Valorization of Fruit Pomace by Enzymatic Treatment and Microbial Fermentation

Abstract

Fruit pomace is a major processing byproduct abundant in fermentable sugars, dietary fibers, and phenolic and other bioactive compounds. This review provides a summary of the latest developments in fruit pomace enzymatic valorization and microbial fermentation, focusing on the enzymes and microbes used, technologies, bioconversion products, and applications. The extraction and structural transformation of dietary fibers, oligosaccharides, and phenolic and other bioactive compounds have been made easier by enzymatic treatments. Microbial fermentation of fruit pomace produces a range of compounds such as prebiotics, organic acids, and polyphenols. Solid-state fermentation and enzyme immobilization allow the scalability and efficiency of these processes. The combination of enzymatic valorization and microbial fermentation may provide a sustainable approach to turn fruit pomace from waste into value-added food ingredients.

1. Introduction

Fruit and vegetable production exceeded two billion metric tons in 2022 [1], of which a major part is processed into juices, purees, jams, frozen pulp, and wines, generating significant wastes (20–50%) [2,3]. Within the zero-waste concept, the valorization of fruit and vegetable byproducts is targeted to reduce food waste and promote circular bioeconomy [4]. Pomace, the residual material accounting for 20–30% of processed fruits, is a major byproduct of juice and concentrate extraction [5,6]. It is mainly composed of cell wall compounds, especially dietary fiber, phenolics, and other bioactive compounds, and minerals [7,8,9]. Pomace can provide versatile raw materials for functional foods, pharmaceutical and cosmetic products, bioplastics, biofuels, and biofertilizers [10]. Fruit pomace like apple pomace [11,12,13,14,15,16], citrus pomace [17,18,19,20,21,22], grape pomace [16,23,24,25,26,27], olive pomace [28,29,30,31,32], tomato pomace [13,33,34,35,36], date pomace [37,38,39,40,41], and pear pomace [12,15,42,43] are currently receiving much interest. Pomace from different fruits may vary significantly in the type of tissues and chemical composition, which affect their valorization strategies. For example, date fruit pomace typically contains fibrous tissue, and it is low in bitterness, making it more suitable for food applications compared to some other fruit pomaces.

There has been an increasing research effort for the utilization of fruit pomace in bakery and meat products. Fruit pomace can be utilized as a component in fiber- and antioxidant-rich functional foods, nutraceuticals, and pharmaceuticals [44]. Bioprocessing techniques, including enzymatic treatment and microbial fermentation, can further expand the potential applications of fruit pomace [10]. There have been reviews on fruit pomace valorization, for instance, Venkidasamy et al. [45] discussed various bioactive compounds extraction techniques from fruit pomace, Sharma et al. [38] discussed natural pigment extraction from fruit pomace and encapsulation technologies, Ikusika et al. [46] described how solid-state fermentation can be used to enhance nutritional and antioxidant activities of fruit pomace, Yuan et al. [47] discussed how to use fermentation to improve the antioxidant capacity of fruit byproducts, etc. However, these reviews did not discuss in detail the main components of fruit pomace, some valorization strategies, mechanisms, and technologies involved in the enzymatic valorization and microbial fermentation of fruit pomaces. Therefore, this current review analyses the latest advances in fruit pomace valorization strategies that use enzymatic hydrolysis and microbial fermentation, highlighting the generation of a wide range of value-added products, the different enzymes and microbes employed, and the technologies used. The review aims to provide insight into optimizing valorization strategies and exploring industrial applications of derived bioproducts.

2. The Main Components of Fruit Pomace

Understanding the composition and the physicochemical properties of the fruit pomace is a prerequisite for designing suitable strategies for its valorization.

Table 1. Proximate composition of selected fruit pomaces (g/100 g DM).

Pomace	Soluble Sugars	Dietary Fibre	Protein	Fat	Ash	References
Apple	1.1	26.8–82.0	1.2–6.0	0.3–8.5	0.5–4.3	[12,52,53]
Orange	-	33.8–46.9	5.0–6.6	1.0–1.5	2.9–3.6	[18,54,55]
Grape	1.3–77.5	17.3–88.70	3.57–14.2	8.2–10.1	4.6–4.7	[24,25,56]
Olive	21–32	53–59	7.4–9.0	5.8–16.6	1.9–6.46	[29,30,57]
Tomato	25.7–26.7	39–74.8	15–30	2.2–16.2	3.1–4.2	[34,35,36,58,59]
Dates	35.3	45.5–77.2	2.1–10.6	0.7–2.0	1.9–4.1	[37,40,60]
Pear	0.3	57.9–90.7	1.8–5.7	0.9–3.7	0.9–2.0	[12,61,62]

shows that fruit pomaces are especially rich in dietary fiber and phenolic compounds and have variable levels of soluble sugars, proteins, fat, ash, phenolic compounds, and organic acids [48,49]. Dietary fiber includes non-starch polysaccharides and lignin, which are not digested or absorbed in the small intestine. The main components of fruit pomace fibers include the insoluble lignin, cellulose, hemicellulose, and the highly soluble pectin, which are the remnants of plant cell walls (Figure 1). Lignin, on the other hand, is a highly branched and cross-linked heterogeneous polymer that makes plant biomasses highly recalcitrant and resistant to degradation [50]. Cellulose is a linear homopolymer of glucose residues connected by β-1,4-glycosidic bonds, while hemicellulose is a mixture of heteropolymers including xylan, mannan, and arabinogalactan backbones [13]. Pectin comprises a complex and heterogeneous mixture of galacturonic acid-rich polysaccharides (homogalacturonan and rhamnogalacturonans I and II) [51].

The relative contents of soluble and insoluble fiber fractions (lignin, cellulose, hemicellulose, and pectin) vary between the different fruit pomaces, processing methods, drying conditions, etc. [13]. In addition, pomace may contain vitamins, essential oils, pigments, and other secondary metabolites of special interest. The dietary fiber content is high in fruit pomace (in g/100 g DM), e.g., apple (82), pear (90.7), olive (53–59), dates (45.5) [16,17,31]. The dietary fiber, which remains as the major component in pomace after the removal of soluble sugars, varies in the relative proportion of its soluble and insoluble fibers [63]. For instance, apple contains a high percentage of soluble fiber (68–73%), whereas date fruit pomace contains 10% or less soluble fiber, which is mainly pectin [14,41]. The lignin content can be higher than that of cellulose and hemicellulose in some pomaces [64,65]. Protein (in g/100 g DM) is present in apple (6), orange (6.5), grapes (3.57–14.2), olive (8–9), tomato (15–24), dates (10.6), and pear (5.7) as shown in Table 1. After processing, the protein might associate strongly with dietary fiber components, especially lignin, via covalent and non-covalent bonds.

