Very recently, phenolic compounds from rich-grape marc (or pomace) from the País winery, an abundant resource suggested as having a possible in animal feed, were considered for potential positive environmental effects on enteric CH4 emissions in Chile by Suescupun-Ospina et al. (2023) [85]. The authors evaluated the effect of the oven- and freeze-drying process and showed that oven-dried grape marc had the highest proanthocyanidin content and lower CH₄ production [83].

The production of cleaner fuels can be accomplished by improving the refining of environmentally friendly technologies and by adding synthetic fuels or ethanol [48]. Within environmentally friendly technologies, we use residual biomass to synthesize bioethanol through the fermentation process of substrates with a high carbohydrate content for degradation. The use of sugarcane juice to synthesize bioethanol is already a reality. However, this substrate competes with the food industry [144]. To resolve this problem, other biomass resources, such as byproducts and wastes from fermented food factories, stand out as substrates since they are also rich in the carbohydrates needed for fermentation [77,79,97].

The cellulose and hemicellulose in lignocellulosic biomass can be chemically or enzymatically degraded, generating hydrolyzed sugars that provide raw materials for microorganisms to act upon during fermentation [145]. Lignin, the third component, is formed by aromatic polymers, and its catabolism generates a mixture of aromatic components that can be enzymatically converted into a set of organic molecules in the form of D-3,2-butanediol [79,143], methane [64], poly(3-hydroxybutyrate (PHB) [97] and PHA [69], and tartrate or tartaric acid [97].

Unlike bread waste, which is rich in fermentable sugars for H₂, ethanol, methane, and recovery, some biomass residues, such as wood bark, are not easily converted into biofuels through the application of microorganisms. The use of the gasification process with these residues produces a synthesis gas (syngas) that offers a solution to this problem, such as the conversion of CO and H₂ (which are present in the syngas) into multicarbon compounds [146]. Such an approach may be extended to other fermented biomass residues.

The production of fuels covers the use of various residues, both from agriculture and the food industry. Food waste occurs in large quantities worldwide; thus, anaerobic digestion can be employed, including H₂ in the fermentation stage [76] and CH₄ in the methanogenesis stage [77]. This process, also called dark fermentation, uses an environmentally friendly approach to solve the problem of food waste and delivery residue to synthesize industrial products. Several microorganisms can be used, including hydrolytic, acidogenic, and acetogenic bacteria, to form VACs, metabolites, CO_2, CH₄ [64] alcohols [77], and H₂ [76]. The use of residues for H₂ synthesis is still being optimized for use on an industrial scale, which brings other possibilities for study and evaluation regarding the topic [147,148].

Moreover, it has already been proven that dual-stage H₂ production achieves 20% more biogas than single-stage production [149]. In the methanogenic phase, the VACs thus produced are transformed into CH₄ and are recovered as biofuel, whereby part of the methane remains dissolved and can be treated for application in biofertilizers [150]. Dark fermentation encompasses the use of waste to synthesize environmentally friendly products for industrial commercialization. Thus, its optimization is essential for food and agro-industrial waste re-usage [151]. Figure 4 exemplifies the synthesis steps of some biofuels via dark fermentation.

To produce a range of compounds such as acetate, ethanol, butyrate, and butanol, fermentation by microorganisms depends on acetyl coenzyme A (acetyl-CoA). The oxidation of H₂ to 2H⁺ with H₂O, or of CO with H₂O to 2H⁺, provides the reducing equivalents for the reduction of CO₂ to form methanoic acid (HCOOH), from methylene tetrahydrofolate (CH-THF) to methenyl tetrahydrofolate (CH₂-THF), from CH₂-THF to methyltetrahydrofolate (CH₃-THF), and from CO₂ to CO. Acetyl-CoA synthase/CO dehydrogenase catalyzes the formation of acetyl-CoA from methyl group bonding, CO group bonding, and coenzyme A (CoA). The metabolic pathways of acetyl-CoA reduction, which are involved in the fermentation of biomass conversion into a range of biofuel compounds, are displayed in Figure 5 [151,152,153].

Hydrogenogenic carboxydotrophic microorganisms (HCM) conserve energy by synthesizing H₂, in which a monofunctional CO dehydrogenase oxidizes CO. The electrons from the oxidation process undergo the reduction of protons in hydrogen molecules through an energy-converting hydrogenase (ECH). Furthermore, ECH couples H₂ to the membrane translocation of protons/sodium ions, which generates an ion gradient, driving ATP synthesis. HCM use is independent of the acetyl-CoA pathway [151,152,153].

