Fruit wastes and/or by-products that food agro-industries accumulate are typically made up of underutilized residual biomasses that are rich in various bioactive functional components ^[1][20]. Fruit wastes have been researched for the extraction of phenolic compounds, dietary fibers, and other bioactive substances, as they are rich sources of phytochemicals. Peels, pomace, and seed fractions make up the majority of fruit by-products, and they have the potential to be a decent source of bioactive compounds with high added value, such as proteins, dietary fibers, polysaccharides, flavor compounds, and phytochemicals ^[2][21]. As a starting point for additional research into the usage of these compounds, researchers and food manufacturers frequently examine the bioactive compounds found in various fruit parts. Therefore, scientific evidence showing the abundance of beneficial components in various fruit parts justifies the consumption of fruit waste in food applications, while also reducing its environmental impact. Studies have shown that sizeable levels of essential nutrients and phytochemicals are available in the peels, seeds, and other parts that are not often utilized, even though most people only eat the pulp of fruits ^[3][22]. In contrast to banana peels, which primarily contain gallocatechin, catechin, and epicatechin, the peels of avocado and custard apples have large concentrations of condensed tannins and flavonoids, including procyanidins ^[3][22]. However, the predominant compounds found in banana peels are gallocatechin, epicatechin, and catechin ^[3][22]. Peels of Prunus cultivars, including the nectarine, apricot, and peach, are abundant in hydroxycinnamates and flavan-3-ols, which may have antioxidant properties ^[3][22]. Onion peel is reported to be rich source of flavonoids, including athocyanins, kaempferol, and quercetin derivatives (quercetin diglucoside, quercetin aglycone, and quercetin 4-O-glucoside) ^[4][5][23,24]. Phenolic compounds are secondary metabolites that are among the major classes of significant bioactive compounds with wide-ranging biological effects. In their basic structure, they have one or more aromatic rings, along with one or more hydroxyl groups. Polyphenolic compounds can be divided into several classes, including flavonoids (subclasses: flavonols, flavanones, flavanonols, flavanols, flavones, isoflavones, and anthocyanidins), tannins, phenolic acids, lignans, and stilbenes ^[6][25]. According to Wolfe, et al. ^[7][26], apple peels can contain up to 3300 mg/100 g of dry matter in terms of their phenolic content. Zadernowski, et al. ^[8][27] noted that mangosteen peel and rinds have been shown to contain around two times more total phenolics and phenolic acid than the aril, whereas the mangosteen pericarp has been observed to have a total level of seven primary xanthones that is eight times greater than that in the aril ^[9][28]. It was formerly reported that the total phenolic content of mango peels is roughly 13–47% higher than that of the flesh and 32% higher than that of the seeds ^[10][11][29,30]. The phenolic content of papaya peels is about 1.2 times higher than that in the seeds ^[12][31]. On the other hand, the biochemical indices of the crude fiber of papaya seeds are much greater than those of the pulp and peel, although they have a lower total fiber content ^[13][14][32,33]. Passion fruit seeds and pulp are known to have much higher total phenolic and flavonoid concentrations, although they have lower total dietary fiber ^[15][34]. Both the peel and pulp of the dragon fruit contain a considerable amount of pectic compounds; however, the peel exhibits a higher level of pectic compounds than the pulp ^[16][35]. According to reports, tomato seeds contain a variety of bioactive substances, including bioactive peptides, flavonoids, carotenoids, pectin, and vitamins (tocopherol) ^[17][36]. Guava seeds are also reservoirs of bioactive components, such as fatty acids, including palmitic, linoleic, and oleic acid, as well as vitamin C, vanillin, and vanillic acid ^[18][37]. An unpalatable byproduct of the fruit is the Jamun seed. However, its high concentration of phytochemicals makes it a valuable source of nutraceuticals. The presence of phytochemical components, such as phenols, tannins, flavonoids, saponins, triterpenoids, steroids, and alkaloids, in the Jamun seed is associated with its bioactivity ^[19][38].

