high cost of enzymes relative to volume and a high dependence of enzymes on pH and temperature	[5,79]

illustrates the advantages and barriers encountered by various methods applied for the extraction of bioactive compounds from FVW.

4.1. Ultrasound-Assisted Extraction (UAE) and Microwave-Assisted Extraction (MWAE)

With the ultrasound-assisted extraction (UAE) method, acoustic waves generated in the solvent induce cavitation bubbles, which burst at the surface of the FVW, causing disruption to the cell wall. Furthermore, this disruption facilitates the release of phenolic bioactive compounds into the solvent. This technique is optimized by controlling the solvent concentrations, solvent-to-sample ratio, extraction time, temperature, frequency, and power [19].

UAE is applied as an environmentally friendly technology to recover valuable components from polysaccharides, pectin, essential oils, polyphenols, flavonoids, anthocyanins, lycopene, carotenoids, and related antioxidant extracts from FVW [18]. Furthermore, ultrasound extraction aids in the elimination of toxic and persistent pollutants, such as aromatics, surfactants, and dyes, from the wastewater produced during the extraction of biocompounds, which is achieved through the oxidative breakdown of these substances.

The extraction of phenolic compounds from mango peels by this technique in the presence of 50% aqueous ethanol solution, with a solid-to-liquid ratio of 1:30, a temperature of 45 °C, and 30% of ultrasound amplitude after a treatment time of 10 min, resulted in the highest phenolic content of 35.5 mg GAE/g and 92% antioxidant activity [120].

Compared to the UAE method, microwave-pretreated ultrasound-assisted extraction was found to release less CO₂ into the atmosphere and had a shorter extraction duration when isolating bioactive compounds from papaya fruit peel [121].

4.2. Supercritical Fluid Extraction (SFE)

Supercritical fluid extraction (SFE) is primarily employed for the extraction of volatile compounds. It involves utilizing supercritical fluids, such as CO₂ and H₂O, at or near their critical temperature and pressure to recover extracts from solid plant matrices [122]. For the optimal extraction of valuable compounds from fruit waste, pre-treatment steps such as microwave, ultrasound, and enzymatic pre-treatments for preparing the substrate for SFE are necessary [122].

In an experiment, 12 g of tomato peel and seed acquired from a ketchup plant in California was used for lycopene extraction using supercritical carbon dioxide (SC–CO₂) at 30 MPa and 80 °C for 4 h as well as the Soxhlet (hexane) method [123]. The bioaccessibility of lycopene in the extracts was 11.5% using the SC-CO₂ technique, compared with 3.5% with the conventional extraction method. This demonstrates the significant potential of the SC-CO₂ technique to enhance the health and wellness benefits of lycopene by improving its bioavailability due to the higher (Z)-lycopene content.

4.3. Pressurized Liquid Extraction (PLE) and Hot Water Extraction (PHWE)

When the efficiencies the of pressurized liquid extraction (PLE), Soxhlet, and maceration techniques were compared during isolation of bioactive compounds from passionfruit rinds, PLE was found to be the most economical at a temperature of 60 °C and 70% ethanol [124]. At this concentration of ethanol, a pipeline blockage was observed due to the gelatinization of pectin found in the passionfruit rinds. A step to extract pectin was proposed.

Pressurized hot water extraction (PHWE) employs only liquid water as an extraction solvent at temperatures exceeding the atmospheric boiling point of water (100 °C, 0.1 MPa), but remaining below its critical point (374 °C, 22.1 MPa) [22]. Pagano et al. [125] found PHWE to be an excellent and environmentally friendly method for reclaiming bioactive compounds from artichoke agro-industrial residues, demonstrating recoveries of 93–105% and 90–105% for caffeoylquinic acids (CQAs) and flavone glycosides (FLs), respectively.

4.4. Ohmic Heating-Assisted Extraction (OHAE)

Ohmic heating is a sustainable and green technology based on the utilization of electric energy, which is converted into thermal energy to heat the substance [25]. Microwave-assisted extraction (MWAE) and ohmic heating-assisted extraction (OHAE) were concurrently employed for phenolic extraction from avocado leaves [126]. Although both methods resulted in high values for total phenolic content (TPC), 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging, and antidiabetic activities, the OHAE approach was found to be superior to MWAE. The optimal OHAE conditions were a 9.38 V/cm voltage gradient, a 6 min extraction time, a temperature of 60 °C, 5 min, and 1 g dried leaf/100 mL water [126].

4.5. Enzyme-Assisted Extraction (EAE)

The need for targeted extraction by specific bacteria, fungi, nematodes, or earthworms arises from their unique abilities to break down complex organic matter and extract valuable molecules, thus efficiently controlling postharvest fungal infection.

To evaluate the extraction performance of oil and polyphenols from raspberry pomace through EAE compared with conventional extraction methods, four proteases—Alcalase 2.4 L, a thermostable alkaline protease from Bacillus licheniformis; Neutrase from Bacillus amyloliquefaciens; pepsin from porcine gastric mucosa; and papain, a cysteine protease from Carica papaya latex—were used simultaneously [127]. Three carbohydrates were also employed: cellulase from Trichoderma reesei, pectinase from Aspergillus aculeatus, and xylanase from Trichoderma viride. The recovery rates of polyphenols and antioxidant activity were 48% and 25% higher for EAE, respectively, compared to extraction with a methanol/acetone/water mixture.