Bread and bakery product wastes have an interesting composition (up to 70% of carbohydrates, mainly starch), which is attractive as a nutrient source for microorganisms and can improve the circular economy by reusing energy-rich food wastes for gas production [77] and energy recovery [76]. Bread waste was employed to produce organic acids by lactic fermentation (Lactobacillus amylovorus DSM 20532) and was sequentially employed for hydrogen (H₂) production (3.1 mol H₂ mol⁻¹ glucose) and energy recovery (54 MJ t⁻¹ dry waste) by photo-fermentation (Rhodoseudomonas palustris 42OL) [76]. Using biorefinery concepts and a circular economic approach, bread waste was employed to produce bioethanol [77], butanediol [78,79], and biomethane [77]. The fermentable sugars from out-of-date bread supplied by local supermarkets in the United Kingdom were first submitted to acid/enzymatic saccharification to recover glucose-rich hydrolysates, which were sequentially fermented with Saccharomyces cerevisiae KL17 to produce ethanol with higher efficiency, using the fed-batch mode of cultivation [77]. In further experiments, Narisetty et al. (2022) [77] employed solid residues (from acid/enzymatic hydrolysis, along with the respective SSF residues after ethanol production) under anaerobic digestion to yield biomethane, revealing a biochemical methanation potential of up to 379 mL CH₄/g. Likewise, other studies have previously reported glucose-rich enzymatic hydrolysate from bread waste being submitted for fermentative production by Bacillus amyloliquefaciens [78] and by Enterobacter ludwigii [79] to accumulate D-2,3-butanediol.

The continuous production of plastics from the petrochemical industry has generated an exorbitant amount of waste for disposal. Such waste is part of approximately 140 million tons of plastics produced in 2022, and an estimate of plastic waste in the ocean has reached a level of 150 million tons [154,155]. Figure 6 shows a forecast for global plastics generation from 2002 to 2050 [156,157].

This challenging scenario has motivated researchers to evaluate biodegradable plastics produced by the fermentation of microorganisms to replace petroleum-based plastics [158,159,160,161]. The production of biopolymers from microbial fermentation is a less expensive, adaptable, and environmentally harmless process when compared to producing plastics made from fossil fuels. However, bioplastics are susceptible to water hydrolysis, which reduces their strength and durability [161,162]. The synthesis of bioplastics, such as polylactic acid (PLA) and polyhydroxyalkanoates (PHAs), derives from renewable sources such as food waste, lignocellulose residues, and vegetable oils [161,162]. The purpose of studies concerning bioplastic production through microbial fermentation is to create plastic with mechanical, physical-chemical, and thermal properties to match the performance of fossil-based plastics. On the other hand, the high biodegradability of bioplastic makes it environmentally friendly, reducing plastic waste due to its shorter product lifetime [162,163].

Although it is possible to produce synthetic biodegradable polymers from petrochemical byproducts such as polycaprolactone (PCL) [164,165], several methods of bioplastic synthesis from renewable resources use natural polysaccharides (lignin, cellulose, and starch), proteins, and lipids. Other examples include polyhydroxyalkanoates (PHA) [154,166], polyhydroxy butyrate (PHB) [155], and poly(3-hydroxybutyrate-co 3-hydroxy valerate) (PHBV) [158,167]. The third alternative uses microbiological fermentation, whereby lactic acid is synthesized and polymerized to form a PLA-biobased product [14].

Under biorefinery concepts, wine lees (the liquid and solid residual fractions, remaining streams, and alcohol-free nutrient-rich liquid) from producing Merlot red wine were employed as a bioresource to produce bioplastic poly(3-hydroxybutyrate) (PHB) synthesis. This included the production of generic fermentation feedstock (ethanol, phenolic compounds, and tartaric acid) [97]. Thus, the crude nutrient-rich hydrolysate from enzymatic hydrolysis was used to initiate PHB production with high efficiency using the strain Cupriavidus necator DSM 7237, while the hydrolysis of pretreated lees was performed using crude enzyme consortia via solid-state fermentation with Aspergillus oryzae [97].

Demirci et al. (2019) and Jung et al. (2022) produced paramylon and xanthan gum biopolymers from fermentable sugars obtained and extracted from bread waste (stale bread that remained unsold and returned goods from shops). After the enzymatic hydrolysis step, the glucose thus obtained was employed in both studies as a bioresource: i) as a carbon substrate for the heterotrophic cultivation of the microalgae Euyglena gracilis to produce the carbohydrate paramylon [80]; ii) as a carbon source in the fermentation of Xanthomonas spp. to obtain the xanthan gum polysaccharide [82]. Xanthan gum is widely used in the food industry as a common additive to stabilize and emulsify food products.

