Microorganisms require a controlled atmosphere to produce better final quality, ensuring optimal productivity and better benefits [30]. SF has some process characteristics which have slight advantages over control in solid-state fermentation, which are shown in Table 1.

In submerged fermentation processes, nutrients are used, and some enzymes such as tannase, amylase, and glucoamylases are released, to name a few [37]. It is the most widely used type of fermentation in the industry due to its simplicity, and its final product is easier to recover. The development of microorganisms is typical, which gives rise to a dormancy phase, growth, stationary phase, and death phase. These phases are present in submerged and solid-state fermentation, divided into several types—batch, continuous, and fed—according to the input and output of the substrate and product, and are used for the bioremediation of industrial effluents [32]. In the SF, the initial water content and inoculum and substrate are important; the conditions of the culture medium can produce a wide range of bioactive compounds that have benefits for human health [38]. It is the most widely used in the food industry to obtain enzymes with different substrates and microorganisms.

Solid-state fermentation is an effective method to produce microbial enzymes and bioactive compounds [46], allowing the accumulation high-value products. Its main products are enzymes produced using agro-industrial waste. Microorganisms grow by joining solid substrates, imitating their habitat, giving a fruitful result [49]. In this process, the substrate and nutrients can be reused, although this type of fermentation does not apply to bacteria because they require a high-water content [30]. However, it is much more attractive because its chemical and physical processes are low cost, simpler, and have better yields; it allows reproducible conditions similar to the environment in which microorganisms live [45]. Solid-state fermentation has advantages in terms of the final product concentration, has a lower water expenditure, requires less sterile conditions, and has a low repression effect [50]. It is a slightly more laborious process because the growth of the microorganism is slower; there is a low transfer of oxygen and carbon dioxide, heat removal, and it can easily be contaminated [32]. A fundamental stage is the culture media preparation to ensure the fermentation process productivity [51]. The problem that may arise is that the culture substrate is uneven, and changes in temperature, pH, humidity, and substrate concentration lead to making it less reproducible [52]. The water content is an extremely important parameter to produce the desired final product [53]. Growth is carried out on a solid substrate almost without water, but with enough to tolerate growth and metabolism. In solid-state fermentation, the particle size and the humidity level are the most critical factors, as well as the initial pH, substrate, aeration, temperature, inoculation, and incubation [12]. Figure 2 shows the production process of enzymes from agro-industrial waste through solid-state and submerged fermentation.

Tannases can be produced from microorganisms, plants, and animals, even though the most widely used microorganisms are fungi, yeasts, and bacteria. Aspergillus and Penicillium are the most used genera in tannase production; around 27 and 24 species, respectively, have been evaluated for tannase production. Paecilomyces variotii is the best tannase producer using grape pomace as substrate [54]. Four species of Trichoderma and three species of Fusarium have been also evaluated. Regarding bacteria, Lactobacillus is the most studied [35], mainly Lactiplantibacillus plantarum, but Bacillus spp. and Corynebacterium spp. have been also studied [6]. Lactic acid bacteria (LAB) degrade gallotannins to gallic acid, but they live under extreme conditions [25]. Bacteria can change and repair the membrane to reduce the effect of tannins on the digestive tract, increasing the number of unsaturated fatty acids, modifying their isomerization [31].

There are a variety of microorganisms that produce tannases that have been studied for a long time in terms of process, development, and application; the microorganisms for the activation and production of tannases are listed in Table 2.

