Legumes belong to the Fabaceae family and fall into 11 main categories: chickpeas, cowpeas, bambara beans, beans, fava beans, lentils, lupins, peas, pigeon peas, vetches, and minor pulses ^[1][44]. The most consumed legumes today include soybeans, peas, chickpeas, lentils, and beans ^[2][3][45,46].

In comparison to cereals, legume seeds contain a high level of protein, ranging from 20% to 45%, while cereals generally contain around 6% to 15% protein ^[4][5][47,48]. In addition, legumes contain dietary fiber (5–37%) ^[1][44], and carbohydrates (6–62%) ^[6][49]. As an example, lupine, an underutilized legume crop, contains 34 g of protein per 100 g of dry matter, while soybeans and fava beans contain 36.5 g and 27.2 g of protein respectively. The carbohydrate content of these legumes is 9.5 g/100 g, 30.2 g, and 46.5 g, respectively ^[7][50]. Legumes generally contain healthy fats and have no cholesterol ^[1][44], and they are also rich in essential minerals particularly Ca, K, P, Cu, Fe and Zn ^[8][9][51,52], and antioxidants, oligosaccharides, polyphenols and bioactive compounds ^[10][53]. Legumes provide a significant amount of folate and thiamine as well ^[8][11][12][51,54,55]. It is reported that, legumes typically contain 380–660 mg/100 g of arginine, 650–820 mg/100 g of aspartic acid, and 975–1150 mg/100 g of glutamic acid ^[7][50] which are the most abundant amino acids in legumes. These compositional attributes highlight the potential use of legumes as functional food ingredients.

Despite the many benefits that legumes provide, they have some qualitative limitations that negatively affect their sensory profile, techno-functionality, and nutritional quality ^[13][14][15][56,57,58] and consequently their use in a variety of food products. 

Flavor (aroma and taste) is one of the main factors that determine the acceptance of food products. It is determined by the chemical composition of the food matrix and the structure related to the release of flavor ^[16][59]. Legumes are often associated with unpleasant aroma compounds (e.g., green, earthy, grassy, leafy, astringent, and metallic) ^[17][18][19][60,61,62]. These are derived from volatile (aldehydes such as acetaldehyde, decanal, hexanal; esters such as acetate, butyrate, caproate; alcohols such as butanol, methylbutanol; and ketones such as acetone, acetophenone) and non-volatile compounds (isoflavones, saponins, phenolic compounds and peptides) ^[19][20][62,63]. In addition, beany flavor is a common off flavor associated with legumes, which intensifies during harvesting, storage and processing ^[21][64]. Beany flavor compounds are formed through the lipoxygenase catalyzed oxidation ^[22][65] of unsaturated fatty acids such as linoleic acid, linolenic acid and oleic acid, which produce volatile aroma compounds ^[23][66]. The intensity of beany flavor is highly species dependent, for instance, soybean has a higher concentration of off-flavor volatiles than lupin ^[24][67]. Other factors such as geographical origin, agronomical practice, processing method, storage and packaging conditions also influence the generation and intensity of beany flavor in legumes ^[25][68].

Similarly, bitterness is a common off flavor associated with legumes, particularly due to the presence of low molecular weight peptides that contain hydrophobic amino acids such as leucine, proline, phenylalanine, and tyrosine ^[26][69]. Non-volatiles derived from saponins, such as triterpenoids and saponins A, B, E, and DDMP saponins, also contribute to bitterness ^[25][27][68,70]. The degree of bitterness is dependent on the type and concentration of saponins. The bitterness of DDMP saponin is significantly higher than that of saponin B, for example ^[27][70].

Furthermore, antinutritional factors (ANF) compounds present in legumes inhibit the bioavailability of many nutrients, including proteins, carbohydrates, vitamins, and minerals, which is a major challenge ^[11][28][29][54,71,72]. Among the ANF found in legumes are tannins, trypsin inhibitors, phytic acid, hemagglutinins ^[11][30][31][54,73,74] and non-digestible carbohydrates such as stachyose, raffinose and verbascose (raffinose family oligosaccharides) ^[32][75]. The phytate content in common beans is reported to be 0.6–0.8 mg/g, while it is 1.2 mg/g in peas ^[33][76]. Similarly, the tannin content of common beans and peas is reported to be 6–28.4 mg/g and 27.8–30.9 mg/g respectively ^[33][76]. There is evidence that multiple ANF work synergistically. For instance, tannins and other polyphenols, oxalates, and fibers synergistically act with phytic acid to inhibit the bioavailability of minerals ^[34][77]. ANF can disrupt protein digestion and utilization, mineral absorption and metabolism and cause gastrointestinal discomfort such as bloating and nausea. All of these contribute to the reduction of legumes’ nutritional value ^[28][29][71,72].

