Phytochemicals in Tofu and Its Health Benefits

Abstract

A diet high in plant-based foods will offer an environment rich in phytochemicals, which are nonnutritive components of plants that have health-protective properties. Genistein and daidzein, two isoflavones, are found in soy as a dietary source. While soy has only recently become popular in the United States and Western Europe, it has been a staple of Southeast Asian diets for about five millennia. Among South east Asian populations, consuming a lot of soy is linked to lower risks of some malignancies and cardiovascular disease. The abundance of phenolic compounds, terpenoids, pigments, and other naturally occurring antioxidants found in nuts, whole grains, fruits, and vegetables has been linked to the prevention and/or treatment of chronic diseases like heart disease, cancer, diabetes, and hypertension as well as other medical conditions. Isoflavones are one of the phytochemicals in soy-based products that may support excellent health and are present in soymilk and tofu.

1. Introduction

Soyabean is one of the most important agricultural products with a high commercial value is soy (Glycine max), which has a high nutritional value. It is simple to grow, resistant to insects and pathogens, and convenient for processing and cooking [1]. Isoflavones, a class of plant oestrogens with structural and functional similarities to human oestrogens, are present in adequate quantities in soybeans [2, 3]. Popular soybean product tofu is formed by coagulating soymilk to form a protein matrix. It is a popular meal in the Far Eastern nations of China, Japan, and Korea. It is also being considered as a meat substitute in Western nations including the United States, the United Kingdom, and France [4]. It is a low-calorie diet since it contains much more water and contains important amino acids, fatty acids, calcium, as well as a number of valuable phytochemicals derived from soybeans [5].

For more than two thousand years, people have ingested tofu, which dates back to the Western Han period [6]. Tofu is highly nutritious and packed with soy protein. Bean curd, commonly known as tofu, has been manufactured in China for a very long time [7]. Due to its comparatively high content of proteins, fats, vitamins, minerals, and isoflavones, tofu offers several significant nutritional and physiological advantages [8, 9]. It one of the best plant protein sources, is also a good source of healthy fats, vitamins, and minerals, as well as other bioactive substances including isoflavones, soyasaponin, and others [10]. As a result, include tofu in a balanced diet may lower the risk of developing conditions such as cardiovascular disease, high blood pressure, diabetes, and hyperlipidemia [11]. Choosing raw soybean seeds, soaking, grinding, heating soymilk, filtering, adding coagulants, pressing, and packaging are all steps in the multi-step process that produces tofu. Figure 1 shows a flowchart for the production of tofu and its nutritional advantages. As a result, a variety of factors influence the quality of tofu products. These factors fall into two categories: internal and extrinsic. The variety of soybean seeds, depending on their genotype and protein composition, is an intrinsic factor. Extrinsic elements include food packing and processing environments [13]. Based on product qualities and the various coagulants employed throughout the tofu-making process, tofu products can be divided into firm/soft tofu, packed/pressed tofu, and fermented tofu. The variety of tofu products has fulfilled the various consumption demands, along with the validation of the theory behind tofu formation and the advancement of cutting-edge food processing methods [13, 14, 15]. The majority of soy’s anti nutritional components may be eliminated through processing, which also greatly increases the soy protein’s ability to be digested. According to studies, the digestibility of whole, ripe soybeans is only 65.3%; after being made into tofu and soy milk, it increases to 85% and 92–98%, respectively [16]. The production of tofu is attributed to the gelation properties of soybean protein because tofu is a well-known highly hydrated gel-type food (Singh et al., 2008). The final flavour, quality, and shelf life of the tofu will depend on the materials used for packing and storage product. Products made of tofu can be purchased in bulk, water-filled tubs, plastic bags, or vacuum-sealed packages [17]. Even under refrigeration, the shelf life of tofu products is only a few days because to the high moisture and protein content, which provides an ideal environment for the growth of bacteria [18]. The shelf life of tofu varies significantly in Japan, though, from 1 to 5 days for raw tofu to roughly a week for packed tofu to 3 weeks to a month for pasteurised tofu to 6 months to 2 years for aseptically processed tofu [19].

Figure 1.

