**1. Introduction**

Food waste (FW) is already acknowledged as a major global issue that threatens the long-term viability of the food supply chain [1]. FW in the European Union is estimated to be at 89 million tonnes per year. This is expected to rise by 40% in the next four years [2]. According to the Food and Agriculture Organization of the United Nations [3] and the International Food Policy Research Institute [4] estimated one-third of food produced globally, or 1.3 billion tonnes, is thrown away each year. Over half is generated at the final consumption stage in food services (e.g., restaurants, school canteens) and households [5,6]. The Sustainable Development Goals (SDGs) recognize the importance of this issue. By 2030, SDG Target 12.3 calls for the reduction by half of the per-capita global FW at retail

**Citation:** Mohd Basri, M.S.; Abdul Karim Shah, N.N.; Sulaiman, A.; Mohamed Amin Tawakkal, I.S.; Mohd Nor, M.Z.; Ariffin, S.H.; Abdul Ghani, N.H.; Mohd Salleh, F.S. Progress in the Valorization of Fruit and Vegetable Wastes: Active Packaging, Biocomposites, By-Products, and Innovative Technologies Used for Bioactive Compound Extraction. *Polymers* **2021**, *13*, 3503. https://doi.org/10.3390/ polym13203503

Academic Editors: Domenico Acierno and Antonella Patti

Received: 15 September 2021 Accepted: 8 October 2021 Published: 12 October 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and consumer levels, diminishing food losses in production and supply chains, including post-harvest losses [7]. The call was agreed to by all 193 UN member states, which comes as no surprise.

Every year, more than 1748 million tonnes of fruit and vegetable waste (FVW) are produced worldwide [8]. Due to rapid economic growth, FW generation in Asia is predicted to increase by 278 to 416 million tonnes [9], resulting in an increase in world anthropogenic (carbon) emissions [10]. Malaysia is expected to produce 6.7 million tonnes of FW per year by 2020 [11]. In the EU, households account for over half of all FW [12]. Around 12 percent of the total EU food production is wasted in the United Kingdom [13], while the United States recorded the highest FW rates per capita at 278 kg [14]. FVW is a major component of FW, particularly in industrialized countries [15,16]. Figure 1 shows the FW management practices and the novel or emerging valorization approaches [17].

**Figure 1.** Food waste management practices and the novel or emerging valorization methods. Reproduced with permission from Ref. [17]. Copyright 2021 Elsevier.

According to Fabi et al. [18], median losses are estimated to be higher than 10 percent in Africa and Latin America for fruits and vegetables. In comparison, they range between 4 and 7 percent in Europe and North America. Figure 2 depicts the amounts of fruits produced globally, including 124.73 million metric tonnes (MMT) of citrus, 114.08 MMT of bananas, 84.63 MMT of apples, 74.49 MMT of grapes, 45.22 MMT of mangoes, mangosteens, and guavas, and 25.43 MMT of pineapples. Besides the production of potatoes, which was recorded at a considerable 3820.00 MMT, other vegetables are also shown in Figure 2, including tomatoes (171.00 MMT), cabbages and other brassicas (71.77 MMT), carrots and turnips (38.83 MMT), cauliflowers, broccolis (24.17 MMT), and peas (17.42 MMT) [19]. Due to losses in sales and transportation costs, such wastage, including subsequent generation of FVW, raises market operation costs, resulting in higher inflation.

The dairy, meat, fishery, and seafood processing sectors are the primary contributors of trash from animal wastes. Numerous types of residues can be identified from vegetable wastes, including grains, roots and tubers, oil crops and pulses, and fruits and vegetables, depending on the source [20]. Typically, FVW is characterized by a high water content and a high concentration of biodegradable organic substances (e.g., carbohydrates, lipids, and organic acids) [21]. As a result of these varied organic wastes, typical solid waste management systems (such as landfills and incineration) may cause serious environmental impacts, such as the discharge of leachate and the production of greenhouse gases [22].

**Figure 2.** A significant amount of fruits and vegetables (in MMT) are produced globally.

FVW is generated in large quantities in open markets, but little information is available on the actual volume of waste generated. Since significant amounts of waste are produced by the fruit and vegetable value chains, biorefinery concepts have been developed to valorize these wastes [23]. Waste from the FVW can be reused or recycled, which is more environmentally beneficial than disposal through open dumpsites or incineration [24]. Wholesale marketplaces, supermarkets, and agricultural activities are the traditional sources of FVW. Fruits and vegetables produce FVW at every stage of the chain, including production, transportation, storage, distribution, and consumption [25]. The valorization of FVW and potential products are shown in Figure 3.