Although up to 50% of residual whey from cheesemaking is being valorized as a source of high-added-value compounds in food/pharmaceutical factories, mainly as proteins with high levels of biological properties, lactose, lactic acid (LA), and minerals, there is still a high proportion of this whey that is discarded, thus promoting environmental pollution [168]. An integrated cheese whey valorization process [69] utilized the wastewater from cheesemaking to produce several added-value products, including lipid recovery via thermocalcic precipitation, protein recovery via ultrafiltration, and lactose valorization through biological processes to obtain polyhydroxyalkanoates (PHA). Therefore, this circular economic approach can provide lactose as a carbon source for biomolecules, lipids, and proteins to produce bioplastic PHA for use as a flame retardant, along with whey protein concentrates for the food industry. It can also potentially obtain lactic acid (LA), a monomer of PLA derived from natural sources. Thus, LA is produced using the bacterial fermentation of fermentable biomass. Likewise, reducing sugars from bread wastes drawn from Indian bakery wastes were simultaneously submitted to saccharification and SSF by LAB, suggesting bread wastes as potential candidates for lactic acid production [84]. Those findings are interesting because PLA (C₃H₄O₂)_n is a biodegradable thermoplastic that can be synthesized using residues such as sugarcane bagasse, corn cobs, starch, and food waste [42,169]. PLA is a well-known biopolymer with many applications and properties, such as biodegradability, biocompatibility, elasticity, moldability, and rigidity. In addition, it is the first polymer produced from renewable sources to be marketed for various applications, such as for grocery bags and food packaging [42]. PLA synthesis has, as an initial step, the production of LA by the fermentation process. Lactic acid comes in the form of a white powder or yellow liquid. Its structural formula incorporates chiral carbon, which leads to the formation of a left-handed (L⁺) and a right-handed (D⁻) structure. The chemical activity of the LA molecule depends on the acidity in an aqueous medium and the functional activities of groups present in the chemical structure, such as the carboxyl and hydroxyl groups, which promote a wide variety of chemical reactions [13,169]. The global market for PLA is constantly expanding, which causes an increase in its demand due to its widespread use in the packaging, agriculture, and transportation industries. Therefore, an efficient manufacturing method with reduced production costs is needed so as to evaluate other methods that can be employed when using agricultural and food waste [161].

Nanotechnology is being applied in several areas and sectors of industry, such as food, cosmetics, and chemistry. Some studies have demonstrated the importance of nanomaterials based on fermented food residues or byproducts and several applications on health and food. A nanocomposite based on kefiran films has garnered research attention due to the electrospinning and film-forming abilities of the kefiran polysaccharides recovered from kefir grains. Kefiran, an exopolysaccharide, derived from the microflora of kefir grains, which are used to produce fermented milk beverages, has recently attracted research attention due to its biological properties and its potential for use in the nanomedicine and food packaging industries [30]. Recent investigations reported a kefiran biopolymer, manufactured via electrospinning, to produce kefiran nanofiber [170,171]. Kefiran nanofibersis potentially valuable for encapsulating bioactive ingredients in food [172], as matrix for probiotic/drug delivery [72,73,74,173,174], andscaffolds for regenerative medicine and tissue engineering [170].

Fermentation residues from Ca/Fe-rich antibiotics, for example, can be considered very hazardous but, at the same time, can be recycled and applied in adsorption resources by using biochar [175]. Despite the possibilities, such hazardous wastes have seldom been explored and require several stages of waste preparation. In one study, residual vancomycin and antibiotic-resistant genes were fully exposed during pyrolysis. The process showed fast kinetics and a maximum adsorption rate of 102 mg p/g. Ca and Fe represented active sites that helped in the adsorption process. Therefore, the use of biochar produced from fermentative residues can be used in the treatment of wastewater to absorb contaminants. Furthermore, biochar can be used as a phosphate fertilizer, as it promotes seed germination (germination rate: 96.7% vs. 80.0% in the control group, p < 0.01) and seedling growth (shoot length was increased by 57.9%, p < 0.01) due to the slow release of the available phosphate. Consequently, hazardous waste was turned into phosphate fertilizer, adding to the benefits of biomass reutilization, and recovering phosphate from wastewater. Biochar from spiramycin (SPI) fermentation residues (SFR) in China, which are dangerous, can also be cited here. The pyrolysis method was adopted to convert SFR to biochar to remove SPI from wastewater, and the results showed no residual SPI, indicating the achievement of SFR detoxification. Furthermore, after recycling 5 times, the SPI removal efficiency was still greater than 80.0%, indicating a promising method for SFR disposal [176].

Lipstatin, a fermentative residue (LFR) from the pharmaceutical industry, can be discarded in the soil after the composting stage due to its high content of organic matter. Generally, residues from such fermentation processes are composed of a high amount of organic matter, which is of high importance in fertilization. The pH value of soil fertilized with composted LFR slightly decreased, without the accumulation of lipstatin. Soil nutrients, including the available phosphorus, potassium, organic matter, and soluble organic matter, increased significantly in soil fertilized with composted LFR, providing excellent application as a biofertilizer [177]. Likewise, the synthesis of doramectin offers another interesting example of the use of fermentative residue from the pharmaceutical industry, which generates a high rate of residues with a large amount of organic matter, resulting in their applicability in agricultural fertilization [178].