Flowchart for the production of tofu that includes several steps, and health advantages [12].

2. Toffu processing

2.1 Preparation of soymilk

Rekha et al. [20] 200 g of cleaned soybeans were allowed to soak for 12 h at room temperature in excess water. Beans were put directly to boiling water with 1% sodium bicarbonate solution, blanched at 85°C for 5, 10, and 15 min, and then drained. After dehulling each batch by hand, the cotyledons were pulverised in a high-speed blender with water (1.6 L). The soybean slurry was continuously stirred while being indirectly heated in a water bath for 45 min at 85°C. After separating the soymilk from the leftover material (okara) using a double-layered cheesecloth filter, the hot slurry was cooled and the fatty layer was scraped off. The Refractometer’s was used to measure the soymilk solid content, which was then corrected with water to 7°, 8°, and 9° Brix.

2.2 Preparation of Tofu

Soymilk (200 ml) was heated to 95°C for 5 min. It is then continuously stirred at ambient temperature while cooling to 80°C. In separate batches, soymilk was mixed with a solution of magnesium chloride (0.2%) and calcium sulphate (0.2%) w/v (1:1) and swirled for 5, 10, 15, and 20 minutes. Without disturbing it, the milk was left to coagulate for 15 min. The coagulated milk was poured into a cheesecloth mould that was lined with porous plastic. The weights used to press the curd were 500, 700, and 1000 g for the first 15 min, followed by 500 g for the following 15 min. After pressing, the cloth was taken off, and the tofu was kept chilled while being preserved in water [20].

2.3 Factors influencing tofu’s quality features

The quality of soymilk and the subsequent coagulation process affect the yield and quality of tofu, whereas the quality of soymilk is influenced by the type of soybeans used and the conditions under which it is prepared. The most crucial process in forming tofu is coagulation, which is dependent on several factors, including the soymilk’s concentration and temperature, the type and proportions of the coagulant, the mixing technique, etc. The most difficult aspect of manufacturing tofu is determining out the precise amount of coagulant to add to the soymilk because it has a significant impact on the product’s yield and quality. The appearance of whey can also be used to estimate the concentrations of coagulants. If the right amount of coagulant is employed, whey turns translucent and has an amber or light yellow tint. However, if too much coagulant is used, the whey will turn yellowish, have a bitter flavour, and have a gritty texture [21].

2.4 Phytochemicals in Tofu

Biologically active, non-nutritive substances from plants are referred to as phytochemicals. The phytochemicals in soybean are very diverse. They consist of trypsin inhibitors, isoflavones, saponins, phytates, phytosterols, and phenolic acids. Diabetes, cancer, and cardio vascular disorders are just a few of the chronic illnesses that phytochemicals may help to prevent [22]. The low prevalence of prostate and breast cancer in Japanese men and women, respectively, is thought to be in part due to soybean consumption. Chinese who frequently eat soybeans and/or tofu have a 50% lower risk of stomach, colon, rectum, breast, and lung cancer compared to Chinese who eat soy or soy products infrequently [23]. The isoflavones genistein and diadzein are found in exceptionally high and unique concentrations in soybeans and non-fermented soy products, such as tofu. In culture, these isoflavonoids prevent the development of cancer cells that are both hormone-dependent and hormone-independent (Figure 2) [24].

Figure 2.

No. of publications during last 20 years (2002–2022) with the term “Phtochemicals in tofu” (source: Science.gov) use scopus or web of science data.

2.4.1 Isoflavones

Isoflavones which are nearly exclusively found in members of the legume family, are only present in significant quantities in soybeans. The two main isoflavones are genistein and daidzein. Dry soybeans contain an average of 1600–2400 mg/kg of isoflavones. Isoflavones are highly soluble in alcohol, mildly water soluble, and heat stable. Because of rinsing, filtering, or dilution with water or flavourings, tofu and soy milk have decreased isoflavone concentrations [25].

