Valorization and food applications of okara (soybean residue): A concurrent review

Abstract Agriculture waste is rising continuously across the globe due to enormous industrial, food processing, and household activities. Proper valorization of this waste could be a promising source of various essential bioactive and functional ingredients. Okara is a major residue produced as result of soybean processing and has a rich nutritional profile. The nutritional profile of okara is affected by the processing conditions, variety, pre‐treatment, post‐production treatments, and processing techniques. Owing to the high fibers, lipids, proteins, and bioactive components, it is being used as an essential industrial ingredient in various food processing industries. The prebiotic potential and nutritional profile can be increased by various techniques, that is, enzymatic, chemical, biotransformation, high‐pressure microfludization, and fermentation. The prebiotic potential of okara makes it suitable as a therapeutic agent to prevent a variety of metabolic disorders such as diabetes, obesity, hypercholesterolemia, and hyperlipidemia. The current review highlights the structural, nutritional, functional, therapeutic, and industrial applications of okara.

Okara has a protein level of about 28%-30% (essential amino acid), a fat level of 8%-10% (polyunsaturated fatty acid), and a small number of other nutrients like starch, sugar, potassium, and some amount of B group vitamins that improve the digestion and make it a low-cost plant-based protein (Préstamo et al., 2007).
Recent reports showed the progress of okara from a residue to a functional or value-added food. Biotransformation, which combines enzymatic saccharification and fermentation, is an efficient method of okara utilization. The biotransformed okara is more suited for additional processing to generate functional food items for human use, such as sauces or functional drinks, due to its higher levels of prebiotic dietary fiber, free sugars, and amino acids (Vong et al., 2017).
The most common method to enhance the the sensory profile and functional qualities of okara is fermentation. Moreover, okara fermentation with probiotic bacteria improves the microbial viable count and storage of food products (Colletti et al., 2020). It contains aglycones and isoflavones as bioactive components. Fermentation of okara enhances its functional properties and may also improve its value because it contains lactose (as a prebiotic source) and probiotics (Voss et al., 2021). In this review, we suggest a possible use of okara as a healthy and functional food to improve gut function and prevent hyperlipidemia, hypertension, diabetes, cancer, and obesity.

| Production technology
The composition of okara is determined by the quantity of waterphase extraction from ground soybean, as well as the production of soybean. There are two methods commonly used for soymilk production as shown in Figure 1. The first is the Chinese method and the second is the Japanese method; there are very slight modifications in both methods. In the Chinese method, the following steps are followed during soymilk production: Soybean is first soaked, rinsed, and then ground and heated, and finally filtered. On the other hand, in the Japanese method, soybean is soaked, rinsed, and then heated, ground, and filtered to obtain soymilk ( Figure 1). After filtration, the solid part that is leftover is known as okara (Guimarães et al., 2018).
Okara is produced throughout the world. A 1 kg of tofu produced from soybean provides around 1.2 kg of fresh okara. Around 800,000 tons of okara is produced in Japan, 310,000 tons in Korea, and 2,800,000 tons in China by tofu industries. European nations too consume soybean in large quantities. Spain alone purchases 2.5 million tons of soybeans every year. These countries produce a large amount of soybean residue, okara (Li et al., 2012). A small portion of okara is recycled as animal feed. Okara-based animal feed has good nutritional and bioactive components that improve the digestion of animals, the quality of meat, and milk production. It is found to be a cost-effective and alternative source of protein and energy for animals (Bo et al., 2011;Rahman et al., 2021).