Fruit pomace consists of particles having heterogeneous shapes and sizes due to their variable contents of different tissues and fiber types [66]. Generally, fruit pomaces have a fibrous and compact texture and structure due to the high amount of insoluble fiber (cellulose, hemicellulose, and lignin) in their cell walls [21,67]. The tightly compact fibrous structures of pomace are due to fiber–fiber, fiber–starch, and fiber–protein interactions through strong internal bonds (e.g., hydrogen bonds, van der Waals forces) [21,68,69]. Fiber protein, especially lignin–protein interactions, strengthens the bonding between these components and negatively affects protein extractability and digestibility [69].

Table 2. Polysaccharides and lignin composition of selected fruit pomaces (g/100 g DM).

Pomace	Cellulose	Hemicellulose	Pectin	Lignin	References
Apple	3.6–43.6	4.3–24.4	11.7	3.0–29.2	[13,14,15,16]
Orange	14.3–21.5	6.3–14.3	-	3.3–10.7	[19,20]
Grape	3.2–14.0	1.7–8.3	4.1–8.8	7.9–53.6	[16,25,26]
Olive	8.6–9.7	8.1–11.3	6.0–6.5	21.8–44.0	[29,31,32]
Tomato	8.6–34.0	5.3–14.4	7.6–8.8	5.9–37.3	[13,36,59]
Dates	13.0–18.0	17.4	1.4–3.7	52.9	[38,39]
Pear	32.4–34.5	18.6–20.2	2.1–13.4	26.0–33.5	[15,70]

summarizes the polysaccharide and lignin composition of selected fruit pomaces.

The phenolic compounds in pomaces can be classified into three types: (i) soluble low-molecular weight compounds that can be extracted in aqueous organic solvents containing methanol, ethanol, or acetone, (ii) hydrolysable compounds that are esterified to the fiber and that can be released by low concentration alkaline solution before extraction by an organic solvent, and (iii) high molecular weight phenolic polymers that can only be extracted with strong alkali [71]. These polymeric phenols include melanin and lignin fractions. Some pomaces and fruit peels, such as those of citrus fruits, contain appreciable amounts of essential oils that can be used as flavor agents [72].

3. Valorization Strategies for Fruit and Vegetable Pomaces

Table 3. Biochemical components derived from fruit pomaces.

Target	Pomace	References
Insoluble fiber	Apple, grapes, peach, orange, pear	[7,73]
Soluble fiber	Apple, mango, pear	[7,73]
Pectin	Apple, orange, tomato	[12,74,75,76]
Phenolic antioxidants	Olive, dates, mango, plum, berry, grape, apple	[7,77,78]
Carotenoids	Mango, tomato, carrot	[75,76,79,80]
Essential oils	Citrus fruits	[81]
Industrial enzymes (Polygalacturonase, tannase, pectinases)	Grape, apple, olive	[82,83]
Prebiotics	Citrus fruit, apple, grapes	[84,85]

shows the biochemical components that can be derived from fruit pomaces.

3.1. Fruit Pomace as a Source of Fiber in Food Products

Fruit byproducts are increasingly being incorporated into food formulations to offer nutritional enhancement and waste reduction. For example, bakery products were supplemented with apple peel and pomace [86], carrot pomace, tomato pomace, and grape pomace [87], and date fruit pomace [88] to improve the dietary fiber content and nutritional value. Alaa et al. [88] found that fortification of the bread rolls with date fruit pomace increased the total dietary fiber, antioxidant capacity, decreased the glucose release during the oral, gastric, and intestinal digestive phases, but had a minimal effect on glycemic index. Similarly, wheat bread supplemented with apple pomace had higher carbohydrates, phenolic content, and antioxidant capacity, but lower protein, fat content, and loaf volume, and similar sensory acceptance as the pure wheat bread [89]. The high insoluble fiber in fruit pomace weakens the gluten network in yeast-leavened bread, leading to reduced bread gas volume and impaired texture quality [90]. The high-water absorption rates of fiber-rich pomace fibers necessitate adjustments of hydration levels, kneading time, and emulsifier usage to maintain desirable dough consistency and sensory properties [91]. Pomace has been successfully incorporated into biscuits, cookies, and muffins [88,92,93]. Nguyen et al. [94] compared the addition of enzymatically and non-enzymatically treated pineapple pomace into biscuits. The enzymatic treatment of the pineapple pomace was carried out with cellulase. Adding either cellulase-treated pomace or crude pomace to the biscuits increased their fiber content, antioxidant activity, and hardness. However, when 30% of the biscuits were made with cellulase-treated pomace, the biscuits were softer, had more soluble fiber, higher antioxidant activity, and were preferred overall compared to those made with crude pomace [94,95]. Excessive addition of pomaces might reduce the cooking quality, textural properties, and overall acceptability of the pasta [96]. Fruit pomace has also found application in dairy; for example, microcapsules of blackberry pomace were used as a supplement in yogurt. The formulated yogurt with the blackberry pomace microcapsules had higher post-gastrointestinal digestion bioavailability compared to the pristine microcapsules [97]. Fruit pomace is included in meat products such as patties and burgers, to improve fiber contents and some functional properties like stability and water-holding capacity [98]. Fruit pomace has been used as a low-cost non-caloric bulking agent to replace flour or sugar as well as to increase the stability of emulsions [99].