The most prevalent class of phytoestrogens, isoflavones are found in high quantities in soybeans and soy-based foods like tofu. Isoflavones naturally occur in forms called glycosides that are less bioavailable than their aglycone counterparts. By increasing the bioavailability of isoflavones and acting as a bio-catalyst for the conversion of isoflavone glycosides to isoflavone aglycones, β-glycosidase can be utilised to enhance the quality of tofu [26]. Processing tofu is a sophisticated physicochemical process, as is widely known. The distribution of nutritional components and the conditions under which they are processed may have an impact on the tofu’s nutritious value. Isoflavones are widely present and explored in beans, grains, and fruits as a form of phytoestrogen with a molecular structure similar to that of oestrogen. Twelve isoflavones, comprising three aglycones (free isoflavones) of genistein, daidzein, and glycitein and their corresponding three glucosidic conjugates, have so far been isolated and characterised in soybean seeds [27]. Because this form of phytoestrogen can reduce the incidence of breast cancer, prostate cancer, cardiovascular disease, osteoporosis, climacteric syndrome and its linked disease, the sources and compositions of isoflavones have been widely explored [28]. The occurrence of intricate physicochemical reactions, the composition of isoflavones in the final product or intermediate product, and the obvious differences between each processing step. By analysing the changes in isoflavone composition brought on by GDL during the preparation of tofu, it was revealed that the processing steps of soaking raw soybean seeds, filtering soybean slurry, and coagulation step had loss ratios of 4%, 31%, and 18%, respectively, of the total mass of isoflavones [29]. After heating soybean slurry to a maximum value for a certain isoflavone, the concentration of aglycones decreased in conjunction with the tofu-making process. Throughout the tofu processing, the amounts of β-glucosides and malonylglucosides were consistently lowered [27]. When soybeans are processed, a huge percent of the isoflavones are lost. This loss occurs in the preparation of tofu (44%), soy isolate (53%), and tempeh (12 and 49% during the soaking and heat processing steps, respectively) [30].

The four chemical forms of soy isoflavones—aglycone (daidzein, genistein, and glycitein), glucoside (daidzin, genistin, and glycitin), acetylglucoside (acetyldaidzin, acetylgenistin, and acetylglycitin), and malonylglucoside—contain a total (malonyldaidzin, malonylgenistin and malonylglycitin) [31]. Various biological properties of soy isoflavones that may include.

2.4.1.1 Soy isoflavones and osteoporosis

Menopause-related ovarian hormone deficit causes an increase in bone turnover and an imbalance between resorption and creation, which speeds up bone loss [32]. In comparison to baseline, soy isoflavones significantly raised bone mineral density in women by 54% and lowered the bone resorption marker urine deoxypyridinoline by 23%. According to a sensitivity study, soy isoflavones had a substantial impact on deoxypyridinoline and bone mineral density. An abrupt decrease in oestrogen levels in ostmenopausal women causes an increase in the pace of bone remodelling, which is linked to a loss of bone mineral density and an increased risk of fractures [33, 34, 35].

2.4.1.2 Soy isoflavones’ effects on memory and learning

High plasma isoflavone levels from dietary phytoestrogens can have a considerable impact on anxiety, learning, and memory as well as sexually dimorphic brain regions [36]. According to reports, the phytoestrogen genistein, which may cross the blood-brain barrier, has an antioxidant action that protects against the harm caused by ultraviolet (UV) radiation and chemicals. Soy’s antioxidant properties may offer protection from neurodegenerative disorders [37].

2.4.1.3 Soy isoflavones’ effects on coronary heart disease

Foods containing soy protein are a good source of the phytoestrogen isoflavones genistein and daidzein. Since a high dietary consumption of soy-containing foods has been linked to lower rates of chronic diseases, including coronary heart disease, there is growing interest in these compounds [38].

2.4.1.4 Effect of soy isoflavones on diabetes

Chronic insulin resistance and a loss of functional pancreatic β-cell mass are the causes of type 2 diabetes. Numerous studies conducted over the past ten years have shown that genistein has anti-diabetic properties that are distinct from its roles as an oestrogen receptor agonist, antioxidant, or tyrosine kinase inhibitor, including direct effects on β-cell proliferation, glucose-stimulated insulin secretion, and protection against apoptosis [39].