**Figure 3.** Valorization of FVW and potential products.

There are several reviews on the applications of fruit and vegetable wastes. A review conducted by Bayram et al. [26] focused on the potential applications of fruit- and vegetablebased by-products such as biopolymers, biocomposites, active or intelligent packaging, and edible films and coatings. The authors discussed the advantages, disadvantages, and applications of these by-products as well. Kadzi´nska et al. [27] reviewed the role of specific chemical compounds found in fruits and vegetables, particularly in designing the physicochemical and functional properties of edible packaging materials. The advantages and disadvantages of using fruit and vegetables as components in matrix-forming solutions

and their potential applications, future trends, and issues to consider when commercializing these products were discussed within the context of sustainable development.

In the fruit and vegetable processing industry, by-products such as peels, seeds, and shells are produced in large quantities [28,29]. These by-products contain high concentration of bioactive components such as antioxidants (polyphenols and dietary fibers), pigments and flavor compounds, proteins, essential oils, enzymes, and dietary fibers [30]. Coman et al. [31] looked at the bioactive potential of fruit and vegetable by-products and how they could be used in the food industry (functional foods) or the health sector (nutraceuticals). Several applications of food vegetable waste incorporated into the meat and its derivatives were reviewed by Calderón-Oliver and López-Hernández [32]. The peels and seeds of fruits, such as grapes, pomegranates, avocados, and citrus, are the most commonly used vegetable by-products because they help inhibit oxidation (lipid and protein) and the growth of pathogenic and deteriorating bacteria in the food supply. Adding these by-products to meat products can sometimes improve the quality of the product while also extending its shelf life. In food processing of waste and biomass by-products, pectin is one of the most common constituents; therefore, improving pectin extraction and recovery is critical to completely valorize these significant feedstock resources [29]. As a result, it is crucial to investigate the composting process of FW, particularly FVW, within this framework.

Given the potential for the valorization of such organic waste, research on the proper use of waste materials obtained from horticulture commodities may generate sustainable development initiatives to minimize environmental problems [19]. Significant products can be produced from these organic wastes, including active packaging, biopolymers, biocomposites, and other by-products. This review also explores the innovative technology (thermal and non-thermal) for bioactive compounds extraction based on the natural resources from fruit and vegetable losses and waste.

#### **2. Active Packaging and pH Indicator Film**

#### *2.1. Active Packaging*

The integration of additives into the packaging system to maintain or enhance the quality of food products and shelf life is referred to as active packaging [33]. Active packaging concepts include moisture absorbers, gas scavengers, carbon dioxide emitters, antioxidant, and antimicrobial-releasing and-containing systems. For example, the addition of antimicrobial additives into the packaging materials can minimize the risk of food spoilage and contamination from microorganisms [34]. Several additives have been successfully incorporated into packaging materials, including organic acids and their salts, bacteriocins, enzymes, chelators, and a range of plant extracts [35–37]. Due to concern about the health hazards of synthetic additives, researchers have been implementing various natural plant extracts to the biopolymers as active components [38]. In addition, active ingredients from FVW have added another level to active packaging systems as a solution for reducing FW. Several studies have focused on utilizing these wastes to make active films and investigated their properties, as summarized in Table 1.

For example, Luchese et al. [39] added blueberry pomace into cassava starch films. The aromatic compounds in blueberry pomace improved the light barrier properties, indicating the films' ability to preserve food against UV lights. At the same time, the films were structurally stable when immersed in water for more than 24 h. The authors suggested the feasibility of the films for packaging aqueous food products.

In another study, fiber and ethanolic extracts from blueberry juice from processing waste were used to make active films from gelatin capsules wastes [40]. The study found that the films with fiber showed reduced tensile strength and increased water vapor permeability with improved light barrier activity against UV light, which effectively reduced the lipid oxidation of sunflower oil. Both films showed significant decreases in light transmission and stability in antioxidant activity over 28 days.


**Table 1.** Utilization of fruit and vegetable wastes in food packaging systems.

Ali et al. [34] utilized pomegranate peel as a filler and an antimicrobial agent in developing films with hydroxypropyl high-amylose starch. The results showed that the tensile properties of the films improved and the pomegranate peel inhibited the growth of both Gram-positive (*S. aureus*) and Gram-negative (*Salmonella*) bacteria. On the other hand, research done by dos Santos Caetano et al. [47] produced biodegradable films based on minimally processed pumpkin residue extract (PRE) (0 to 6%) incorporated with cassava starch, glycerol, and oregano essential oil (OEO) (0 to 2%). The addition of pumpkin residue is crucial, since it provides opacity to the films, but it was not effective in improving the antioxidant and antimicrobial properties.