3.2. Fruit Pomace as a Source of Phenolic Compounds

Fruit pomace is a rich source of health-beneficial bioactive compounds, which include phenolic compounds, secondary plant metabolites with antioxidant capacity, pigments, flavoring agents, and aromas, that can be used in nutraceuticals, medical, and food industries [100]. Among these compounds, the phenolic compounds are the most extracted bioactive compounds from fruit pomaces [101]. The amount and type of phenolic compounds extracted greatly depend on the type of fruit pomace, extraction methods, and conditions. For instance, grape pomace (68.8–96.3 mg GAE/g) has greater phenolic content compared to olive pomace (20.1–34.1 mg GAE/g), orange pomace (18–21.8 mg GAE/g), etc. [102,103,104]. The main phenolic compound in apple pomace is phloridzin (

Table 4. Phenolic compounds in fruit pomaces.

Pomace	Total Phenolic Content (mg GAE/g)	Identified Compounds	References
Apple	1.1	Phloridzin, chlorogenic acid, gallic acid, catechin, rutin, ursolic acid, oleanolic acid, etc.	[105,108]
Orange	18–21.8	Hesperidin, naringenin, hesperitin, apigenin, quercetin, isorhamnetin, p-coumaric acid, sinapic acid, etc.	[101,103,109]
Grape	68.8–96.3	Quercetin, peonidin, cyanidin, catechin, epicatechin, procyanidin, etc.	[100,104]
Olive	20.1–34.1	Quinic acid, oleuropein, hydroxytyrosol, caffeic acid, vanillin, rutin, luteolin, apigenin, etc.	[102,110]
Tomato	0.9–1.0	Chlorogenic acid, gallic acid, caffeic acid, rutin, quercetin, ferulic acid, naringenin, etc.	[111,112,113]
Dates	0.2–2.6	Nil	[37]
Pear	3.8–6.6	Chlorogenic acid, quinic acid, protocatechuic acid, gallic acid, isoquercitrin, rutin, epicatechin, etc.	[106,114,115]

), pear pomace is chlorogenic acid, orange pomace is hesperidin, olive pomace is oleuropein, etc. [102,103,105,106]. Pollini et al. [105] extracted polyphenols from apple pomace with two methods (ultrasound-assisted technique and accelerated solvent extraction), and it was found that the ultrasound-assisted technique showed the highest yield, but the accelerated solvent extraction method was more effective for extracting phloridzin, which is a major phenolic compound. In the same vein the research of Cea Pavez et al. [107] extracted polyphenol from olive pomace with two methods (pressurized fluid extraction and traditional organic solvent extraction), it was found that pressurized fluid technique extract had higher total phenolic content (1659 mg GAE/kg) compared to the traditional technique (282 mg GAE/kg), and the secoiridoids and flavonoids in the pressurized fluid technique is 3–4 times that of the traditional solvent extraction.

3.3. Fruit Pomace as a Source of Prebiotics

Prebiotics are non-digestible food ingredients that support the growth and activity of beneficial gut bacteria (e.g., Lactobacilli, Bifidobacteria, etc.) [116]. Fruit pomaces are rich in dietary fiber, polyphenols, and residual sugars that support gut health [117]. The insoluble and soluble fibers (e.g., pectin, hemicellulose) resist digestion in the small intestine, and soluble fibers are fermented in the colon to support the growth of beneficial gut bacteria [118,119]. Polyphenols like flavonoids, phenolic acids, and tannins in fruit pomace modulate the gut microbiota (e.g., enhancing the growth of Lactobacilli, Bifidobacteria, etc.) [116,120]. The individual and combined prebiotic potential of three fruit pomaces (apple, banana, and mango) was evaluated [121]. Three probiotic strains (Lactobacillus rhamnosus, L. casei, and Bifidobacterium lactis) were tested, and different concentrations of the fruit pomaces were used (0%, 2%, and 4%). All the fruit pomaces increased the probiotics viable counts (>10logs) after 24 h incubation. The prebiotic potential of different fruit wastes (yellow watermelon, honeydew, and papaya) was tested on probiotics (L. rhamnosus and Bifidobacterium bifidum) [122]. Watermelon and honeydew waste better promoted the growth of L. rhamnosus and Bifidobacterium bifidum compared to papaya waste, which is related to better favorable substrate, e.g., more fermentable fiber (soluble fiber) and sugars (glucose and fructose). Similarly, Li et al. [123] found that soluble dietary fiber remarkably enhanced the growth of probiotic bacteria species. Apple pomace and pectin modulated beneficial gut bacteria and promoted the production of short-chain fatty acids, which was related to the galactose and rhamnose contents [124]. The released bound polyphenols from Rosa roxburghii fruit pomace showed significant prebiotic potential by reducing the Firmicutes to Bacteroidetes ratio, improving the relative abundances of beneficial bacteria, and improving the short-chain fatty acids production [125].

4. Enzymatic Treatment and Fermentation of Fruit Pomace

Enzymatic treatment and fermentation are effective methods for improving the nutritional and functional properties of fruit pomace. These processes loosen the complex cell wall structure, selectively break down the target components, and can transform the existing compound into a new product. Enzyme like cellulase selectively cleaves cellulose in the cell wall, while lactic acid bacteria fermentation transforms glucose into lactic acid. The final products or processes carried out depend on the types of enzymes and microorganisms used. Figure 2 shows a practical example of an integrated/novel fermentation unit, and a continuous self-cycling strategy used to increase lycopene yield and reduce waste [126].

4.1. Enzymatic Valorization of Fruit Pomace

The use of enzymes for the valorization of agro-industrial residues offers several advantages [127,128,129], including greater extraction efficiency, greater selectivity, higher stability, and environmental sustainability. Moreover, the efficiency of the enzyme depends on the temperature, extraction duration, action mode, pH, and substrate availability [127]. Enzymes facilitate the degradation of the cell wall fibers, thereby enabling the extractability of the target component [130]. For example, the release of phenolic antioxidants from Dimocarpus longan was increased by up to four-fold when treated with pectinase [131]. Figure 3 provides a summary of the different lignolytic and glycosidic enzymes that can be used in the selective degradation of pomace.