It has been previously reported that pineapple skin contains substantially more lutein, α-carotene, and β-carotene than the core ^[20][39]. Notably, it has been reported that both the fresh and dried pulp of rambutan has higher levels of ascorbic acid than the fruit’s peel and lower levels of carotene ^[21][40]. Contrarily, despite the content of total carotenoids derived from mango peels being much higher than that found in the kernel, they are poorer in terms of their total phenolic content and antioxidant activity ^[22][41]. Among plants, raw grape leaves (16.19 mg/100 g) are considered a key source of β-carotene ^[23][42], and β-carotene is widely utilized in the food additive, cosmetics, health care, and pharmaceutical industries. Markedly, it has demonstrated several advantages, including improved human immunity, antioxidant activity, protection against various malignancies, and a reduced risk of cardiovascular illnesses due to its ability to manage cholesterol levels ^[24][43]. Along with β-carotene and lutein, lycopene is one of the primary carotenoids extracted from tomato waste ^[25][44]. Lycopene is a phytonutrient with a significant impact on human health and it has long been recognized for its range of biological properties, including antioxidant, anti-inflammatory, etc. ^[26][27][45,46]. Lutein is a yellow–orange carotenoid that is a member of the xanthophyll family and is frequently found in fruits ^[28][47]. The main anthocyanin found in many fruits is cyanidin 3-O-glucoside, which is the most prevalent anthocyanin in plants and has been linked to anti-obesity, anti-inflammatory, antioxidant, and anti-tumor characteristics ^[29][30][48,49]. Non-anthocyanin phenolic chemicals, such as flavonols (myricetin, quercetin, and kaempferol) and flavones (luteolin and apigenin), are a promising family of natural food colorings. In fruits, they are present mainly as quercetin ^[31][50]. For instance, elderberry contains a significant amount of quercetin derivatives, and quercetin is reported to have positive benefits on health; typically, they are well-known for their antioxidant, anti-obesity, and anti-inflammatory properties, and can be used in preventing cardiovascular illnesses ^[32][33][51,52]. Apple pomace is typically discarded as waste material in processing industries after the juice has been extracted. This waste can, however, be an excellent source of nutritional fiber. According to reports, apple peel contains more dietary fiber than apple pulp. The amounts of soluble and insoluble dietary fiber in apple pomace are 15% and 36%, respectively ^[34][53]. Apple seeds are also reported to be a rich source of bioactive compounds ^[35][54]. The pomace powder of blackcurrants, red currants, gooseberries, rowanberries, and chokeberries is also reported to have a high fiber content (>550 g/kg) ^[36][55]. The total amount of dietary fiber found in grape pomace is around 78%, of which 9.5% is soluble and the remaining 68% is insoluble ^[37][56]. Ajila and Prasada Rao ^[38][57] evaluated the total dietary fiber in mango peels and revealed their content to be 40–72%, with glucose, galactose, and arabinose being the main neutral sugars in the soluble and insoluble dietary fibers. The dietary fiber concentrations in the pulp and peels recovered as a byproduct of the extraction of peach juice range from 31–36% (dry weight), with 20–24% insoluble dietary fiber making up the majority.