Other researchers also used pomegranate peel as an antibacterial additive with sodium caseinate powder [48]. The peel resulted in a decrease in tensile strength and increased water vapor permeability (WVP). The films showed profound growth inhibition for Grampositive (*S. aureus*) rather than Gram-negative (*E. coli*) bacteria. Shukor et al. [41] prepared tapioca-starch-based films incorporating thymol, jackfruit skin, and straw. Improvement in mechanical and barrier properties was observed for the films. The incorporation of skin and thymol retarded bacterial and fungal growth due to the antimicrobial activity of the films.

*2.2. pH Indicator Films*

Researchers are currently attracted to intelligent packaging systems, whereby active ingredients such as dyes and pigments are added into the film as pH indicators to trace and monitor food freshness throughout the storage period. This is related to the interactions between the food and its environment and the packaging material [49]. Table 2 shows some of the fruit and vegetables used as pH indicators in packaging films.

**Table 2.** Utilization of fruit and vegetable wastes as pH indicator in packaging films.


The colorimetric pH indicator films display apparent color changes with alterations of the food pH due to food deterioration and extrinsic environmental changes [53]; thus, from these color changes, the consumers receive authentic information regarding the food's quality and its edibility, which is not achievable from the expiry date written on the package. As a pH indicator dye replacing synthetic pigments, natural dyes are now used mainly in biodegradable packaging materials [54,55]. Interestingly, the natural dyes and pigments from waste and by-products from fruits and vegetables have also been implemented as pH sensing dyes into packaging films to ensure food safety [56].

A study was conducted by developing colorimetric indicator film from mulberry based on gelatin and polyvinyl alcohol (PVA), whereby anthocyanin extract from the residue of mulberry processing was incorporated. This study showed that the mechanical properties were improved and visible color changes were shown when monitoring fish spoilage [57]. In addition, an earlier study produced films from cassava starch and blueberry residue

that were rich in anthocyanin. Insertion of blueberry residue produced less compact films with high oxygen permeability. The films exhibited visual color changes in the pH range of 2 to 12 [56]. Luchese et al. [50] developed biodegradable pH indicator films based on cassava starch, utilizing the blueberry residue obtained after juice processing. The researchers deliberated on two different particle sizes for the blueberry residue powder for film preparation. They found that the films with smaller particles were more uniform and homogenous in appearance and the color change with pH was more intense than films with large particles.

In addition, the tensile strength of the films decreased whereas elongation increased; simultaneously, the water vapor permeability of the films increased due to the presence of the particles and their heterogeneity. In another study, blueberry agro-industrial waste, a co-product from juice processing, was successfully used to develop pH indicator films based on corn starch [51]. The films changed their colors in response to different pH, and the color changes were visually perceptible to the human eye.

Black chokeberry (*Aronia melanocarpa*) pomace extract (a residue material after juice pressing) was chosen as a pH sensing dye of chitosan films. The addition of pomace extract reduced the solubility and swelling of chitosan, and these indicator films maintained integrity in acidic pH environments [58]. Sohany et al. [52] utilized sweet potato peel powder (SPP) as a filler to develop sweet potato starch (SPS)-based pH indicator films incorporating purple sweet potato anthocyanin. The films with peel exhibited reduced tensile strength with higher swelling and WVP values. Visually, the films were dark maroon and changed their color in response to various pH buffers, as shown in Figure 4. The films successfully indicated chicken freshness by changing their color with changes in pH of the deteriorated chicken.

**Figure 4.** Visual appearance of SPS and SPS/SPP films at CA of 0, 1, 1.5, and 2%. Reproduced with permission from Ref. [52]. Copyright 2021 Taylor & Francis.

#### **3. Biocomposites**

Several biocomposites products are produced from agro-industrial wastes whereby the fibs, proteins, carbohydrates, organic acids, and oils are extracted from the wastes, followed by fermentation and enzymatic processing [59]. Biocomposites produced from natural fillers/fibers and biodegradable plastics are examples of biodegradable materials. Starch can be obtained through the extraction process [60], whereas polylactic acid (PLA), polybutylene succinate (PBS), and polyethylene (PE) are produced by fermenting, chemical processing, and polymerizing their monomers [61,62]. The polyhydroxyalkanoate (PHAs)

are synthesized by bacterial fermentation [63]. The carboxymethyl cellulose (CMC) can be produced via etherification [64]. Starches are partially modified to make biopolymer; PLA is widely used in food packaging; PHAs used for water-resistant packaging and injection molding [65].

