(PART I ) Prebiotic Impacts Of Soybean Residue (Okara) On Eubiosis/Dysbiosis Condition Of The Gut And The Possible Effects On Liver And Kidney Functions

It is well established that soybeans possess an abundant source of protein owing to their high nutritional value as well as chemical and physical properties. Besides, soybean and its by-products are evidenced in the literature as a rich source of phytochemicals/bioactive compounds, i.e., non-nutrient components of a plant with health-promoting functions and qualities. These compounds include but are not limited to, lunatic, lectin, phytic acids, saponins, omega-3-fatty acids, phytates, trypsin inhibitors, proteins, peptides, Bowman-Birk’s protease inhibitor, phytosterols, and isoflavones, mainly daidzein, genistein, and glycitein [1,43,44]. Traditionally, all these nutritional constituents have been regarded as anti-nutrients. However, the recent advances in knowledge have yielded a better understanding of their therapeutic and beneficial health functions, from cholesterol-lowering functions to anticancer properties, diabetes-mellitus-controlling effects, and postmenopausal osteoporosis reduction [1,45]. 

The main components of this residue are the coating of the beans and the broken cotyledon cells [1], which are made of crude fifiber, total dietary fifiber, insoluble dietary fifiber, and soluble dietary fifiber, and are suggested in several reports to play vital roles in multiple biological processes and as well aid in the fight against syndromes of varied origins. Hence, this residue is regarded as an important source of dietary fifiber owing to its major composition and low cost. However, the chemical composition is dictated by the method of soy processing or extraction, thus the number of water-soluble components obtained from ground soybeans and whether the residual extractable constituents have been extracted or not, and the cultivar of soybean used. Varied cultivars differ in lipid and crude protein contents, fatty acid compositions, and lipoxygenase activities [46,47]. Moreover, the variation in the nutritional profile of both wet and dry soybean residue has been attributed to cultivar differences, the incidence of sunshine, analysis methods, and the production or processing conditions used. Hence, the characteristics of water-soluble constituents may vary owing to the raw material used [48,49]. However, this lies beyond the scope of this review and, hence, interested readers can refer to these articles [10,47,50–52]. The sequence and procedures for processing the beans are as well very essential and it dictates the fate of all water-soluble extract in the beans. For instance, there is variation in the way the Chinese and Japanese process their soymilk and soybean curd. In the Chinese way, the soaked beans are rinsed, and the raw beans are then ground and the residue is then filtered off with water, and then heat the extract; in the Japanese system, the soaked beans are first cooked prior to grinding and filtering [7,30,46]. Figure 1 displays the schematic illustration of the steps involved in processing soy milk and the production of soybean residue/okara [46,47,52]. 

Although the substrates (okara) generated from soybean processing are suggested to have a high moisture content of nearly 70–80% and are mostly bound to the dietary fifiber, yielding to a clumpy texture and appearance similar to wet sawdust, with mainly insoluble fifiber i.e., cellulose and hemicellulose accounting for almost all it dry matter content (i.e., about 40–60%), which can be fermented by the gut microbiota in the large intestine, even though it cannot be fully digested in the small intestine. In contrast, the proportion of free carbohydrates (including, galactose, arabinose, fructose, sucrose, glucose, stachyose, and raffinose) is low (4–5%) and the lack of fermentable carbohydrates is the core factor inhibiting efficient fermentable bacterial growth in the residue. In particular, soybean residue contains 1.4% raffinose and stachyose, which may yield flatulence and bloat in some individuals. The monomers constituting the cell wall polysaccharide of the residue are mainly galacturonic acid, arabinose, glucose, galactose, fucose, xylose, and a small amount of mannose and rhamnose [53]. However, the moisture-free/dry residual content of soybean is reported to contain about 10% fats, 30% protein, and 55% total dietary fifiber, thus, 5% low-soluble dietary fiber and 50% insoluble dietary fifiber [48,54]. A recent study on the impact of high hydrostatic pressure (HHP) on the functionality of dietary fifiber in okara was reviewed. The authors observed that subjecting HHP to soybean-derived dietary fiber increased soluble dietary fifiber content (i.e., more than 8-fold), which is important in ensuring that soybean residue has anti-carcinogenic and anti-inflammatory effects on the host digestive tract [55]. 