| NUTRITIONAL COMP OS ITION
Fresh okara is high in moisture (putrefies very fast) and dietary fiber content. Okara also is a rich source of protein and carbohydrates.
Linoleic acid, palmitic acid, stearic acid, oleic acid, linoleic acid are the most common essential fatty acids present in okara. Okara contains monosaccharides, oligosaccharides, and polysaccharides such F I G U R E 1 Extraction methods (Japanese and Chinese) of okara from soybean. as arabinose, glucose, galactose, fructose, stachyose, raffinose, sucrose, and starch. Phytochemicals that are present in okara are phytates, saponins, coumestans, phytosterols, lignans, and isoflavones (genistein and daidzein). These chemicals have a variety of therapeutic and physiological properties, such as their antioxidant action, cardiovascular disease prevention, and are excellent chemopreventive agents for cancer patients (Li et al., 2012). Okara is also a rich source of minerals; every 100 g of okara contains 126 mg of Ca, 4.45 mg of Fe, 0.77 mg of Cu, 313 mg of phosphorus, 286 mg of potassium, and 3.14 mg of zinc (dos Santos et al., 2019;Kamble & Rani, 2020). Now a days, different varieties of soybean are grown (Black soybean, Yellow soybean); hence, the extracted okara has also different forms. The chemical compositions of black okara and yellow okara are shown in Table 1.
Okara is a rich source of dietary fiber. Dietary fiber is widely recognized to have a vital function in numerous physiological processes as well as in preventing health maladies (Li et al., 2012).
There are four forms of fiber: crude fiber (CF), total dietary fiber (TDF), insoluble dietary fiber (IDF), and soluble dietary fiber (SDF) (Brownlee, 2011). Okara, like other vegetable wastes from the food sector, is high in IDF, but low in SDF. Different techniques such as enzymatic treatment, fermentation, micronization, and highpressure treatment improve the SDF concentration of okara (Han et al., 2008). Ultrasonication is a technique used to enhance the protein yield from okara. Extracted protein has a more zeta potential and yield (%); therefore, the protein isolate from okara is added in different food items (Eze et al., 2022).

| Bioactive components
Some of the bioactive components of soybean are phenolics and isoflavones. Soy isoflavones are a part of flavones, an estrogen-like plant chemical called phytoestrogen. These soy flavone compounds contain glycosides, for example, ferulic acid, genistein, chlorogenic, daidzein, and syringic acid. These phenolic components are important for physiological and health-promoting functions. The amount of bioactive components in soybean depends upon the method for extraction and the temperature during processing. Okara contains 30% isoflavones, 28% glucosides, 15% aglycones, and 0.89% acetyl genistin.
Isoflavones (genistein and daidzein), aglycones, and glycosides have strong antioxidant properties compared to other polyphenols. It is interesting to note that these antioxidants prevent DNA damage and low-density lipoproteins oxidation. From the above discussion, it is clear that okara is a dominant source of bioactive components (Kamble & Rani, 2020).

| S TRUC TUR AL CHAR AC TERIS TIC S
The structural properties of okara has been studied by various researcher using different techniques such as scanning electron microscope (SEM), Fourier transforms infrared spectroscopy (FT-IR), X-Ray diffraction (XRD), and energy dispersion X-Ray analysis (EDX).
Surface microstructure such as the arrangement of starch granules in the protein matrix and dietary components of okara was identified by SEM images as shown in Figure 2. The SEM images showed that okara powder has a rough, hollow, irregular, and porous structure and these describe the particle properties of the product. The rough and hollow structure represents okara containing IDFs (cellulose, hemicellulose); on the other hand, the irregular and porous structure of okara dietary fiber accounts for high water-holding capacity and oil-holding capacity. This describes okara as having hygroscopic properties. Dietary fiber extracted from okara mostly showed the crystalline and amorphous regions due to the presence of cellulose, hemicellulose, and lignins. Ostermann Porcel et al. (2017) studied the SEM images of bread enriched with different concentrations of okara. They determined that micrographs showed the relationship between bread and its composition. Due to the presence of protein and dietary fiber, okara-enriched bread showed different micrographs.
The functional properties of okara may be identified by FT-IR spectra, which showed structural similarities and differences in the product. The FTIR spectrum of frozen okara was studied by

| OK AR A A S A SUBS TR ATE
The functional, sensory, and microbial properties increased by the fermentation of okara ( Table 2). The fermentation technique of plant media by certain lactobacillus species such as Lactobacillus plantarum and Lactobacillus acidophilus enhances the free radical scavenging activity and increases the nutritional value as well as the sensory, physical, and technological characteristics of okara (Razavizadeh et al., 2021). Fermentation had no discernible effect on the density of okara. After lactic acid fermentation, the  Table 2.

| FUN C TIONAL CHAR AC TERIS TIC S
Okara has isoflavone bioactive components which improve cancer resistance, protect from osteoporosis, reduce antimicrobial inflammation, and control cardiovascular disease. Intake of soy foods has been associated with a decrease in plasma cholesterol, the protection of heart disease, a lower risk of cancer (colon, breast, and prostate), osteoporosis, cognitive performance, and menopausal symptoms (Kamble & Rani, 2020). Fermented okara is used as a nutraceutical such as fucoxanthin and ecosapentaenoic acid (EPA) (Kim et al., 2023). Some prebiotic, nutritional and therapeutic effects of okara are shown in Figure 3.