Further, the peel/skin, seeds, and pomace of fruits and vegetables contain fibers used as filler in the film matrix to improve the biopolymer properties [39,52]. Pectin is another compound extracted from wastes and used in the polymeric matrix [35]. Also, peel and seeds contain more phenolic compounds than their fleshy parts [26]. For example, mango peel has a high level of phenolic content compared to its flesh [66]. The presence of phytochemicals and bioactive compounds provide preservative effects such as antimicrobial activity with anti-inflammatory and antioxidant attributes [67]. The blending of additives to polymers is found to improve the mechanical and barrier properties. Depending on the presence of additives, the polymers also function as pH indicators (sensor) or antimicrobial films [34,39]. Moreover, abundant organic wastes are available in skin and pulp, including citrus fruits such as orange, grapefruit, pineapple, mandarin/tangerine, lemon, and lime; seed waste from mango, grape, and pumpkin; skin from potato, sweet potato, jackfruit, pomegranate, and banana [65]. The utilization of these wastes into packing materials offers an essential alternative to conventional plastic packaging and contributes to a sustainable environment.

Despite substantial research on natural fiber composites, little is known about incorporating FW into biocomposites. Most biocomposites from FW research have focused on the biomass such as olive, pineapple, and banana. The mechanical and thermal properties of composites made from these biomasses and from other fruits are thoroughly investigated. Fiber treatment, type of polymer matrix, amount of fiber, compatibilizer, and process techniques have significant impact on the properties of biocomposites. Table 3 summarizes the residues from fruits and vegetable waste used in the production of biocomposites.


**Table 3.** Residues derived from fruit and vegetable waste.

#### *3.1. Lignocellulosic Fiber*

Fibers extracted from agricultural residues such as fruit and vegetable waste, woodland residues, and farming deposits are rich in cellulose, hemicellulose, lignin and are termed lignocellulose. These lignocellulosic fibers are obtained from biosources such as bast, foliage, fruit, kernel, timber, farmed excess, lawn, etc. Natural fibers possess comparable or even better mechanical properties like glass or aramid fibers [82]. Nanocomposite films based on lemon waste, 3% cellulose nanofiber (CNF), and 3% savory essential oil (SEO) are fabricated and are shown to enhance the barrier and mechanical properties. Film from lemon waste showed antibacterial properties against five foodborne pathogens [83].

Szyma´nska-Chargot et al. [36] evaluated the mechanical, hydrophilic, thermal, and antibacterial properties of nanocomposite made of PLA and nanocellulose. The nanocellulose is a carrot CCNF isolated from carrot pomace modified with silver nanoparticles. The nanocellulose modified with metal nanoparticles at a concentration of 0.25 and 2 mM was prepared earlier before combining with PLA. Composite containing CCNF with 2 mM of AgNPs showed the most significant improvement in mechanical properties. The degradation temperature was lower for PLA with CCNF/AgNPs, and this addition also increased hydrophilicity. The addition also improved transmission rates of oxygen, nitrogen, and carbon dioxide. It also acquired antibacterial function against *Escherichia coli* and *Bacillus cereus*, suggesting the lack of migration of nanoparticles from the composite.

Mohd Nordin et al. [68] studied the effect of freeze-dried durian skin nanofiber on the physical properties of PLA biocomposites. Durian skin nanofiber (DSNF) was developed using a freeze-drying (FD) process from durian skin fiber (DSF), and cinnamon essential oil was added as a plasticizer for PLA biocomposites. The tensile strength of these composites showed significant changes in the presence of DSF and DSNF in PLA.

Fibers from bananas have the potential to be incorporated into sound insulation composites. Singh and Mukhopadhyay [69] studied the effect of hybridization on sound insulation of coir-banana-PE hybrid biocomposites. These were prepared as shown in Figure 5 with chopped and randomly oriented coir and banana fibers. PP was used as a matrix, and a compression molding technique for composite fabrication. Hybrid and nonhybrid composites from coir and banana fibers were prepared at total fiber loading of 5, 10, 15, 20, and 25% by volume. The ratio of both fibers in hybrid biocomposites was maintained at 1:1. It was found that an increase in fiber loading considerably improved sound insulation up to a certain limit. The difference in transmission loss at the minimum and maximum fiber loadings for nonhybrid was higher for the finer banana fibers.

**Figure 5.** The process includes (**a**) fiber treatment followed by layering of fibers to form (**b**) fibrous bed, composite sample preparation through (**c**) compression molding, and (**d**) final composite samples. Reproduced with permission from Ref. [69]. Copyright 2021 Taylor & Francis.

Anuar et al. [70] developed durian skin fiber (DSF)-reinforced PLA biocomposites with the addition of epoxidized palm oil (EPO). The amount of energy required for the production of the biocomposites was studied. The results showed that the PLA/DSF/EPO biocomposites had lower negative impacts as compared to the PLA/DSF biocomposites because the EPO improved the workability and processability of the biocomposites. They concluded that the plasticized PLA/DSF biocomposites could be potential biodegradable food packaging materials, as they have acceptable properties and produce no waste.