4.1.1. Cellulases

These include the inter alias endoglucanase (EC 3.2.1.176), a non-selective enzyme able to cleave the β-1,4-d-glycosidic bonds in cellulose and xylan, exoglucanase or cellobiohydrolase (EC 3.2.1.91) that cleaves off cellobiose units from the cellulose chain, and β-glucosidase or cellobiase (EC 3.2.1.21) that hydrolyzes the released cellobiose to glucose. Cellulases comprise three types of enzymes: (i) endoglucanases, which act on less crystalline cellulose regions, resulting in free chain ends; (ii) exoglucanases or cellobiohydrolases, which attack the free chain ends, producing cellobiose units; and (iii) β-glucosidase, which depolymerizes cellobiose into glucose units as in Figure 4. It facilitates the release of polyphenols [133] and produces organic acids such as lactic acid [134], liquid polyols, and oligomeric mixtures by liquefaction and saccharification of apple pomace [135]. The incorporation of cellulase and gluco-amylase increases the polyphenol extraction from grape pomace [136]. Likewise, refined enzymatic digestions were used to extract polyphenols from both wet and dried red grape pomace with a celluclast enzyme, yielding the largest number of extracted polyphenols [137]. Enzyme-assisted extraction of grape pomace using a combination of commercial enzymes Cellubrix (cellulase and β-glucosidase activities), Neutrase (protease and α-amylase activities), and Viscozyme (cellulase, hemicellulase, and pectinase activities) resulted in a 25–65% increase in phenols and a 20–45% surge in the yields of soluble solids. Enzyme-assisted pectin extraction is always more beneficial than acid extraction, in which acid randomly hydrolyzes pectin by breaking glycoside bonds into smaller molecular weight pectin, but enzymes are more specific to certain chemical bonds, resulting in the extraction of larger pectin molecules [138]. Researchers have employed specific enzymes to hydrolyze cellulose to extract pectin from the cell wall of apple pomace [139]. For example, Wikiera et al. [140] used a commercial enzyme Celluclast (endo-1,4-β-glucanase, exo-1,4-β-glucanase, and β-glucosidase) at dosages between 25 and 75 μg/g pomace and obtained the high pectin extraction yield (19%) at optimum conditions: temperature (59 °C), pH (4.5), stirring rate (200 rpm), and reaction time (18 h).

4.1.2. Hemicellulases

Hemicellulases include a broad range of enzymes able to catalyze the hydrolysis of galactans, xylans, mannans, and arabinans. The most common enzymes in this group are xylanase (EC 3.2.1.8), which hydrolyzes the β-d-xylano-pyranosyl linkages in xylans to release xylo-oligosaccharides, and β-xylosidase or xylobiase (EC 3.2.1.37) that catalyze the hydrolysis of xylo-oligosaccharides into d-xylose sugars. Hemicellulases also include β-mannanase (EC 3.2.1.78) and arabinofuranosidase (EC 3.2.1.55) that catalyze the hydrolysis of mannans and arabinogalactans, respectively. Wikierra et al. [142] then updated the optimum extraction condition by employing pH 5.0 and 40 °C for 10 h constant stirring, along with the enzymes’ endo-β-1,4-glucanase and endo-β-1,4-xylanase produced by filamentous fungus Trichoderma viride.

4.1.3. Pectinases

Pectinases comprise many enzymes that degrade pectin through the catalysis of depolymerization (hydrolases and lyases) and de-esterification (esterase) reactions [143]. Enzyme-assisted pectin extraction was carried out in 1991 from apple pomace using arabinanase, galactanase, and pectin lyase [144]. The incorporation of endo-β-(1,4)-glucanase into these enzyme formulations increased the amount of high-molecular-weight material in the extracts. Cellulase and pectinase are often used for the enzyme-assisted extraction of bioactive compounds from citrus residues. Pectinase, along with cellulase and hemicellulase, can increase phenolic compounds and antioxidant activity in raspberry residue [145]. The enzyme pectinase is composed of four types; (i) protopectinases that convert insoluble protopectin into soluble pectin; (ii) pectinesterase responsible for removal of methoxyl esters, resulting in acid pectins and methanol; (iii) polygalacturonases, hydrolyzing the polygalacturonic acid chain and (iv) pectate lyases that depolymerize pectic substances by a trans-eliminative split at C-4 [135].

4.1.4. Laccases

Laccases (EC 1.10.3.2) can polymerize or depolymerize lignin via C oxidation, C-C cleavage, and alkyl-aryl cleavages. Laccases are an effective and low-cost technique for lignin removal. Certain microorganisms produce extracellular enzymes known as ligninase or lignin-modifying enzymes, which contain three lignin peroxidase enzymes. These include lignin peroxidase (EC 1.11.1.14) which target peroxidation of the non-phenolic lignin component accounting for up to 90% of the lignin, manganese peroxidase (EC 1.11.1.13) which target the phenolic component of lignin, and versatile peroxidase (EC 1.11.1.16) which target both phenolic and non-phenolic component of lignin [146]. However, the widely explored enzyme for lignin degradation is laccase, which is a multicopper-containing oxidase. Laccase is composed of four copper atoms bound to a reducing substrate and molecular oxygen [127]. Laccase is capable of splitting aromatic rings, monomer cross-linking, and breaking down polymers through the oxidation of phenolic compounds and metal complexes [147]. Laccases may be applied in many industries (e.g., food, pharmaceuticals, etc.) since they can act on a wide range of substrates [148]. For example, Nazar et al. [149] used Bacillus ligniniphilus L1 laccase for delignification of lignocellulosic biomass prior to its conversion to bioethanol. Laccase treatment reduced lignin (9%) and phenolic content (44.8%), increased the release of glucose, and reduced sugars for subsequent bioethanol production. Another study compared the use of laccase from bacteria (Streptomyces ipomoeae) and fungi (Trametes villosa) sources for delignification of lignocellulosic biomass [150]. The fungal laccase reduced the phenol content (71%) in the biomass more than the bacterial laccase (35%), which might be due to enzyme structure, activity range, substrate affinity, and redox potential. Free and co-immobilized laccase was used for depolymerization and conversion of lignocellulosic biomass to vanillin [151]. The free or co-immobilized laccase successfully produced vanillin from the lignocellulosic biomass; however, the vanillin yield was higher for the co-immobilized system.

4.2. Valorization of Fruit Pomace by Microbial Fermentation

The word fermentation originated from the Latin word “fervere”, meaning to boil and indicating gas production. Microbial fermentation of fruit pomace involves the action of microorganisms (bacteria, yeast, fungi) to convert the various fruit pomace components (sugars, fiber, proteins, etc.) into value-added products and several metabolites (Figure 5). Fermentation can be carried out under aerobic or anaerobic conditions [152]. A major challenge in the utilization of pomace is the recalcitrance imposed by lignin that inhibits the conversion of carbohydrate polymers into constituent sugars [153]. The steps, products, and byproducts from bacterial, yeast, and mold fermentation of fruit pomace are shown in Figure 6 and discussed below.