In terms of allergenicity, compared to other legumes, allergic reactions to soybeans and peanuts are relatively common and the allergic reaction can be severe ^[35][36][78,79]. Lentils, chickpeas, peas, mung beans, and red grams have also been reported to have allergenic potential ^[37][38][80,81]. What is most challenging is the fact that legumes exhibit immunological cross-reactivity among themselves as well as with other sources, increasing the severity of an allergenic response, as reported with peanut and lupin ^[38][39][81,82]. For instance, patients who are allergic to peanuts develop allergies to lupin, demonstrating the ability of cross-reactivity to trigger an immune response to molecules found in closely related species ^[39][82].

2. Microbial Fermentation of Legumes

2.1. Traditional Fermentation of Legumes

Traditional fermentation of legumes and other substances is a complex process involving several microorganisms at different stages of the process. Many of these fermentation processes are not well investigated. The main microorganisms that are involved in the traditional fermentation of legumes are fungi such as Aspergillus spp. and Rhizopus spp. and bacteria such as Bacillus spp. and LAB. The LAB that are predominant in such processes are Leuconostoc, Lactiplantibacillus, Streptococcus, and Pediococcus species. Traditional legume fermentation involves back slopping, i.e., using a portion of the previously fermented material as a fermentation starter inoculant in subsequent fermentation. The microbial profile and inoculum concentration vary depending on the fermentation technique, type of legume, temperature, pH, moisture, oxygen availability, season and region. For instance, traditional soybean fermentation is dominated by fungi such as Aspergillus and Rhizopus while the traditional fermentation of African locust bean is dominated by Bacillus species ^[40][41][124,125]. In the traditional fermentation processes, it takes a relatively long time to stabilize microbial populations. During this time, there is a risk of contamination by other microorganisms, which can lead to product spoilage. In addition to affecting the safety of the final product, competition for nutrients in the substrate under such conditions might change the outcome of fermentation from a quality perspective. The pre-treatment processes before fermentation might also affect the techno-functional properties of the final product. Conducting the soaking stage of soybeans under controlled conditions, for instance, can lead to a more predictable acidification result ^[42][128]. Depending on the soaking temperature, the resultant product might have different characteristics, since the dominant microorganism varies ^[43][129]. Pre-treatments facilitate the activation of enzymes, therefore expediting the breakdown process resulting in accelerated fermentation ^[44][97]. Establishing standards and using defined pre-treatment and fermentation methodologies in traditional fermentation are essential for improving the quality and safety of traditional fermented foods. Some of the ways that can improve traditional fermentation include: (i) selecting substrate varieties suitable for fermentation, (ii) isolating, selecting and preserving cultures with desirable traits for use as starter cultures, (iii) developing optimized fermentation processes with appropriate process and quality control for faster production, safe and consistent product and scale of operation. During traditional fermentation, microbial growth, metabolism, and different types of interactions among the microbial consortia (mutualism, favorism, competition) result in the production of unique flavor attributes and a rich nutrient profile ^[45][130], which are difficult to reproduce in industrial fermentation processes employing defined starter culture(s).

2.2. Modern Fermentation

Even though traditional fermentation has been practiced for thousands of years, it is still a very uncontrolled process susceptible to variable and inconsistent outcomes. Modern advances in microbiology and processing technology have presented possibilities for more precise and controlled fermentation processes. By controlling the type of starter culture and dosage and environmental factors such as temperature, pH, and nutrient levels, modern industrialized fermentation can produce more consistent and predictable outcomes than traditional methods. Furthermore, the use of specific starter cultures that have been optimized for certain food products or food ingredients allows for greater control over the fermentation process and can lead to the development of new and innovative functional food products. These developments have already led to the industrialization of the manufacture of traditional fermented products such as beer, spirits, cheese and yogurt. More recently, kefir and kombucha have also become mainstream products manufactured at a commercial scale in response to consumers’ renewed interest in fermented foods for health and wellness ^[46][37]. There are also several research and development efforts both by industry and academia to modernize the fermentation of legumes and other substrates with the use of defined starter cultures. Fermented soymilk and other plant-based yogurts incorporating pea and fava bean protein are already available in the market as dairy alternatives. There are also a few fermented soybeans (e.g., JeollaNamado fermented soybean powder, Kalustyan’s Mejugaru fermented soybean powder) and pea powders (e.g., Phytopea SF sprouted fermented pea protein powder, Nutrasumma fermented pea powder) in the market for use as condiments in Asian cooking or as dietary supplements.