Agricultural production currently creates substantial amounts of organic waste from agricultural wastes and the industrialization of the output, such as food industry waste. This industrialization process engenders large quantities of co-products that are difficult to preserve because of their chemical and physical–chemical properties. Historically, these co-products have been utilized for animal food or compost. In their composition, however, it is likely that several compounds with high added value will be identified that, after undergoing an appropriate conversion process, could be transformed into marketable products as ingredients for the development of new food products to obtain the benefits of the vast quantity of potentially valuable compounds that they contain. Some of the food industry’s by-products include fruits, skins, seeds, and membrane residues that have been discarded. These fractions are rich sources of many bioactive compounds, such as dietary fiber (pectin, cellulose, hemicellulose, and lignin), minerals (potassium, calcium, magnesium, and selenium), organic acids (citric, oxalic, and malic acids), vitamins (vitamin C, thiamine, riboflavin, and niacin), phenolic acids (chlorogenic, ferulic, and sinapic acids), flavonoids (hesperidin, narirutin, didymin, hesperetin, and diosmin), terpenes (limonene), carotenoids (lutein, β-carotene, and zeaxanthin), etc. ^{[39][40][41][42][43]}[15,165,166,167,168]. Numerous health benefits have been linked to these bioactive substances, including antioxidant, antibacterial, anti-inflammatory, anti-hypertensive, neuroprotective, and antiallergenic activities ^{[41][44][45][46]}[166,169,170,171]. As a result, the creation of several products employing by-products from agro-industrial waste is gaining interest in the food industry.

2.1. Food Fortification

The consumption and processing of a variety of fruits, such as apples, mangos, grapes, and citruses, generate numerous by-products that frequently contain a high concentration of useful bioactive compounds. One of the biggest by-products of processing fruits is the fruit pomace. Fruit pomace can be used in food items as a cost-effective, low-calorie bulking agent to replace some of the sugar, fat, or flour. It frequently improves food functionality by enhancing emulsion stability and water and oil retention ^[47][172]. Fruit pomace often combines the usual fruity and baked taste and aroma of the finished products to improve the aroma and flavor of baked goods. By using 30% (w/w) apple pomace, researchers developed several high-fiber, functional baked and extruded snacks. The product’s chemical composition remained unchanged when compared with the control ^[48][173]. In another study, up to 20% (w/w) mango peel powder enhanced the soluble dietary fiber and hardness while reducing spreading in soft dough biscuits. Contrarily, it was discovered that adding mango peel powder up to 30% (w/w) improved the nutritional value of cookies without impairing their sensory or textural qualities ^[49][174]. Similar to bakery products, the use of fruit pomace in meat products has also been investigated by several researchers. To increase the dietary fiber content of meat products, fruit pomace has been added to different meat products. For example, apple pomace in meat could make up for the lack of fiber in our diets. A study developed beef patties with 2–8% apple pomace as a beef substitute ^[50][175]. The water-holding capacity, cooking yield, meat emulsion stability, and textural qualities, such as the firmness, toughness, and hardness, of patties were improved with higher apple pomace powder incorporation. However, only the addition of apple pomace powder up to 6% was deemed acceptable based on a sensory examination of the patties. Similarly, it was reported that red grape pomace could enhance the color stability and acceptability of pork burgers by reducing lipid oxidation. When the percentage of fruit pomace replacement exceeded 6%, a decrease in hardness and cohesiveness was found ^[51][176]. Fruit pomaces are sometimes also used in dairy products as a natural texturizer and stabilizer. Apple pomace was added to skimmed milk in three different concentrations (0.1%, 0.5%, and 1%) and then fermented at 42 °C by Lactobacillus bulgaricus and Streptococcus thermophiles. The outcomes showed that adding 1% pomace caused a higher onset pH and quicker gelation. Additionally, after 28 days