4.2.1. Bacterial Fermentation and Fermented Products

Bacterial fermentation is a complex bioconversion process involving many metabolic pathways depending on the bacterial species, fermentation conditions, and substrate composition [154]. During anaerobic bacterial fermentation, the residual sugars can be converted to organic acids and other byproducts [155]. For example, during homo-lactic fermentation, lactic acid bacteria convert glucose into lactic acid, and during hetero-lactic fermentation, glucose can be converted to lactic acid (Figure 7) and other byproducts (ethanol, carbon dioxide), and this increases the acidity of the system [37,156]. The ethanol produced can be converted to acetic acid using acetic acid bacteria under aerobic conditions. Furthermore, bacteria species like Bacillus spp. under aerobic conditions can secrete enzymes (cellulase, pectinase, xylanase) that can degrade carbohydrate polymers (e.g., cellulose, pectin, xylan) into glucose, galacturonic acid, and xylo-oligosaccharides [157]. The glucose monomer can be used to produce organic acids.

Production of organic acids, especially lactic acid, from residual sugars in pomace has been widely explored; however, a more recent study used a mixture of probiotics (L. plantarum, S. cerevisiae, and B. subtilis) to replace the use of commercial enzymes (cellulase) for the valorization of pomace-cellulose to lactic acid. Probiotic bacteria significantly increase lactic acid production and other metabolites and antioxidant activity [158]. Another study successfully used thermotolerant bacteria (Heyndrickxia coagulans and Geobacillus stearothermophilus) to produce lactic acid from pomace, however, with supplementation for nitrogen source (e.g., yeast extract or other inorganic chemicals like urea, ammonium chloride, etc.) [159]. Moreover, external supplementation for nitrogen source can be eliminated in thermotolerant bacteria fermentation by mixing different fruit pomaces, e.g., apple pomace with tomato pomace [159].

Figure 7. Lactic acid bacteria homo-fermentation and hetero-fermentation mechanism (Modified from Papadimitriou et al. [160]).

Figure 7. Lactic acid bacteria homo-fermentation and hetero-fermentation mechanism (Modified from Papadimitriou et al. [160]).

Bacterial fermentation increases phenolic content (

Table 5. Bacterial fermentation of selected fruit pomaces.

Pomace	Strains	Findings	References
Apple	Lactiplantibacillus plantarum KKP 1527, L. rhamnosus	Increased phenolic and flavonoid content, especially gallic acid, procyanidin A2, protocatechuic acid, and procyanidin B2. Increased volatile compounds by 80%.	[162,163,169]
Orange	L. plantarum P10, L. plantarum M14, Bacillus subtilis BF2 and L. acidophilus LA-5	Co-fermentation with B. subtilis and L. plantarum raised the phenolic content by 133%. Improved dietary fiber and organic acid by 47% and 1429%, respectively.	[155,170]
Grape	L. plantarum, L. casei, and Bacillus subtilis natto DSM 17766	Increased antioxidant and antibacterial activities of the pomace. Bacillus subtilis fermentation efficiently produces cellulase.	[156,157]
Olive	Komagataeibacter intermedius	Bacterial cellulose was successfully synthesized. The bacterial cellulose has improved antioxidant activity.	[171]
Tomato	Propionibacterium shermanii	Fermentation produced vitamin B12 (11.1 mg/L).	[172]
Date fruit	Lactobacillus casei 431	Produced a high concentration of lactic acid.	[37]
Pear	Komagataeibacter rhaeticus	Produced bacterial cellulose and vinegar simultaneously.	[173]

) [161]. The high polyphenol content is due to the deposition of β-glucosidase during fermentation, which helps in catalyzing the hydrolysis of complex polyphenols, thus releasing more polyphenols, especially bound polyphenols [162]. Moreover, it is necessary to note that not all bacterial strains increase polyphenol; for example, Lactiplantibacillus plantarum ZFB 200 reduced polyphenol. This might be because of the consumption of polyphenols by bacteria through a detoxification mechanism [163]. The phenolic acid content varies over fermentation time and strains. For example, L. plantarum fermentation produced greater amounts of chlorogenic acid and caffeic acid on the second and fourth day, whereas L. acidophilus fermentation produced the lowest phenolic acids on the fourth day [164]. An increase in pH (1.5–3.5) increased anthocyanin content because of the partial degradation of pro-anthocyanidin oligomers to anthocyanins [165]. There are other possible transformations of polyphenols, e.g., the transformation of p-coumaric acid into flavonols, and this is catalyzed by enzymes such as reductase, decarboxylase, dehydrogenases, and β-glucosidase [164]. The accumulation of bioactive compounds, transformation of flavonoids to flavonol aglycones, is enhanced by the synergistic fermentation with lactic acid or acetic acid bacteria and yeast, thereby improving pomace appearance and decreasing anthocyanin breakdown [166]. The interaction between anthocyanins, lactic acid, acetic acid, and acetaldehyde preserves the fermented pomace color [167,168]. Bacterial fermentation produces volatile compounds (e.g., esters, alcohols, and aldehydes) which enhance the flavor and aroma of downstream products (e.g., beer flavoring agent, etc.) [169].

4.2.2. Yeast Fermentation and Fermented Products

Yeast fermentation is an anaerobic biochemical process in which yeast converts pomace residual sugars (glucose, fructose, or sucrose) into alcohol and other byproducts [174]. Glucose is first converted to pyruvate via glycolysis (Embden–Meyerhof–Parnas pathway), pyruvate is converted to acetaldehyde and carbon dioxide, which is then finally converted to ethanol and other byproducts (Figure 6). There are saccharomyces-based yeast species such as Saccharomyces cerevisiae, S. bayanus, etc., and non-saccharomyces species such as Pichia kudriazevii, Hanseniaspora uvarum, etc. Yeast can also secrete β-glucosidases. However, the amount varies for different yeast species and strains; for instance, H. uvarum has strong β-glucosidases activity while S. cerevisiae has low-to-moderate β-glucosidases activity. β-Glucosidase cleaves β-glycosidic linkages in cellulose, releasing glucose, which is converted to alcohol, or the β-glycosidic linkages in non-sugar moieties (e.g., phenolics, alcohol, terpenes, etc.), releasing glycosylated phenolic compounds (e.g., quercetin, kaempferol, resveratrol, etc.) and glycosylated aroma compounds (e.g., linalool, geraniol, etc.). Moreover, some yeast strains can secrete minor amounts of pectinmethylesterase, arabinofuranosidase, and β-xylosidase during fermentation [175]. Yeast fermentation is affected by several factors, including strain, pH, temperature, sugar source, nutrient availability, oxygen availability, and ethanol tolerance [176].