F I G U R E 3
The prebiotic, nutritional, and therapeutic effect of okara.
physiology as well as in disease prevention (Corpuz et al., 2019 Figure 4 represents the effects of okara on human metabolism, especially in the small intestine, pancreas, adipose tissue, and liver.

| Anti-obesity
Okara is beneficial as a weight-loss dietary supplement with a possible prebiotic impact, because okara is responsible for lowering body weight and more fecal fermentation (Préstamo et al., 2007).
When Perez-Lopez and his colleagues experimented and gave hypercholesterolemia diet with SDF-enriched okara to rats, it resulted in various health-promoting advantages. Due to the prebiotic potential of okara, it is needed for weight management and lipid reduction, increasing short-chain fatty acid synthesis, improving mineral absorption, and modifying gut microflora. In comparison to untreated samples, in vivo colon digestion of okara results in lower pH, greater fecal weight, and increased short-chain fatty acid (SCFA) synthesis in okara-fed rats. For 10 weeks, mice that were fed an elevated fat diet (14% crude fat) or a dry okara-added raised diet (10%, 20%, or 40%), it reduced the growth of body mass and endometrial white adipose tissue and avoided a rise in lipid levels, comprising cholesterol levels, relatively low cholesterol, and non-esterified fatty acids (Matsumoto et al., 2007). Okara consumption also avoided hepatic steatosis.

| Antidiabetic
Diabetes is categorized as a frequent metabolic disorder with complicated pathophysiology, described by persistent hyperglycemia.

F I G U R E 4
Effect of okara dietary fiber on human physiology (gut, pancreas, adipose tissue, and liver).
Diabetes is thought to be caused by several factors, such as impaired insulin production, impaired insulin physiological activity, and inflammatory response. Diabetic individuals' metabolisms may be altered, resulting in a variety of metabolic problems including sugar, protein, fat, water, and electrolytes (Chan et al., 2019). Furthermore, hyperglycemia may cause chronic damage and malfunction of different tissues including the eyes, kidneys, heart, blood vessels, liver, and nerves, resulting in catastrophic consequences. Therefore, diabetes is responsible for cell membrane damage, and changes in serum biochemical markers and lipid profile in diabetic patients.
These changes can be reduced by the intake of an okara whey diet that reversed hyperglycemic conditions while also improving liver and pancreatic abnormalities. The chemical composition of soybean residue consists of 10 fat, 25% protein, 50% dietary fiber, and 15% other nutrients. Cytokines, lactoperoxidase, lactoferrin, lactoalbumin, and β-lactoglobulin, as well as protein insulin, are called whey proteins and it includes different amino acids like valine, leucine, and isoleucine that are necessary for tissue repair and development. It also contains cysteine that has a free radical-scavenging activity due to the presence of glutamic acid, glycine, and thiol groups all of which combine to make glutathione reductase (one of the cell's key antioxidants) (Kim et al., 2016;Usman et al., 2023). They also include vitamins, good protein fraction with strong water-retaining and emulsifying properties, and an antihypertensive peptide. As a result, the combination of okara and whey protein has a therapeutic effect to reduce the hyperglycemic condition. So, okara can also be utilized

| ANTIH YPE RTE NS IVE AND ANTI OXIDANT
Antioxidant amino acids such as phenylalanine, tyrosine, and methionine were found in higher quantities in all fermented okara products (Hwang et al., 2015). Biotransformation of okara improved its oxygen-scavenging capacity by increasing the quantity of fiber in the diet, bioactive components (phenolic acids, isoflavones, aglycones), and amino acids (Vong et al., 2017). β-carotene (0.411 mg/100 mL) and isoflavones (0.15 mol/gFM) were found in okara-enriched food products which have antioxidant properties (Guimarães et al., 2018).
Okara drinks (hydrolysates) inhibited ACE activity. Furthermore, antioxidant and ACE inhibitor action during in vitro intestinal digestion increased, whereas total phenolic components remained constant throughout all okara drinks (Sanjukta et al., 2015). Moreover, the current study has focused on the technique of boosting the antihypertensive action in milk or soy, but few studies have been undertaken using soy waste (Voss et al., 2021). Vital et al. (2018) showed that okara is an excellent source of phenolic acids, and flavonoids and are 106.7 mg GAE/100 g and 32.7 mg QE/100 g, respectively. They reported that the addition of okara to omega-3 enriched milk can be used as an antioxidant supplement. Therefore, okara decreased lipid oxidation and increased the free-radical scavenging activity.