Okara is suggested to be a potential source of less-priced plant protein used in human food owing to its recently confirmed high nutritive value and superior protein efficiency ratio [56]. Notably, the dry matter fraction of okara is shown to contain 15.2–33.4% protein (i.e., mainly 7S globulin and 11S globulin) [57,58]. These residual protein isolates contain all important amino acids although less soluble in water [57,59]. Again, the protein has been demonstrated to resist complete digestion by the gastrointestinal enzymes, pancreatin, and pepsin, and the latter is made of mainly steapsin, trypsin, and amylopsin. However, these low molecular weight component (less than 1 kDa) of the digestible-resistant peptides is highly potent in hindering angiotensin-converting enzyme (ACE) and hence exhibiting great antioxidant activity, possibly owing to its high fraction of hydrophobic amino acids [60]. About 5.19–14.4% of the residual protein content is made of trypsin inhibitors and can be inactivated with sufficient heat treatment [61]. Microbial bioconversion of soybean residual proteins may present few merits. Thus, its bioconversion into smaller proteins may elevate its solubility and hence generate bioactive peptides and/or amino acids. Trypsin inhibitors are suggested to be degraded by microorganisms to encourage their residual nutrient quality. However, microorganisms can catabolize residual proteins and amino acids, yielding a reduction in the number of essential amino acids present in the residual fraction. A recent study suggests that it is vital to put into account all potential effects of fermentation on the molecular weights of peptides, amino acids profile as well as inhibitory activity of trypsin since they play a role in impacting the overall functional characteristics including solubility and foaming properties, as well as the bioactivity of residual soy content [1,46]. Table 1 presents a summarized report on the effect of heat, fungi, and bacterial treatments on soy-based products. 

A study by Chan and Ma [57] reported a significant improvement in the emulsification, solubility, and foaming properties of okara protein by acid modification. The authors also discovered a distinct variation in the techno-functional properties of okara through different pre-treatments (i.e., ultrasonic, homogenization, and steam-cooking treatment), thus, the drastic improvement in the content of hydrophobic amino acids, decreasing hydrodynamic diameter, promoting surface hydrophobicity as well as enhancing solubility and oil holding capacity [73]. However, a significantly enhanced antioxidant capacity of residual protein concentrate was expressed after enzymatic hydrolysis using a combined mixture of flavorsome and alcalde [74]. A recent study on the impact of acid precipitation (mainly HCL, malic acid, and citric acid) on the structural and functional properties of okara was thoroughly reviewed. The authors recorded a variation in the functional properties of the residual protein from soybean (mainly, 7S globulin) influenced by the acid precipitation. It was observed that citric acid yielded an increase in the size of residual protein in contrast with HCL and malic acid. HCL resulted in high solubility, foaming ability index, water holding capacity, and foaming stability index. Malic acid recorded the lowest foaming stability index, emulsifying stability index, and foaming ability index. Citric acid-induced highest emulsifying stability index and oil holding capacity. The authors concluded that acid precipitation was able to modify the functional property of the okara protein by impacting the structure, which facilitated protein extraction from the poorly soluble raw materials and hence has widened the possible application of the obtained protein in the food industry [75].