Yeast species like S. cerevisiae, S. bayanus, and H. uvarum were more effective in converting the residual sugars in pomace to alcohol than Saccharomycodes ludwigii, H. valbyensis, Metschnikowia pulcherrima, and Pichia guilliermondii [177]. The reasons for low efficiency by some yeast species is probably the inhibition of strain by ethanol, and/or the differences in the metabolic pathway, e.g., M. pulcherrima and P. guilliermondii have aerobic metabolism, which generates low alcohol amount and great amount of volatile esters, and the presence of Crabtree-negative strains of these species enables sugar consumption through the respiratory pathway, thereby reducing alcohol production. Ethanol production from apple pomace was greater during Saccharomyces fermentation than Hanseniaspora fermentation (

Table 6. Yeast fermentation of selected fruit pomaces.

Pomace	Strains	Findings	References
Apple	Saccharomyces cerevisiae, S. bayanus, Saccharomycodes ludwigii, Hanseniaspora uvarum, H. valbyensis	Increased protein, fat, dietary fiber, and phenolic compounds. Hanseniaspora showed lower ethanol production than Saccharomyces. Increased the aromatic richness of the apple pomace.	[175,177]
Orange	S. cerevisiae	Sterilized orange pomace supported yeast fermentation and produced greater flavor compounds. Immobilized cell technology protected yeast cells from hydrolysate inhibitors.	[179,180]
Grape	S. cerevisiae, Kluyveromyces marxianus	Produce white wine with better sensory characteristics. The yeast immobilized system had higher ethanol levels and volatile compounds.	[181,182,183]
Olive	S. cerevisiae	Increased crude protein content by 85%.	[184]
Tomato	S. cerevisiae	Increased soluble protein concentration.	[185]
Dates	S. cerevisiae and Pichia kudriavzevii	Increased malic acid production and volatile compounds. Increased antimicrobial activity and reduced antioxidant activity.	[186,187]
Pear	S. cerevisiae	Increased fruit pomace protein content and phenolic compounds.	[188]

) [175]. Pretreatment of pomace with diluted acid and enzymes (cellulase, pectinase, β-glucosidase, and laccase) improved ethanol production, because of increased enzymatic saccharification [135]. Ethanol yield is favored by acidic pH, but higher pH promotes the production of glycerol instead of ethanol [178]. The aromatic volatile compounds in pomace increased with fermentation, about 44 new volatile compounds were observed, especially esters [175]. Protein, fat, and dietary fiber increased with yeast fermentation; the fiber increase was majorly in insoluble fiber, while the soluble fiber decreased, and this is because of the pectinase activity of some Saccharomyces and non-Saccharomyces yeasts. Phenolic content in the pomace increased with S. cerevisiae fermentation but decreased when lactic acid bacteria and acetic acid bacteria were added to the fermentation system, which may be due to the reduction in enzyme activity. However, the addition of lactic acid bacteria or acetic acid bacteria to yeast fermentation increases flavonoids and decreases the degradation of anthocyanins [166,167].

4.2.3. Mold Fermentation and Fermented Products

Fermentation by filamentous fungi involves complex enzymatic degradation of plant biomass, leading to the release of fermentable sugars, phenolic compounds, organic acids, and other metabolites [189]. Mold fermentation is more effective than bacteria because of its ability to secrete a cocktail of extracellular enzymes (Figure 6) [190]. The extracellular enzymes secreted by molds include: cellulases (endo- and exo-glucanases and β-glucosidase), hemicellulases (arabinofuranosidase, xylanases, and β-Xylosidase), pectinases (pectinmethylesterase, polygalacturonase, pectate lyase), lignin-degrading enzymes (e.g., laccase, lignin peroxidases and manganese peroxidases), feruloyl esterase, and proteases [191]. Endo- and exo-glucanases break down cellulose into smaller units that are further broken into glucose monomeric units by β-glucosidase [192]. Arabinofuranosidase debranches hemicelluloses (e.g., xylan and arabinogalactan) by removing the arabinose side chains, thereby making the backbone more accessible. Xylanase cleaves the xylan backbone into xylo-oligosaccharides that can be hydrolyzed by β-xylosidase into xylose monomers [193]. Pectinmethylesterase first de-esterifies pectin, then polygalacturonase and pectate lyase cleave the de-esterified backbone [194]. Lacasse oxidizes phenolic lignin units to quinones under aerobic conditions, manganese peroxidase oxidizes Mn²⁺ to Mn³⁺, which in turn oxidizes lignin structures, while lignin peroxidases directly oxidize non-phenolic lignin components [195]. Feruloyl esterase cleaves the ester bonds between ferulic acid and arabinoxylan. Protease cleaves the pomace protein into short peptides and amino acids [196]. β-glucosidase can also cleave the glycosidic links in glycosylated phenolics and aromatic compounds, thereby releasing more phenolics and volatile aromatic compounds [197]. The released sugars and other compounds can be metabolized to value-added products such as organic acids, ethanol, etc.