Lactic acid bacteria (LAB) have important applications in the food industry and other sectors, as shown in Table 3, for the added value they give to products, influencing the taste, odor, texture, nutritional value, and sensory characteristics. The genus Leuconostoc, Enterococcus, Streptococcus, Pediococcus, and Lactobacillus [42] LAB have been used in submerged fermentation to produce tannases [6]. They are made up of a large group of Gram-positive, non-sporulated bacteria and are the largest producers of lactic acid; they have preservative power because they have antimicrobial properties and have a high content of organic acids and bacteriocins that inhibit the growth of bacteria [58]. LAB are present in groups of microorganisms called starters or lactic cultures, where lactic acid is their main metabolite. LAB are unable to synthesize some vitamins, specific peptides, and amino acids for their growth but can produce antimicrobial substances known as bacteriocins and hydrogen peroxide [59]. They are mesophilic, but some can grow in refrigeration at 4 °C, and others at 45 °C, with a preference for an acidic pH (4–4.5), although sometimes tolerate up to pH 9. They play an important role in biotransformation because they can act on the isolation of new chains of tannases. Among the LAB, the first to be tested to produce tannases was Lactiplantibacillus plantarum [6]. One of the least used microorganisms to obtain tannins through fermentation by LAB [60], there are reports that they degrade and hydrolyze natural tannins and tannic acid efficiently [29]. Their cell wall contains glycerol, teichoic acids, and peptidoglycans [61]. On the other hand, Lactiplantibacillus plantarum was used to produce tannases from olive mill residues using a basic medium, plus 100 mL supplementation with tannic acid as a carbon source and inoculated with 1.38 mg of cells which were incubated at 37 °C for 24 h [13]. It is an optional heterofermentative; it can produce lactic acid and other by-products such as acetate and ethanol, depending on its growing conditions and its carbon source [62]. Lactiplantibacillus plantarum is the most widely used bacteria because it has the greatest yield on plant fermentations where tannins are abundant due to their strains having tannase activity [31], playing an important role in food microbiology and human nutrition for their fermentative power and probiotic functions. It also has effects on cholesterol levels, inhibiting inflammation by acting on interleukins IL-1α, IL-1β, IL-6, and tumor necrosis factor α [59]. The importance of the growth of Lactiplantibacillus plantarum stands out in the technological and physiological characteristics of the probiotic strains, the amino acids valine, arginine, tyrosine, and tryptophan, pantothenic acid and nicotinic acid which are essential, as well as manganese and iron [63]. While the growth during a Lactobacillus fermentation occurs at a pH of 4.5 to 5.8 [59]. Tannase production with Lactiplantibacillus plantarum occurs at 37 °C, with pH 6.0, within 24 h, with tannic acid as a carbon source and the presence of nitrogen ammonium sulfate [64]. Several studies have been carried out obtaining the optimized fermentation process with Lactiplantibacillus plantarum with a maximum tannase activity of 6 U ml-1 [13].

Agro-industrial wastes rich in polyphenolic compounds can be used as a source of carbon and energy so that the microorganism can develop and produce tannases. This section describes some research papers where they used agro-industrial waste to produce tannases with the help of microorganisms. For example, coffee pulp rich in polyphenolic compounds as a source of carbon and energy were evaluated through a solid-state fermentation process for tannase production. The authors concluded that the utilization of this wastes for tannase production has potential applications in the food industry [74]. In another study, solid-state fermentation was used for tannase production, using a medium containing pomegranate peels (10%) and other salts with pH adjusted to 7.0. These production conditions were compared to define the optimal ones, and various carbon sources, including Moringa cake, palm kernel, palm fronds, wheat straw, olive cake, and pomegranate peels were evaluated [75]. Tannase production from solid-state fermentation using coir waste as a substrate was performed. In the reactors, 10 g of the substrate were added and subsequently incubated at 30 °C for 48 h [76]. The tannase-biotransformed grape pomace extracts were evaluated to prove intestinal anti-inflammatory activity using an in vitro colon model. This research mentioned that the biotransformation improved the anti-inflammatory effect in Caco-2 cells, indicating that the application of a bioprocess in grape residues may contribute to the development of new ingredients and nutraceuticals with anti-inflammatory potential [77]. Various agricultural wastes (coffee husk, tamarind seed powder, bagasse, groundnut oil cake, wheat bran and rice bran) and solid substrates (bengal gram powder (Cicer arietinum), black gram flour (Vigna mungo L.), green gram flour (Phasleolus aureus), barley (Hordeum vulgare), millet flour (Peninsetum glaucum), ragi (Eleusine coracana), oats (Avena sativa), corn flour and rice flour) were evaluated to produce tannase by Lactiplantibacillus plantarum [78]. In another study, agro-industrial wastes were shown to be generally rich in tannins and can be used as low-cost substrates for tannase production. Agro-industrial wastes as potential substrates were evaluated for enhanced tannase production using solid-state fermentation. Rice bran showed the highest tannase activity during solid-state fermentation, followed by brewer’s rice, spent coffee grounds, and desiccated coconut waste [48]. The cost-effective substrate Triphala was used as a tannin and energy source for tannase production through solid-state fermentation. In this research, statistical experimental design with Triphala at optimized conditions showed 6.1-fold higher tannase activity than that obtained in submerged fermentation [43]. Tannase production was optimized using pomegranate husk as a solid substrate. Easily available and low-cost substrates such as cow dung, coffee husk, apple peel, and pomegranate peel were initially screened for tannase production [79]. On the other hand, a cocktail of hydrolytic enzymes was performed by solid-state fermentation using as substrate a mixture of grape pomace and wheat bran. They concluded that the use of solid-state fermentation is a less costly alternative to obtain an enzymatic cocktail that can be used in the food industry [80].