The defatted soybean residue, which is generally obtained from the production of soybean protein isolate and soy oil, is usually made of 14–25% proteins, 70–85% fibers, and less than 1% lipids [73]. The residual content of soybean is suggested to contain a considerable amount of lipids at 8.3–10.9% (dry matter). Most of the fatty acids are poly- or monosaturated and are made of linoleic acid (54.1% of total fatty acids), stearic acid (4.7%), palmitic acid (12.3%), oleic acid (20.4%), and linolenic acid (8.8%) [76]. During the grinding of soybeans, unsaturated fatty acids, mainly linoleic acid, react with soy lipoxygenase and hydroperoxide lyase yielding to the formation of aromatic compounds like hexyl and nonyl aldehydes and alcohols. These formed odors with low detection thresholds signify the aromas/off-flavors in raw soymilk. Since these enzymes are generally denatured at a temperature above 80 ◦C, the Chinese way of processing soymilk (i.e., the raw soybeans are ground before the filtrate is heated) is likely to generate residue with greener character and beany flavor [77]. Hence, the variant obtained using the Japanese way of soymilk processing is relatively more palatable and is likely to have a lower content of trypsin inhibitor, thus it can easily be reused during cooking and processing [61]. However, this may define the reason why soybean okara is common in the Japanese market but rarely found in the Chinese markets. Fermentative microorganisms could metabolize fatty acids and their respective derivatives to produce much desirable aroma compounds. A recent study on the recovery of okara oil components through supercritical carbon dioxide extraction modified with ethanol indicated that at a pressure of 20 MPa and a relatively low temperature of 40 ◦C in the presence of 10% mol EtOH yielded to the recovery of about 63.5% oil-component. The obtained oil component was made of phytosterols, fatty acids, and traces of decadence. EtOH retained its dignity by elevating the yield and composition of phenolic compounds in the extracts, mainly soy isoflavones (i.e., genistein and daidzein). Soy isoflavones are well-known antioxidants that can increase both the value and stability of the oil, making the process attractive for food, cosmetics, and even in the pharmaceutical industries [78]. On the other hand, soybean residue is evidenced to contain a variety of minerals, a fair amount of iron, calcium, and potassium [53,79]. 

CISTANCHE WILL IMPROVE KIDNEY/RENAL INFECTION

2.2. Anti-Nutritional/Bioactive Constituents of Soybean Co-Products; with Emphasis on Polyphenols (Soy Isoflflavones)  Soy food such as soymilk contains a combination of balanced nutrients that is comparable to that of cow milk, but with gluten and lactose, and is embedded with promising phytochemical compounds that are linked with health-promoting functions. Soy foods and products are evidenced in several reports to possess a relatively high and diverse group of phenolic compounds, including phenolic acids, flavonoids, and non-flavonoids. Their vital role in our daily diet as bioactive compounds have been widely explored, with growing evidence revealing their role in decreasing chronic disease risks, such as cardiovascular diseases, diabetes, immune dysfunction, age-related eye problems, and cancers, which are all associated with the antioxidative effects of these phenolic compounds [80]. Bioactive compounds are molecules evidenced to portray therapeutic potentials with an impact on metabolic disorder, energy intake, oxidative stress, as well as reducing proinflammatory state [81]. The main bioactive components of soybean are proteins or peptides, saponins, phytosterols, isoflavones, protease inhibitors [82,83], tocopherols, and carotenoids [84]. Vong and Liu [46] reported on biologically active constituents of okara and they include acetyl glucosides (0.32%), saponins (0.10%), phytic acid (0.5–1.2%), malonyl glucosides (19.7%), isoflavone aglycones (5.41%), and isoflavones glucosides (10.3%). Past research has revealed that soybean is rich in polyphenol mainly isoflavones. Soy isoflavones are regarded to depict essential biochemical properties as part of the flavone compounds. Its role as an estrogen-like plant chemical (phytoestrogens) [85], has made them a topic of much interest and have been under surveillance by researchers, as they are accredited with important activities against hormone-derived cancers, menopausal syndrome disorders, osteoporosis [86–88], blood cholesterol, cardiovascular syndrome, and cognitive function [89]. Isoflavones are well-known polyphenols with a chemical structure similar to that of flavones. Both isoflavones and flavones are subclasses of flavonoids, which are in the largest polyphenolic groups [81,90–92]. Soybean contains up to 12 varied categories of isoflavones, which can be segregated into three (3) main groups (i.e., glycitein, genistein, and daidzein), all of which can take four varied forms: β-glucosidase, aglycones, malony lglucosides, and acetyl-glucosides, which constitute the main phenolic components and have been attributed to performing many health-promoting functions [86,89]. A recent study on the composition of okara was reviewed, and the authors reported that the total isoflavones content of okara is 355 mg/g (dry weight basis). The concentration of aglycones, malonyl glucosides, isoflavone glucosides, and acetyl glucosides in the residue were found to be 54.1, 196.8, 103.2, and 3.2 mg/g, respectively [89]. As mentioned above, okara may contain the same 12 isoflavones, although the processing conditions during soymilk production may impact the original isoflavones profile [86]. Another factor that can impact the profile of isoflavone in okara is their associations with other food matrix components, including non-covalent interactions between macronutrients and polyphenols, mainly proteins [81,93,94]. However, approximately 12–30% of the isoflavones contained in soybeans are suggested to be retained in the residue during soymilk processing. The main residual constituent of soy isoflavones is aglycones (15.4%), glucosides (28.9%), and a small fraction of acetyl genistin (0.89%) [95]. β-glucosides and malonyl-glucosides are the basic forms in soybeans, which can be transformed into acetyl glucosides and aglycones during processing due to thermal stress or enzymatic conversion [96]. The study by Izumi et al. [97], on the absorption rate of soy isoflavone aglycones in a human, was reviewed. The authors reported that β-glucosidase may enzymatically hydrolyze isoflavone glucosides into their aglycone forms, depicting a greater bioavailability in humans. Additionally, selected fermentative microorganisms are evidenced by experts to secrete β-glucosidase, hence the bioconversion of isoflavone glucosides in soy residue into aglycones via fermentation provides an opportunity for further value-addition [98].