Gisan et al. [71] investigated the tensile, water absorption, and biodegradation properties of PLA/durian husk fiber (DHF) biofilms. The PLA/DHF biofilms with different DHF contents (0, 5, 10, 15, and 20 wt.%) were prepared via a simple solvent casting method. The results revealed that the tensile strength and modulus of elasticity of the biofilms increased with increasing DHF content from 5 wt.% to 10 wt.%. The tensile strength and modulus of elasticity of the PLA/DHF biofilms decreased compared to the neat PLA film due to the plasticized effect in the biofilms; however, the enzymatic degradation with α-amylase and the water absorption properties of the PLA/DHF biofilms increased with the DHF content.

Sea mango (SM) fiber was used as a filler in PP polymer biocomposites. Ong et al. [72] investigated the flexural and thermal properties of 5 to 25 weight percentages of SM added into the PP. The results showed an improvement in the flexural strength and stiffness when the SM content increased. The thermal stability and degree of crystallinity results were positive when a compatibilizer (such as PP-g-MA) was incorporated into the biocomposites.

#### *3.2. Extract*

Several natural extracts have been used as active additives to develop antioxidantenriched films for food packaging applications. Natural antioxidants of plant extracts (PE) derived from various non-edible portions of fruit and vegetable by-products, such as peels, stones, and seed extracts, often contain a high amount of phenolic substances [84] and have been used as active ingredients in the manufacture of active films.

The valorization of fruit pomace (chokeberry, blackcurrant, apple, and raspberry pomace) in biocomposites was achieved by Zelazi´nski [ ˙ 73]. The mechanical and physicochemical characteristics were studied. The results showed that adding 30% chokeberry, apple, raspberry, and currant pomace substantially contributed to the improvements in flexural strength (between 11.1 and 12.3 MPa) and the increase of the water contact angle of the surface by 40%.

Tanwar et al. [74] investigated the characteristics of PVA starch incorporated with coconut shell extract and sepiolite clay as an antioxidant film for active food packaging applications. An active antioxidant film was fabricated using polyvinyl alcohol (PVA) and corn starch (ST) and incorporated with 3, 5, 10, or 20% (*v*/*v*) coconut shell extract (CSE) and sepiolite clay (SP) for the first time. It was found that the addition of CSE to films enhanced their antioxidant activity properties by up to 80%. Further, increasing the amount of CSE resulted in color changes in the active films and improved their thermal properties.

Rangaraj et al. [75] investigated the effects of date fruit syrup waste extract (DSWE) on the physical properties of gelatin films. The results showed that the loading of DSWE did not affect the thickness of the material. The moisture content and water solubility, on the other hand, increased with an increase in DSWE from 5 to 25 wt.%. PE/sour cherry shell powder biocomposite was investigated as a potential food packaging by Farhadi and Javanmard [80]. The addition of 2.5% sour cherry shell increased the elastic modulus, tensile strength, and mechanical properties of the composite. The increased sour cherry shell powder loading from 2.5 to 7.5 wt.% increased the moisture absorption and water vapor transmission.

Grapefruit and pomelo are commonly consumed fruits with higher levels of essential oils in the peels than other fruits [85]. Previous studies have reported that 10% extract of *Citrus paradisi* (grapefruit) peel obtained by microwave-assisted extraction (MAE) incorporated with multilayer low-density polyethylene (LDPE)/polyethylene terephthalate (PET) showed high antioxidant levels and acts as a free radical scavenger [86]. Pumpkins seeds

and peels are the waste types generated from the processing industry, with high potential for utilization as biodegradable films. Defatted pumpkin seeds (DPS) and pumpkins peels (PP) with glycerol and lecithin are produced in the process. The films exhibited the highest tensile strength values (1401 ± 5.4 kPa) and good elongation (9.74 ± 0.46%). The properties of the films were improved using ultrasound treatment. Generally, waste from the pumpkin processing industry is successfully developed as biodegradable films [87].

### *3.3. Powder and Husk*

The incorporation of pomegranate (PMG), papaya (PPY), and jackfruit (JF) peel into gelatin/PE bilayer films led to significant increases in thickness, opacity, and moisture content (*p* < 0.05) but reduced film solubility in water. Films incorporated with pomegranate (PMG) exhibited high antimicrobial and antioxidant properties [88].

Mardijanti et al. [76] examined the material characteristics of cocopith and evaluated the potential of a mycelium-based biocomposite as an insulator. Dry cocopith, which is the residue from the coconut coir milling industry, was mixed with wood powder, pollard (bran), lime, tapioca, and Ganoderma mushroom seeds and then put into baglogs. The solid baglogs were then removed from the molds, dried, and compacted to the desired size using a hot press. The potential as an insulator was validated via a thermal conductivity test at temperatures of 13 to 40 ◦C. The test showed a thermal conductivity value range of 0.0887241 to 0.002964 W/mK. A value ranges of 0.01 to 1.00 W/mK is recommended for thermal conductivity insulators.