5. Techno-Economic Assessment

Within the scope of the circular economy, valorization of fruit and vegetable waste would be advantageous for reducing the amount of these wastes, recycling them into innovative value-added materials, and addressing environmental problems. Due to the high moisture content of this waste, which affects its perishability, quality, storage, and transportation, a proposed strategy involves reducing the water content through pressing [15]. Consequently, various drying methods have been explored as means to mitigate or prevent potential waste, such as sun-drying, solar drying, convective air-drying, hybrid drying, spray-drying, drum-drying, vacuum-drying, freeze-drying, and refractance window drying for food valorization [4]. Drying techniques have been effectively utilized to convert different types of fermented vegetable residues into nutrient-dense flours. For example, fermented grape residue has been processed into a fortified flour that can be used to boost the nutrient content of various foods [128]. Likewise, this approach can be applied to other vegetable residues, such as beetroot, red pepper, and carrot pomace, transforming them into valuable components for food enhancement [129].

Soltan et al. [62] performed fermentation and pyrolysis experiments with fruit and vegetable peels in 200 mL batch reactors with working volumes of 150 mL and found a maximum net profit for multi-fermentation and pyrolysis processes of USD 4.11 per kg _feedstock for a mixture of 25% pea, 25% tomato, 25% banana, and 25% orange, and USD 4.13 per kg _feedstock for the digestate of orange, respectively.

Fermentation and freeze-drying processes are old approaches to avoid spoiling and the need for refrigeration storage for pickled dried vegetables [130]. These methods contribute to reducing vegetable waste as well as the prolonging the shelf lives of these vegetables.

At an industrial scale, lettuce waste has been efficiently converted using innovative technologies into antioxidant extract through the implementation of a decision support system (DSS) [131]. The structure of the DSS model employed for the valorization of lettuce waste comprised three main phases: an investigative phase, incorporating tools such as material flow analysis (MFA) and techno-economic and profitability assessments; a design phase; and scenario analysis. It was demonstrated that, through the application of homogenization, supercritical CO₂ drying, and bioactive extraction processes on 40% of lettuce waste, a reduction in greenhouse gas emissions (CO_2RED) of 124.4 tons CO₂ per year and a primary energy saving (PES) of 55.1 tons per year were achieved [131]. These innovative processes maximized the economic performances of lettuce waste valorization compared with traditional approaches, such as anaerobic digestion, composting, and carbonization.

Green extraction methods applied to extract bioactive compounds from fruit and vegetable waste processing address market demands and align with the global goals of Agenda 2030 concerning the sustainability of food systems. They boast shorter extraction times for biologically active compounds, resulting in lower energy consumption. Moreover, they eliminate the need for petrochemical solvents, greatly reducing environmental harm and eliminating the issue of disposing of toxic waste afterward.

A life cycle assessment (LCA) demonstrated that, in the EU-28, for an annual production exceeding 0.1 million metric tons (Mt), the quantities of anthocyanins, quinones, and carotenoids were 56 kg/t of dry weight, 18 kg/t of dry weight, and 593 kg/t of dry weight, respectively [5].

Joensuu et al. [132] proposed a holistic approach using carrot peel compounds, such as vitamins A and E, to nourish the skin by producing artisan soap. The profit margin for a 100 g soap piece was estimated to be approximately EUR 5.5, with a selling price of EUR 9 per piece (compared to similar products in the market).

However, some limitations need still to be resolved in this area. Most of the eco-friendly technologies proposed for the recovery of bioactive compounds such as polyphenols, flavonoids, colorants, pectin, and essential oils are still either at the laboratory scale or the pilot scale, showing a moderate technology readiness level (TRL 5 to 7) [133]. The surface areas of large-scale bioreactors, flow dynamics, and purification expenses serve as limitations in designing a scale-up process for enhanced productivity [75].

Managing FVW in the food processing industry is a significant challenge that involves substantial capital investment, primarily due to the transportation costs. Transporting waste to disposal or processing facilities is costly, largely due to the bulk and weight of the waste, which often contains a high water content [52].

Special attention should be paid to the education of consumers and businesses on reducing FVW, and regulatory bodies and policymakers should encourage more sustainable waste valorization practices in circular economy models.

Overall, the valorization of fruit and vegetable waste can open great market opportunities if it is efficiently exploited.

6. Conclusions and Perspectives

Fruit and vegetable waste is highly perishable, leading to quick degradation and potential environmental hazards if it is not managed promptly. Improper disposal can lead to emissions of methane, a potent greenhouse gas, and other environmental issues such as water pollution.

The primary sources of fruit and vegetable waste arise from industrial processing, which can be exploited for the valorization of bioactive functional molecules.

As compared with traditional extraction methods of valuable compounds from FVW technologies, the green extraction methods investigated herein are promising for the separation of various bioactive compounds (for example, pectin, carotenoids, polyphenols, proteins) from FVW, in line with the circularity of resources and sustainability. Other potential solutions for processing and valorization of FVW involve converting them into biogas and organic fertilizers, developing animal feed and innovative food packaging that extends the shelf lives of fruits and vegetables, and creating high-performance materials with applications in construction and water purification.

However, more research is needed to optimize the extraction process parameters, as they directly impact the final quality of bioactive compounds. In addition, the industry should address more sustainable and cost-effective FVW management practices. Some data have shown promise, and exploitation is limited with a low amount of raw material.

Implementing the possible solutions described in this paper would not only reduce environmental impact, but would also open up new economic opportunities through the valorization of FVW.