2.2.1. Effect of Fermentation on Flavor Profile

Fermentation with appropriate starter culture can be used to substantially improve the flavor profile of legumes. For instance, fermentation of soy drink with Lycoperdon pyriforme fungi for 28 h incubation period at 24 °C under dark conditions while being agitated on a rotary shaker operating at 150 rpm resulted in a noteworthy decline of “green” odor elements derived from hexanal, (E)-2-nonenal, and (E,E)-2,4-decadienal, and a reduction in the sensory strength of “green odor” in comparison to unfermented soy milk ^[47][164]. It is worth highlighting that, following 28 h of fermentation, 60% of the sensory panelists were unable to detect off-notes ^[47][164]. This is further supported by a study conducted with pea protein isolates ^[48][165]. When fermentation was carried out with LAB and yeasts (Kluyveromyces lactis, Kluyveromyces marxianus, or Torulaspora delbrueckii) as starters, the levels of hexanal, butanal, and nonanal in pea protein decreased to below the limit of detection. Additionally, the most predominant volatile, 2-pentylfuran, which has been characterized as “earthy/musty”, “green”, and “floral”, was reduced to one-sixth of the concentration found in the non-fermented samples while yeast triggered the generation of esters which improve appealing flavous ^[48][165]. Flavor development during fermentation is a result of a sequence of biochemical processes where the starter culture produces enzymes and other metabolites that modulate the flavor profile of the product. During the early stages of fermentation, enzymes produced by the microorganisms play a crucial role in breaking down complex nutrients into simpler compounds ^[49][168]. Proteases break down proteins into peptides and amino acids, while lipases break down fats into fatty acids and glycerol. Amylases break down starches into simple sugars such as glucose and maltose, and galactosidases breakdown lactose into glucose and galactose. These simpler compounds act as precursors for the development of flavor and aroma compounds later on in the fermentation process ^[50][169]. 

2.2.2. Effect of Fermentation on Techno-Functional Properties

Food matrix interactions are disrupted during fermentation, allowing nutrients and phytochemicals such as polyphenols to be released in more bioactive forms ^[46][51][37,122]. Concurrently, carbohydrates, fats, and proteins undergo several biochemical processes that alter their functional properties. It is these functional properties that determine the use of food ingredients in final products. As an example, legume ingredients that have increased solubility are suitable for preparing protein-based beverages ^[52][107] such as plant-based milk, while legume ingredients that are good at water and oil absorption are suitable for meat-like products. The major storage carbohydrate in legumes is starch, which accounts for 40–50% of their total weight, followed by dietary fibre, oligosaccharides, and simple sugars ^[53][54][21,181]. During fermentation, carbohydrates are transformed into monosaccharides or disaccharides, ethanol and pyruvate by enzymes produced by the fermenting microorganisms. Legumes contain semicrystalline granules of starch that are composed of two principal polysaccharides: amylose and amylopectin ^[55][182]. They are polymers of α-D-glucoses that are linked together in two different configurations, with their structure influencing the gelatinisation, dextrinisation and swelling capacity ^[55][182]. Starch granule swelling capacity, one of the carbohydrate related functional property, is shown to be improved by fermentation, as reported for chickpeas, pigeon peas and soybeans ^[56][57][40,183]. Legumes are renowned for their high protein content, which varies significantly across different species. For example, common beans contain 16.7 to 27.2 g/100 g protein, whereas cowpea’s protein content ranges from 20.9 to 24.7 g/100 g ^[55][58][182,187]. During fermentation, the activity of microbial proteases hydrolyse legume proteins into peptides and free amino acids resulting in changes in protein properties such as hydration, surface activity (charge distribution and hydrophobicity) and protein structure (primary, secondary, tertiary and quaternary structure) ^[59][188]. LAB decomposes food proteins through proteolysis, which degrades proteins into oligopeptides. Surface changes in protein could lead to the exposure of hydrophobic groups, cause a change in surface charge, and promote protein-protein interactions that influence techno-functional properties during fermentation ^[60][191]. Functional properties such as solubility, aggregation, wettability, ability to form foams and emulsions, and rheological properties such as viscosity, elasticity, and adhesiveness are affected by changes in surface charge.  The effect of fermentation on the water-holding capacity of legumes is variable and influenced by protein content ^[61][193]. For instance, some research studies have reported an increase in the water binding capacity of chickpea-based sourdough after fermentation ^[62][63][33,177]. This was attributed to protein hydrolysis which makes more hydrophilic groups exposed ^[62][33]. The results were similar to the findings of another study conducted on chickpeas and pigeon peas, which found that microbial protease enzymes break down peptide linkages leading to increased water binding capacities ^[56][40].  Fermentation is also accompanied by a change in the fatty acid profile, since fermenting organisms may produce lipases that catalyze the hydrolysis of lipids into free fatty acids. Concomitantly, the degradation of proteins into low molecular weight peptides occurs. The peptides can migrate easily to the oil-water interface, increasing in emulsion stability and emulsifying capacity ^[56][64][40,167]. The emulsifying capacity and emulsion stability increased by 30–37% and 15–30% respectively, following fermentation by Rhizopus oligosporus of chickpeas, pigeon peas and soybeans ^[56][40]. This was attributed to protein hydrolysis, which leads to the exposure of hydrophilic and/or hydrophobic regions resulting in higher surface activity ^[56][40]. Fermentation of chickpea flour with Cordyceps militaris SN-18 revealed that proteolytic activity during fermentation unmasks hydrophilic and/or hydrophobic regions of proteins, resulting in a greater surface activity ^[63][177].