Secreted enzymes can be collected and commercialized by industries. However, it is necessary to note that the types and amounts of enzymes secreted vary for different fungi strains, substrates, and fermentation conditions. For instance, enzymes like xylanase and exo-polygalacturonase were produced from grape pomace using Aspergillus awamori in a solid-state fermentation, and xylanase and exo-polygalacturonase activities reached maximum at 36 h and 48 h, respectively (Table 7) [198]. High moisture pomace (>65%) has lower enzyme activity because of the sticky texture, lower porosity, particle structure alteration, and oxygen movement, while low moisture pomace (<65%) had higher enzyme activity because of the nutrient’s diffusion in the solid substrate, lower swelling degree, and increased water tension [198]. Polygalacturonase production from strawberry pomace was greater than from apple and cranberry pomaces due to its higher pectin content [199]. Enzymes (xylanase, pectinase, cellulase, β-glucosidase) production from unmilled pomace was higher compared to milled pomace [200]. High temperature (above 50 °C) and pH (7.5 or above) inactivated polygalacturonase. Additives added during mold fermentation can either stimulate or inhibit polygalacturonase activity; for example, manganese and iron supplementation stimulate the polygalacturonase activity, but magnesium, Tween 80, and Triton X-100 inhibit polygalacturonase activity [201]. The addition of yeast extract during submerged fermentation increased polygalacturonase production [201]. The addition of glucose (more than 6%) repressed the xylanase and exo-polygalacturonase activity. Supplementation of pomace with wheat favors enzyme production, because some mold strains like A. fumigatus require nitrogenized material for their growth [200].

Citric acid was produced from apple pomace using A. niger, and optimum yield was achieved on the fifth day of fermentation, incubation temperature (30 °C), and methanol concentration (4%) [202]. Mold fermentation has been used as a medium for the enrichment of pomace nutrients; for instance, Candida utilis and Pleurotus ostreatus were used individually and sequentially for the nutritional enrichment of apple pomace [203]. Fermentation with Candida utilis increased the protein content and minerals by 100% and 60%, respectively, while sequential fermentation with C. utilis and P. ostreatus increased the protein level of the apple pomace by 500% after 60 days of fermentation. The protein increment was not favored by low pH (<5.5), which was why fermentation with P. ostreatus alone showed less protein increment compared to C. utilis fermentation. Four different Aspergillus species (A. aculeatus, A. japonicus, A. niger, A. tubingensis) were used in the enhancement of the phenolic compounds in apple peel [204]. The phenolic and flavonoid contents in fermented apple peels by A. niger and A. tubingensis were higher compared to A. japonicus and A. aculeatus. Furthermore, the Aspergillus species affect the production and concentration of new phenolics, for example, A. niger and A. tubingensis produced catechin and eriodictyol isomers whereas, A. aculeatus and A. japonicus produced taxifolin isomers.

Table 7. Mold fermentation of selected fruit pomaces.

Pomace	Strains	Findings	References
Apple	Aspergillus aculeatus, A. japonicus, A. niger, A. tubingensis, and A. niger	Reduced quercetin and its glycosides. Increased phenolic and flavonoid content. Maximum citric acid yield (4.6 g/100 g pomace) was achieved at 5 days of fermentation, 30 °C, and with 4% methanol.	[202,204]
Orange	Sporotrichum thermophile Apinis, A. niger, and Paecilomyces variotii	Increased the production of polygalacturonase, pectinase, tannase, and phytase. Increased antioxidant activity.	[205,206]
Grape	A. awamori, Actinomucor elegans, and Umbelopsis isabellina	Substrate particle size did not affect enzyme production. High moisture substrate had lower enzyme activity. Phenolics, flavonoids, and antioxidant activity were higher in A. elegans fermented pomace.	[198,207]
Olive	Pleurotus ostreatus, P. pulmonarius, A. niger, and A. ibericus	Improved protein content and reduced polyphenols and hemicellulose. Increased enzymes (xylanase and cellulase) production.	[193,208,209]
Tomato	A. awamori, P. ostreatus, and Phanerochaete chrysosporium	Produced higher xylanase compared to exo-polygalacturonase, cellulase, and α-amylase. Aeration during fermentation enhanced xylanase and cellulase production but inhibited exo-polygalacturonase and α-amylase.	[189,210]
Dates	A. niger	Efficiently produced endopectinase and increased phenolics and flavonoids.	[211]
Pear	A. niger and A. oryzae sp.	Enriched the bioactive compounds, particularly tannins.	[212]

Table 7. Mold fermentation of selected fruit pomaces.

Pomace	Strains	Findings	References
Apple	Aspergillus aculeatus, A. japonicus, A. niger, A. tubingensis, and A. niger	Reduced quercetin and its glycosides. Increased phenolic and flavonoid content. Maximum citric acid yield (4.6 g/100 g pomace) was achieved at 5 days of fermentation, 30 °C, and with 4% methanol.	[202,204]
Orange	Sporotrichum thermophile Apinis, A. niger, and Paecilomyces variotii	Increased the production of polygalacturonase, pectinase, tannase, and phytase. Increased antioxidant activity.	[205,206]
Grape	A. awamori, Actinomucor elegans, and Umbelopsis isabellina	Substrate particle size did not affect enzyme production. High moisture substrate had lower enzyme activity. Phenolics, flavonoids, and antioxidant activity were higher in A. elegans fermented pomace.	[198,207]
Olive	Pleurotus ostreatus, P. pulmonarius, A. niger, and A. ibericus	Improved protein content and reduced polyphenols and hemicellulose. Increased enzymes (xylanase and cellulase) production.	[193,208,209]
Tomato	A. awamori, P. ostreatus, and Phanerochaete chrysosporium	Produced higher xylanase compared to exo-polygalacturonase, cellulase, and α-amylase. Aeration during fermentation enhanced xylanase and cellulase production but inhibited exo-polygalacturonase and α-amylase.	[189,210]
Dates	A. niger	Efficiently produced endopectinase and increased phenolics and flavonoids.	[211]
Pear	A. niger and A. oryzae sp.	Enriched the bioactive compounds, particularly tannins.	[212]

5. Technologies Used in the Enzymatic Treatment and Microbial Fermentation of Pomace

Enzyme immobilization is a technique where enzymes are physically confined or localized in a defined region of space while retaining their catalytic activity [213]. The main methods of enzyme immobilization include adsorption, covalent binding, entrapment, encapsulation, and cross-linking. Advantages of the enzyme immobilization technique include enzyme recyclability, ease of separation of enzymes from the reaction mixtures, improved enzyme efficiency, controlled reaction, greater thermal and pH stability, and less cost-intensive [214]. Immobilized enzymes have been used for the extraction of phenolic compounds, where co-immobilization of multiple enzymes improves the extraction yield and shortens the reaction time [213]. Extraction of phenolic compounds from fruit peels by co-immobilized nano-biocatalyst and mixture of immobilized enzymes provided a two-fold higher yield than conventional solvent extraction. Enzyme micro-ionization involves reducing enzymes into micro-sized particles (micro-ionization) to enhance their solubility, surface area, and activity, especially in formulations where enzymes must disperse efficiently. Micro-ionized cellulase and xylanase increased the extractability of soluble dietary fiber and improved its structural and functional properties [215].