A study on the enhancement of the mechanical behavior of a PLA matrix using tamarind and date seed micro fillers was conducted by Nagarjun et al. [77]. The composites were manufactured using the compression molding technique. The tensile results showed that the seed filler reinforcement significantly improved the tensile strength of the PLA matrix. The maximum tensile strength values achieved with TI/PLA and PD/PLA were 72.42 MPa and 61.39 MPa, respectively. The particulate reinforcements of both tamarind and date almost doubled the flexural and impact strengths of the PLA matrix. Moreover, the date seed powder-incorporated composites showed a 34.68% improvement in microhardness. The uniform dispersion of the filler was evident in TI/PLA and PD/PLA with 2 wt.% filler, which contributed to their better tensile strength. Conversely, increasing the filler content to 4 wt.% resulted in agglomeration of the fillers and subsequently contributed to the low mechanical strength of the composites.

Reinaldo et al. [78] investigated the effects of grape and acerola residues on the antioxidant, physicochemical, and mechanical properties of cassava starch biocomposites. Various concentrations of grape skin (Gr) and acerola (Ac) residues (0.1, 1.0, 5.0, and 10.0 wt.%) were prepared using extrusion and injection molding processes. The large size distribution of cassava starch may favor the plasticization stage to obtain TPS compared to the different types of starch obtained from other plant sources. The results showed that the addition of grape skins and acerola residues to the cassava thermoplastic starch resulted in better antioxidant characteristics. The addition of grape residue in TPS resulted in decreased in elongation at the break compared with pure TPS, which was more significant with higher concentrations of grape residue (5.0 and 10.0 wt.% of Gr). The similar mechanical behavior was recorded by Gutiérrez et al. [89] and Deng and Zhao [90].

Marzuki et al. [79] studied the effects of jackfruit skin powder (JSP) and fiber bleaching treatment in PLA composites incorporating thymol. The insertion of 30 wt.% jackfruits fibers gave the best tensile performance. The elongation at the break decreased with increased fiber loading, regardless of treatment, but no significant changes from 10 to 30 wt.% loading of powder were observed. All JSP or bleached jackfruit skin fiber (BJSP) composites showed higher tensile modulus than pure PLA, and the results were in agreement with Suradi et al. [91]. The SEM micrographs in Figure 6 show the fiber surface differences between unbleached JSP and BJSP, respectively. Following bleaching treatment, a rougher fiber surface is shown, indicating effective removal of the non-cellulosic components' potential for good mechanical fiber locking with the matrix.

**Figure 6.** Scanning electron microscopy (SEM) micrographs of (**a**) unbleached jackfruit skin powder (JSP) and (**b**) bleached jackfruit skin powder (BJSP). Reproduced with permission from Ref. [79]. Copyright 2021 Elsevier.

Torres et al. [81] evaluated the influence of chestnut husk content on the mechanical properties of novel starch/chestnut husk biocomposites. This was developed by incorporating 2.5, 5, and 7.5 wt.% chestnut husks via an extrusion molding procedure. The results showed that the pure starch samples had an average elastic modulus of 3.30 MPa, while starch samples with 7.5 wt.% chestnut husks displayed an average elastic modulus of 4.85 MPa. The ultimate tensile strength value was independent of the chestnut husk content, while the elongation at the break point decreased as the filler content increased. These results were in agreement with those of previous reports on starch-based biocomposites reinforced with cellulosic fillers such as cotton, hemp, and winceyette fibers [92–94].

Othman et al. [43] developed tapioca-starch-based biodegradable film incorporating banana pseudostems (waste) powder for the starch-based films. The mechanical and optical properties of the films were reduced but the barrier activity improved. The optimum percentage composition of 40 wt.% pseudostem powder can be used for incorporation into starch-based films as food packaging.

Cassava starch-based films have been investigated by Leites et al. [95] to determine the effect of waste from the production of orange juice on the properties of the films. The orange waste was added in two different forms, which are powder and aqueous extract (by soaking the powder in water followed by filtration). When comparing the moisture content, water solubility, and thickness of the extract to that of the powder orange residue, it was discovered that the extract had higher values.

#### *3.4. Isolation of Fiber from FVW*

For the extraction of cellulose from carrot waste, polysaccharides were extracted in acidic and alkaline environments before being treated with sodium hypochlorite solution to remove lignin and other lignin-containing compounds. With this treatment method, carrot cellulose was obtained as a 4% concentration in water suspension after the treatment. The nanocellulose was created by homogenizing cellulose with ultrasonically homogenized cellulose. Because the sonication system included a temperature probe, an ice bath was used to keep the samples from becoming too hot. The amplitude of the ultrasonic homogenizer was maintained at 90% of its maximum. Finally, carrot CCNF samples containing 0.1 wt.% carrots were obtained [36].