of storage, yogurt supplemented with fruit pomace showed enhanced cohesion and consistency ^[52][177]. Similarly, the addition of 3% pomace to stirred yogurt caused a noticeably lower level of syneresis and an increase in the matrix’s stiffness, cohesion, and viscosity ^[53][178]. Citrus fruits (orange, lemon, mandarin, and grapefruit) are also among the most widely grown crops that produce a huge quantity of co-products, such as peel and pulp (seeds and membrane residues). Soluble dietary fiber and insoluble dietary fiber, which can be found in citrus co-products, are outstanding sources of dietary fiber. Several studies reported very intriguing technological–functional properties of citrus co-products due to their high dietary fiber content, including their water-holding capacity (WHC), oil-holding capacity (OHC), swelling capacity (SC), foam capacity (FC), and emulsion capacity (EC). Citrus co-products can be used to increase the dietary fiber content or serve as a fat substitute in meat products. In this regard, a study examined the impact of adding lemon fiber at 2, 4, and 6% on the amount of cholesterol in low-fat beef burgers. The researchers discovered that adding lemon fiber lowered the amounts of cholesterol and saturated fatty acids in a concentration-dependent manner ^[54][179]. Similar to this, low-fat Frankfurt sausages were supplemented with various amounts of citrus fiber (1, 2, and 3%). According to these authors, the sausage samples that had citrus fiber added to them had reduced levels of saturated fatty acids and better water-binding properties ^[55][180]. Citric acid, one of the by-products of kiwi processing, can prevent browning and maintain color characteristics during the osmotic dehydration of kiwifruit slices ^[56][181]. Another compound of interest from kiwi is Actinidin. Actinidin has potential applications as a cost-effective coagulant in milk. According to a study, kiwi extract caused a casein clot to form that was isolated from the serum and remained stable for up to two months at room temperature ^[57][182]. Beyond fulfilling fundamental nutritional needs, bioactive substances have positive health effects on the host. Due to the GRAS (Generally Recognized as Safe) status of medicinal herbs, extracts, or essential oils, they can be added to a variety of food products. The effect of flaxseed extract, which is high in linolenic acid, lignans, and fiber, on the development and survival of kefir-isolated lactic acid bacteria was demonstrated in an in vitro investigation by ^[58][183]. The growth of Lactobacillus kefiranofaciens DN1, Lactobacillus bulgaricus KCTC3635, Lactobacillus brevis KCTC3102, and Lactobacillus plantarum KCTC3105 was reported to be considerably higher after treatment with crude flaxseed extract than that in the control. Similarly, the characteristics of kefir drinks that had been supplemented with yam, sesame seed, and bean extracts were examined ^[59][184]. Upon the application of different concentrations (25, 50, and 75%) of these extracts, the results demonstrated that the fermentation of yam, sesame, and bean extracts by water kefir grains was acceptable for the preparation of fermented vegetable beverages. In addition, the formulation enhanced with 50% beans was the finest base for producing kefir beverages, as well as a protein-rich beverage. To partially replace the fat in an emulsified meat system, the impact of orange peel addition, employed as a fat substitute, on the oxidative stability of low-fat beef burgers was examined ^[60][185]. The authors claimed that the samples in which orange peels were used as a fat replacer had peroxide levels that were lower than those of a control sample, with reductions of >90%. Given that its dietary fiber can aid in regulating colon bacterial populations and lower the synthesis of mutagens following the fermentation of food chemicals by intestinal bacteria, the prebiotic capacity of kiwis is one of their most researched characteristics ^[61][186]. It has been demonstrated that eating cooked starch with kiwis delays the digestion and absorption of carbohydrates and has hypoglycemic effects ^[62][187]. The use of kiwi seed oil as a component of dietary supplements intended to lower cholesterol and prevent obesity has been suggested. It would have an anti-inflammatory effect, enhance the intestinal flora, lower blood sugar levels, and promote a lipid-lowering effect ^[63][64][188,189].