2.2.3. Effect of Fermentation on Nutritional Profile

Through fermentation, starch-hydrolyzing enzymes such as amylase and glucoamylase, which are secreted extracellularly by the fermenting microorganisms, degrade starch into monosaccharides and disaccharides ^[65][195]. As expected, the fermentation of legumes results in a degradation of carbohydrates while increasing the concentration of soluble carbohydrates that are more easily absorbed by the gut ^[66][67][43,196]. When chickpea was fermented with Cordyceps militaris SN-18 there was a 6.7% reduction in total carbohydrate content ^[63][177]. Fermentation generally increases the concentration of amino acids due to the degradation of proteins as well as a microbial synthesis of amino acids and proteins. For example, protein content of pigeon peas, chickpeas and red beans was reported to be increased through fermentation due to the microbial synthesis of enzymes such as lipases and proteases ^[56][40].

2.2.4. Effect of Fermentation on Antinutritional Factors (ANF) and Toxins

During microbial fermentations, ANF is transformed into complex macronutrients which are organoleptically and biochemically useful products (e.g., short-chain fatty acids, peptides, organic acids) as a result of enzymatic and non-enzymatic microbial reactions ^[28][29][51][71,72,122]. Fermentation with Lactiplantibacillus plantarum significantly reduced the tannins and phytate content of soybean, indicating that fermentation is a suitable strategy for reducing antinutrients ^[68][203]. Fermentation by LAB strains of chickpea sourdough reduced the concentration of α-galactosides, which can cause flatulence, bloating, and digestive discomfort. More specifically, raffinose and stachyose contents were reduced by 88.3–92.3%, and 97.7–99.1% respectively ^[62][33]. Similarly, a 17% reduction of phytic acid was observed in chickpeas sourdough fermented by Pediococcus pentosaceus and Pediococcus acidilactici. The stachyose and raffinose concentrations were reduced by 88.3–99.1% while verbascose amounts became undetectable ^[62][33].

2.2.5. Effect of Fermentation on Allergenic Profile

Fermentation has been demonstrated to reduce or eliminate allergens with minimal detrimental outcomes. For instance, the allergenic sequences were hydrolyzed and the binding capacity of IgE was reduced upon fermentation of soybean meal with Lactobacillus casei, yeast, and Bacillus subtilis as fermenting organisms ^[69][208]. One of the main mechanisms of reduction of allergens during fermentation is enzymatic hydrolysis, where enzymes produced by microorganisms break down the allergenic proteins into smaller peptides and amino acids, rendering them less allergenic or non-allergenic ^[70][209].