The extraction of fiber from durian skin required only a few simple procedures. The durian skin was cut into smaller pieces and thoroughly washed with tap water to remove any dust or adhering particles before being prepared. Next, the skin was dried at 70 ◦C for 24 h. The dried skin was ground to obtain fibers ranging in length from 100 to 150 µm. Approximately 300 g of raw durian skin fibers was used in this process. The alkali treatment of DSF was carried out with the help of sodium hydroxide (NaOH). [70].

The chemical treatment was used to isolate the orange peel. Eight grams was kept at room temperature under mechanical stirring for 16 h in a 5% KOH solution. Following the

alkaline treatment, the insoluble residue was bleached with NaClO<sup>2</sup> solution for one hour at 70 ◦C, pH 5.0, which was adjusted with 10% acetic acid. The residue was neutralized, washed, and centrifuged at 6000 rpm for 20 min at 25 ◦C. The second alkaline treatment was repeated with a concentration of 5% KOH. The insoluble residue was subjected to acid hydrolysis for an hour at 80 ◦C with 1% H2SO<sup>4</sup> for one hour. Following centrifugation and washing of the final residue, the diluted suspension was stored at 4 ◦C in a sealed container. The cellulose suspension was dried by lyophilization and stored at 4 ◦C, where it was designated as NFC [96].

The cellulose fiber from banana peels was obtained by removing the hemicellulose and lignin. The banana peel was treated with 10% (*w*/*v*) natural lye, which was made by soaking wood charcoal ash in water for 48 h before being applied. The extracted cellulose fiber was then bleached twice with 10% (*v*/*v*) hydrogen peroxide at 90 ◦C for 10 min each time. As previously described, the white cellulose fiber was retreated with a lye solution. Finally, cellulose was obtained [97].

#### **4. By-Products**

Fruits and vegetables are crucial for human nutrition, providing significant amounts of daily vitamin, mineral, and fiber needs. The fruit and vegetable processing industries generate significant amounts of waste in the form of liquid and solid, which contain several reusable high-value substances with significant economic potentials. Fruit and vegetable by-products accumulated from industrial operations. such as bagasse, peels, trimmings, stems, shells, bran, and seeds, make up more than half of all fresh fruit and have nutritional and functional contents that are sometimes higher than the finished product [98]. These by-products can be applied in food and other non-food applications, including medical, pharmaceutical, energy, and chemical production.

Typically, the fruit processing activities generate solid wastes (peel, skin, pulp, pomace, seeds, bunch, stems, and shells) and liquid wastes (crude extracts and wash water). With proper management, these wastes can be utilized for the production of by-products. Efforts have been made to explore the potential of these by-products in many applications, including in food and non-food industries.

Based on recent literature, most of the progress on utilizing fruit waste has been towards food and polymer applications, particularly in the production of flour, fiber, and pectin as fillers. Khoozani et al. [99] used banana pulps and peels to produce flour by varying the drying process using oven drying, a spouted bed drier, ultrasound, a pulsed vacuum oven, a microwave, spray drying, and lyophilization. Additionally, flour made from ripe "Prata" banana (*Musa* spp.) peels was used in the development of edible coatings, as shown in Figure 7 [100].

**Figure 7.** Development of edible coatings from banana peel. Reproduced with permission from Ref. [100]. Copyright 2021 Elsevier.

Reißner et al. [101] processed the pomace of different berries (blackcurrant, redcurrant, chokeberry, rowanberry, and gooseberry) into flour and fruit powder. de Andrade et al. [102] used orange, passion fruit, and watermelon to produce flour. The prebiotic potential of

the fruit-based by-product flour obtained from the solid waste from fruit processing was evaluated after undergoing in vitro gastrointestinal digestion process.

In addition to flour, fiber is another food application for fruit waste by-products. Sharma et al. [103] processed apple pomace to produce fiber to prepare fiber-enriched products. The dried pomace was packed in gunny and PE bags and stored at low (0–4 ◦C) and ambient temperatures (13–26 ◦C) after washing, blanching, and drying in a polytunnel drier (45 ± 8 ◦C). The apple fiber can be used in product formulations with good cholesterol-lowering characteristics and for the establishment of polymer composites such as reinforcements with epoxy resin to form hybrid composites [104].