2.2. Food Preservation

Bioactive compounds such as phenolics, which comprise terpenes, aliphatic alcohols, aldehydes, ketones, acids, anthocyanins, and isoflavonoids, are the most important group of chemicals with antimicrobial activity ^[65][66][190,191]. The fundamental function of phenolics is in plant defense against biotic and abiotic stressors, pathogens, and pests ^[67][68][69][192,193,194]. Flavonoids are a wide category of phenolic compounds found in several fruits, vegetables, and roots, among other foods ^[70][71][195,196]. The subclasses of flavonoids include flavanones, flavonols, flavones, flavonols, isoflavones, and anthocyanidins ^[72][197]. Grape seed extracts are by-products of winemaking or grape juice production and are high in proanthocyanidins and other phenolic compounds ^[73][74][75][198,199,200]. The use of the Isabel and Niagara varieties of grape seed extracts as natural antioxidants in amounts of 40 and 60 mg, respectively, delayed the lipid oxidation of processed, cooked, and refrigerated chicken meat for 14 days, with effects comparable to those of the synthetic antioxidant butylated hydroxytoluene (BHT). Similarly, the combination of grape extracts with vacuum packaging has been shown to be an effective method for enhancing the lipid stability of cooked chicken ^[76][201]. Several studies have also reported the antibacterial effectiveness of grape extracts against lactic acid bacteria, foodborne pathogens, and wine-rotting yeasts ^{[77][78][79][80][81][82]}[202,203,204,205,206,207]. Grape seed extracts suppressed the growth of foodborne pathogens, such as Staphylococcus aureus, Salmonella sp., Escherichia coli, Listeria monocytogenes, and Campylobacter sp. ^[83][84][85][208,209,210]. Depending on their composition, citrus peels are abundant in several nutrients that serve as functional and antimicrobial compounds. These by-products contain secondary metabolites, such as terpenoids, carotenoids, coumarins, furanocoumarins, and flavonoids, particularly flavanones and polyethoxylated flavones ^[86][211]. The addition of citrus oil in combination with milder heat treatments has been reported to have an impact on the control of spoilage bacteria in apple and orange juices ^[87][212]. On the other hand, mango seed biowaste has also been characterized by a high concentration of bioactive components, including phenolic compounds, carotenoids, and vitamin C ^[88][89][213,214]. A study reported an array of antibacterial properties for mango seed ethanolic extracts and reported their efficacy against Gram-negative bacteria ^[90][215]. Various mango peel extracts were evaluated for their antibacterial effects against Gram-positive Staphylococcus aureus and Gram-negative Pseudomonas fluorescens. Different levels of antibacterial activity were present in the extracts against both. In general, Gram-positive bacteria are more sensitive to natural substances than Gram-negative ones. The peel of the Langra mango variety showed the greatest zone of inhibition for both organisms when it was extracted with 70% ethanol and 80% acetone. Due to the existence of various cell wall architectures, Gram-positive and Gram-negative bacteria exhibit diverse antimicrobial properties. More potent antibacterial substances may include those that can fluidize the membrane and successfully diffuse the lipid bilayer ^[91][216]. Avocado peels and seeds contain many bioactive components, including phenolic acids, condensed tannins, flavonoids (including procyanidins and flavonols), and hydroxybenzoic and hydroxycinnamic acids ^[92][93][94][217,218,219]. Studies have demonstrated the antibacterial action of avocado seed extract components against microorganisms. A recent study demonstrated the biocidal impact of avocado seed extracts against L. monocytogenes, suggesting that this action was caused by an increase in cell membrane permeability. Avocado seed ethanolic extract (104.2–416.7 μg/mL) was found to exert antibacterial effects against L. monocytogenes (Staphylococcus epidermidis, and Zygosaccharomyces bailii ^[95][220]. Table 1 depicts some additional food-preservation effects from food waste.

Food Waste/Bioactive Compound	Food Preservation Effect	Reference
Apple pomace	Inhibitory effect against pathogens Helicobacter pylori	[96][221]
Kiwi leaves (alcoholic and hydroalcoholic extracts)	Antimicrobial effect against S. aureus	[97][222]
Olive mill wastewater (phenols)	Antimicrobial action against E. coli, P. aeruginosa, S. aureus, and B. subtilis strains	[98][223]
Tomato wastes	Antimicrobial activity of tomato waste extracts against S. aureus correlated moderately with isochlorogenic acid content	[99][224]
Acetone and methanol carrot peel extracts	Growth inhibition of Shigella flexneri, E.coli, S. aureus, and Klebsiella pneumoniae	[100][225]
Jabuticaba seeds	Ellagitannins and ellagic acid in the extracts contained antimicrobial and antioxidant properties.	[101][226]