Technologies used for microbial fermentation can be categorized based on the physical state into submerged fermentation and solid-state fermentation [216]. In solid-state fermentation, microorganisms, mostly fungi, grow on the solid substrate in a low moisture environment [217]. Merits of solid-state fermentation over submerged fermentation include less generation of wastewater, minimal or no pretreatment of material for bioconversion, growth of microorganisms in non-water-soluble material, stability, high concentration, and low cost [155,197]. In industries, solid-state fermentation has been used as an effective strategy to lessen the environmental problem caused by solid waste disposal [197]. The performance of solid-state fermentation is affected by several extrinsic factors, including moisture content, temperature, particle size, air flow, etc. [218]. For a typical solid-state fermentation, the substrate is first dried to a low moisture content (5–10.5%), and milled, followed by the incubation or inoculation of the substrate and selected microbial spores in an Erlenmeyer flask using specific process conditions. Solid-state fermentation facilitates the release of substances that are not easily extractable from the material and can also enrich the substrate by forming new compounds that are not originally in the material, or the production of new products. For instance, solid-state fermentation increased the extraction of polyphenols from pomegranate peel by 5.96-fold [219].

Submerged fermentation uses free-flowing liquid substrates like broths and molasses. It is primarily used for recovering secondary metabolites that are required in liquid form. During submerged fermentation, substrates are rapidly consumed; therefore, they need to be supplemented and replaced with nutrients constantly. Bacteria are more adaptable to the submerged fermentation technique because of their high moisture requirement. An advantage of the submerged fermentation technique is its ease of purification [220]. For typical submerged fermentation, the substrate was first completely soaked in water, sterilized, and then inoculated or fermented in a microbial suspension in a screw-capped bottle under selected conditions [221]. Submerged fermentation has some unique advantages over SSF, such as less fermentation time, large volume processing, ready scale-up, simple parameter control processing, and lesser labor requirement; however, the high viscous substrate used in submerged fermentation can hinder the heat and mass transfer, which affects yield and efficiency [222,223]. For example, submerged fermentation required less fermentation time to achieve 10% protein hydrolysis compared to solid-state fermentation using L. plantarum, although the protein yield of the solid-state fermentation was higher than that of submerged fermentation [224].

6. Challenges and Concerns of Utilization of Fruit Pomaces in Food Industries and Possible Solutions

The utilization of fruit pomaces generated in food industries supports sustainability, innovation, and nutrition; however, it is faced with challenges and technical concerns. For instance, the high moisture content in fruit pomaces makes it susceptible to microbial deterioration and causes storage and transportation difficulties [225,226]. It is necessary to reduce the moisture content of the pomace with cost-effective drying methods such as solar or hot air driers, or store with a modified atmosphere packaging technique. The short shelf life of pomace is caused by enzymatic activity and microbial growth due to the residual sugars and other fiber components [226]. This can be curbed by instant stabilization techniques (drying, blanching, etc.) or refrigeration/freezing until the time of usage. Most fruit juice processing industries produce juice from different fruits, which might require different processing methods. Also, the batches of fruits processed in the industries might vary in cultivars; therefore, the factory generates pomaces having varying nutrient and fiber content [227,228]. Establishing a standard baseline composition and blending the various batches might reduce such challenges. The recalcitrant nature of pomace fiber, caused by the lignification of the cell wall, limits the complete utilization of the fiber components to value-added products, for example, the enzymatic hydrolysis of cellulose to glucose, which can be utilized by microorganisms during fermentation to produce lactic acid. Some delignification techniques like deep eutectic solvent, Organosolv, alkaline pretreatments, physical processing, etc., can be used [226]. Anti-nutritional components, contaminants (e.g., heavy metals, pesticides, natural toxins, etc.), and phenolic inhibitors can be present in the pomace, and this can be reduced by following strict quality control and the use of decontamination and detoxification techniques (e.g., enzymes, fermentation, etc.) [226,229]. The bitterness, grittiness, or astringency of some fruit pomaces might affect their valorization, especially when incorporating into food products, and this can be solved by adding complementary flavor, encapsulation, and milling to fine powder [226,229]. There are regulatory and labeling issues affecting the utilization of byproducts; industries wanting to utilize pomaces, especially in food products, must comply with the food safety requirements [226,229,230]. Collection, storage, and transportation of fruit pomaces is cost-intensive; the onward valorization unit should be close to the pomace generation site [225,226,230]. Finally, consumer awareness of the huge benefits of pomace utilization is low; strategies like marketing, branding, and approval from food regulation agencies can serve as a boost [230].

7. Concluding Remarks

Valorization of fruit pomaces and other food processing byproducts is currently highlighted to reduce environmental pollution and support the idea of the circular economy. In this review, we have discussed pomace valorization methods and strategies, with a focus on enzyme treatment and microbial fermentation. The composition of fruit pomace remaining after juice extraction includes fibers, residual sugars, protein, and other compounds (polyphenols, waxes, etc.). The valorization strategies include direct incorporation into food, extraction of bioactive compounds, isolation of specific fiber components, production of prebiotics, enzymatic extraction and modification of fiber components, and microbial fermentation. Microbial fermentation promotes the bioconversion of fruit pomace into carbohydrates, organic acids, alcohols, enzymes, phenolic and other bioactive compounds, etc., whereas enzymatic hydrolysis enables the production of prebiotics, more nutritious food, and isolation of fiber components. In summary, the valorization of fruit pomace is a promising strategy that reduces waste and adds value to an often-neglected resource. Several bioactive compounds can be produced from pomaces and can be used as functional ingredients, nutraceuticals, and pharmaceutical products. Future research is required for developing more innovative sequential processes for the total utilization of pomace, as this will help to combat the growing tonnage of edible food waste.