| Antiaging
Aging is a biological process defined as the organism's cellular and functional degradation with time and decreases the quality of life.
Aging causes harmful modifications in tissues and cells with growing age, which increases the risk of diseases and death. Aging in accordance with this is the major hazard for the development of numerous ailments, including cardiovascular disease (e.g., stroke), cancer, and neurological disease (e.g., Alzheimer's disease) (Aman et al., 2021).
The microbiota gut-brain connection is a bilateral communication channel that links the cognitive and emotional brain areas with pe-

| Anti-hyperlipidemia
Okara has potential for the management of hyperlipidemia. A male golden Syrian hamster was fed a high-fat supplemented diet with okara for three weeks (Kitawaki et al., 2009). The diets comprised either 13% or 20% of okara fiber (OK-13 and OK-20), low-protein okara containing 13% fiber (OK1-13), and isoflavones-free okara with 13% fiber (OK2-13). In hamsters which were given OK-20, plasma concentration of triglycerides, extremely low-density lipids, as well as cholesterol and low-density lipid cholesterol levels dropped considerably (p < .05). In all tests, however, no substantial differences between low-density lipid cholesterol and high-density lipid cholesterol plasma (p > .05) concentrations were identified (Villanueva et al., 2011). The OK-20 diet decreased total triglycerides, fats, and esterified cholesterol levels within the liver. All the okara foods tested the improved fecal output of total circulating free cholesterol, triglycerides, lipids, and overall nitrogen (p > .05).
The findings revealed that the primary contents of okara such as fiber and protein might be responsible for the reduction of cholesterol and lipids within plasma and liver, and an enhancement in fecal excretion in high-fat supplemented hamsters.

| Food applications of okara
Okara has been utilized as a dish or supper in China and Japan for numerous years. It is quite simple to add fiber and protein to food to help meet nutritional content claims. Okara can bind moisture and oil, making it an appealing low-price additive for increasing meat product production. The addition of okara (5%) in chocolate chip cookies increased its shelf life and decreased the syneresis during freezing and thawing. Okara fortified and supplement meat and bread can be used because it cannot change the flavor and texture of food (Mateos-Aparicio et al., 2010). Ibrahim et al. (2022) reported that 2% and 3% okara fortification with probiotics (L. plantarum) and ice-cream significantly improved its chemical, nutritional, microbial, physical, and sensory properties. They called this food product synbiotic ice-cream because it enhances the growth of probiotics as well as nutritional composition. At the same time, Roslan et al. (2021) fortified yogurt with a probiotic (L. Plantarum) and okara (1%, 2%, and 3%). They also concluded that okara dietary fiber improves the probiotic count and chemical properties of yogurt with storage time.
Therefore, okara can be used in food industries as a value-added food and as a supplement food.
Furthermore, freeze-dried okara has the highest swelling, lipidbinding, and water-holding abilities, usually accompanied by hot air drying and vacuum drying. Hot air-dried okara has the highest cation exchangeability, as compared to freeze-drying and vacuum-drying okara (Li et al., 2011). Okara can be used in soy flour, wheat flour, and other components in food manufacturing to boost fiber and protein content. It has been used in the production of bread, pancakes,

| CON CLUS ION
Even though okara is a byproduct of soybean, it includes numerous other useful components. According to the findings of many studies, okara can be utilized as a functional food. It has a high-quality nutritional profile, phytochemicals, and prebiotics potential. In this article, okara has been assessed as a food ingredient enriched with prebiotics, high fiber, protein, fat, digestible carbohydrates, moisture content, and bioactive components, as well as food industry application. Benefits of okara include weight loss, blood glucose management, cognitive performance, ACE inhibition, free radical scavenging activity, and lipid reduction as well as a prebiotic-like effect due to its capacity to increase SCFA synthesis, improve mineral absorption and modify gut flora. Okara can be used as a value-added food, as a supplement, and as a fortified food.

AUTH O R CO NTR I B UTI O N S
Aasma Asghar: Methodology (equal); writing -original draft (equal).

ACK N OWLED G M ENTS
The authors are thankful to Government College University Faisalabad for providing literature collection facilities.

FU N D I N G I N FO R M ATI O N
The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

CO N FLI C T O F I NTE R E S T S TATE M E NT
The authors declare that they have no conflict of interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
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E TH I C S S TATEM ENT
This article does not contain any studies with human participants or animals performed by any of the authors.

CO N S E NT TO PA RTI CI PATE
The corresponding author and all the co-authors participated in the preparation of this manuscript.

I N FO R M E D CO N S E NT
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