Begum and Deka [105] used banana bract to extract dietary fiber (DF) using an ultrasound-assisted extraction method combined with alkaline extraction. This antioxidant DF is vital because it is linked to various health benefits, including preventing and treating chronic and degenerative diseases. Additionally, Mir et al. [106] produced gluten-free crackers with high fiber from brown rice flour and apple pomace. Apple pomace flour blends were made by combining brown rice flour with 0, 3, 6, and 9% apple pomace.

Another common use of fruit waste is that of pectic substances for use in other applications. Khamsucharit et al. [107] extracted pectin from the peels of five banana varieties through a conventional hot acid extraction method with a citric acid solution. Another study on pectin extract from banana peel was conducted by Maran et al. [108] using an ultrasound-assisted, citric-acid-mediated extraction method, optimized through the response surface methodology (RSM) approach. Moorthy et al. [109] used jackfruit peels to produce pectin using an ultrasound-assisted extraction method, a similar approach to that used by Hosseini et al. [110] and Guandalini et al. [111], who respectively extracted pectin from orange and mango peels. Chaiwarit et al. [112] found that pectin extracted from mango peel (Figure 8) can be regarded as a potential biopolymer for an edible film for food packaging.

**Figure 8.** Fabrication of thin polymer films from mango peel waste. Reproduced with permission from Ref. [112]. Copyright 2021 Elsevier.

Fruit waste has also been used for the production of non-food by-products. One of the most common applications of fruit waste is for the production of bioadsorbents. Laysandra et al. [113] used durian skin to produce adsorbent by mixing it with acidactivated bentonite (AAB) and natural surfactant (rarasaponin). They found that the rarasaponin/bentonite-activated biochar from the durian shell composite (RBAB) was effective for use as a new composite adsorbent for the removal of crystal violet and Cr (VI) from aqueous solutions. Meanwhile, fruit waste is also utilized to make films. Mango peel

was used in the production of fish-gelatin-based films [114]. The films were prepared via the solution casting method with three different concentrations of the mango peel extract (1 to 5%). The films produced were then tested for their physical, barrier, mechanical, and antioxidant qualities. Based on their findings, the addition of mango peel extract in the formulation produced a thicker, denser, and continuous structure with outstanding free radical scavenging activity (Figure 9).

**Figure 9.** Gelatin films with various concentrations of mango peel extracts. Reproduced with permission from Ref. [114]. Copyright 2021 Elsevier.

Fruit waste has also been used in the production of edible coatings to improve their properties. Blackberry pomace has reported been used for its the antioxidants and antimicrobials sources in the production of edible coatings for foods and consisting of carboxymethylcellulose, bacterial cellulose fibrils, and pectin sweets, with high lipid contents [115]. Meanwhile, grape seed extract has been included in pectin–pullulan edible film production for the storage of peanuts [116]. This coating successfully extended the shelf life of the stored peanuts by delaying rancidity.

Similar to fruit wastes, flour is the common by-product produced from vegetable waste. de Andrade et al. [102] used solid products from eight types of vegetables, including carrot, courgette, cucumber, lettuce, mint, rocket, spinach, and taro, to produce flour, then tested the modulatory effects on the gut microbiota composition. Amofa-Diatuo et al. [117] produced flour from cauliflower stems and leaves as a source of isothiocyanates (ITC) in an apple juice beverage. Cauliflower waste was also utilized in producing fermentable sugar. Majumdar et al. [118] pre-treated cauliflower stalks and leaves with dilute phosphoric acid prior to enzymatic hydrolysis to better release fermentable sugars. Vegetable waste is also a source of fiber and pectin. Iwassa et al. [119] produced and characterized fiber concentrates from asparagus by-products. They concluded that the fiber concentrates had high potential for use in formulating functional food products. Kazemi et al. [120] used an ultrasonic extraction method to extract pectin from eggplant peels. Eggplant peels provide substantial extraction yields and are considered to have high potential in pectin production

Waste from vegetables was also used for the extraction of essential oils. Chiboub et al. [121] extracted essential oils from the tops of carrots. The result showed that the essential oil produced was a good source of natural antimicrobials or aromatic agents. A similar finding was reported by Caliceti et al. [122] for peptide extract from cauliflower leaves. In contrast, the tops and roots of carrots were reported to be used to produce biodegradable composite films [123]. To improve the composite properties, they mixed and optimized the formulation with hydroxypropyl methylcellulose (HPMC) and high-pressure microfluidized cellulose fibers. Garlic peel extract was reported to be added in the formulated gelatin film as a source of antioxidant and antibacterial agents [124]. The gelatin film was proven to maintain the qualities of the rainbow trout fillets during refrigerated storage. Additionally, vegetable waste was also used for the production of the highly antioxidant edible coatings. Asparagus waste extract was incorporated into a hydroxyethyl cellulose–sodium alginate edible coating to significantly extend the postharvest life and retain the quality of strawberry fruit [125].