CISTANCHE WILL IMPROVE KIDNEY/RENAL PAIN

The health effect of isoflavones, including anti-inflammatory and anticancer properties, cardiovascular defenses, as well as enzyme inhibitory roles of isoflavone are mainly linked with their antioxidant capacity, which is comparable to or better than that of other polyphenols [85,92]. These health effects have as well depicted to be useful against type 1 and type 2 diabetes mellitus, and have been widely proven in numerous reports [56]. An antioxidant is classified as an organic compound, which when available in small concentration/quantity in contrast with an oxidizing substrate, can significantly combat that substrate’s oxidation [92]. Although the term officially defines compounds that react with oxygen, it can also adhere to compounds that guard and/or protect against free radicals (i.e., molecules with unpaired electrons) that deny these radicals from harming healthy cells [47]. Daidzein and genistein are the strongest soy isoflavones for antioxidant activity. Genistin is evidenced in numerous reports to protect against oxidative DNA damage yielding from hydroxyl radicals, such as superoxide anion scavenging ability [56] as well as prevention of low-density lipoprotein oxidation [99]. The research on the impact of different storage conditions and thermal treatments on the stability of okara revealed genistin as the most dominant residual glucoside (0.33 mg/g) along with daidzin (0.25 mg/g), genistin (0.32 mg/g), genistein (0.02 mg/g), and daidzein (0.02 mg/g) of dried okara in a high-performance liquid chromatography study [100]. However, the exploitation of soy residue/by-products for the recovery of bioactive compounds has aroused much interest focusing on the contribution to food production and sustainable agriculture [101]. In fact, these by-products from soybean are often very rich in phenolic compounds, owing to their presence in the seeds and peels, which are often retained in the residues. Their tendency and relatively low water solubility to associate with other components may impact these by-products with rich polyphenolic content. Numerous potential applications of phenolic compounds have been reported, such as antioxidant stabilizers, food flavorings and colors as well as bioactive ingredients for health. Several unconventional and conventional techniques have been suggested for the separation of these high-value components. The extraction for conventional solid-liquid commonly uses hydroalcoholic mixtures [102]. Moreover, many other solvents, including acetonitrile, acetone, ethyl acetate, and methanol, are still under intensive study in the extraction of polyphenols, owing to the relatively easy solubilization portrayed by these solvents and mixtures [103]. Alkaline, acid, and subor supercritical fluid extraction are known alternatives. Modern technologies, including pulsed electric fields, microwave-assisted extraction, and ultrasound-assisted extraction have been proposed as means to encourage yields as well as overcome some challenges in polyphenol extraction. Examples of possible difficulties include component instability and solvent residues in the final product, as well as the kinetic limitations in cell-matrix extractions [1].

The preparation of soy molasses, i.e., a by-product of soy-protein concentrate, is a common starting material for the production of isoflavone. Being a known alcoholic extract of soy flakes, it is embedded with isoflavones in a slightly more concentrated form. Nevertheless, many patented processes use soybean and soybean meal as a commencing material during the recovery of isoflavones from side products, such as okara, which would require fewer valuable resources. Table 2 presents the general nutritional constituents of okara [46]