2.4.2 Genestein

High concentrations of the phytoestrogen genistein, an isoflavone, can be found in soy products. Numerous ailments, including cancer, cardiovascular disease, osteoporosis, and postmenopausal symptoms, are helped by genistein. Genistein has been shown to inhibit cytokine-induced signal transduction processes in immune system cells, which led us to believe that it may also have anti-inflammatory properties. Since as genistein binds to oestrogen receptors and shares structural similarities with oestrogen, it is possible that it exerts an estrogenic effect [40]. Foods from leguminous plants contain genistein in large quantities. The most genistein has reportedly been found in soybeans, a cholesterol-free, high-protein legume. Other legumes like chickpeas (garbanzo beans) contains a small amount of genistein. Soy-based foods, such as tofu, soy milk, soy flour, textured soy protein, soy protein isolates, tempeh, and miso, contain genistein in varying levels [41]. The physiologically active glucoside genistin is the principal dietary source of genistein. The sugar molecule from the isoflavone glycoside, genistin, is released during the fermentation or digestion of soybeans or soy products, leaving the isoflavone aglycone, genistein [42].

2.4.3 Flavnoids

The flavonoids, which constitute up the largest group of plant phenols, are low-molecular-weight chemicals that typically exist linked to sugar molecules. Anthocyanins and anthoxanthins are two types of flavonoids. The molecules of red, blue, and purple pigment known as anthocyanins. Anthoxanthins are colourless or white to yellow compounds that include flavonols, flavones, flavanols, and isoflavones [43].

2.5 Saponins in Tofu

The term saponin has the same origin as soap. When heating raw soymilk to make tofu (bean curd), a lot of bubbles are produced. These bubbles contain significant amounts of saponin components. These bubbles, which have a harsh taste, are skimmed off during regular cooking. In Japan, however, an antifoamer is used throughout the tofu making process rather than their removal. Because saponins are preserved as a result, the traditional tofu production method may have contributed to our good health and longer than average lifespan. Recently, a method without antifoamers that keeps the saponin elements in tofu curd has been developed [44]. Triterpene or steroid glycosides, including steroid alkaloid glycosides, are a more precise definition of saponins. The saponins with pentacyclic triterpenes as the aglycone, especially those of the oleanane type, are the most common [45]. Depending on how many sugar chains are connected to the agylcone, saponins can be further divided into different groups. Saponin concentrations in processed soy foods like tofu and soymilk are only slightly lower than those in soybeans, which contain 1–5% of saponins [46]. Soybeans account for over 80% of all dietary saponins in a normal Japanese diet [47]. Group A saponins have a hydroxyl group at position C-21, whereas group B saponins contain a hydrogen atom there. The carbonyl group at position C-22 distinguishes group E saponins from groups A and B. Group B saponins are referred to as DDMP saponins if they include a 2,3-dihydro-2,5-dihydroxy-6-methyl-4H pyran-4-one (DDMP) moiety at the C-22 position [48].

3. Health effects of soya saponins

Soya saponins aid in antioxidant, hepatoprotective, and anti-cancer properties as well as cardiovascular and cardiovascular protective effects. Their chemical composition heavily influences how they affect human health. Due to this class of soy chemicals’ limited absorption, it’s possible that the GI tract’s indirect activities are what trigger their bioactivity [49].

Cholesterol and some saponins with clearly defined structural properties interact to produce insoluble complexes. This complex-forming activity in the gut prevents both endogenous and exogenous cholesterol from being absorbed via the intestinal tract. (2) By producing mixed micelles, saponins can obstruct the enterohepatic circulation of bile acids. Effectively prevented is the reabsorption of bile acids from the terminal ileum [50].

4. Conclusion

Many people have indicated a strong desire to reduce back on the amount of animal products in their diet for moral, environmental, and health grounds. Tofu is consumed because it is high in nutrients and may be included into vegetarian and hypocaloric diets. Therefore, it is crucial to comprehend the crucial aspects that could affect tofu quality in order to produce high-quality and superbly flavoured tofu goods in order to satisfy the growing client expectations. The variety of soybeans used in the production process, the composition of the soybean protein, its structural characteristics and nutritional value, the type of coagulant used, production technology, and packaging materials are just a few of the variables that affect the final tofu product’s quality..Soybeans are unique among the legumes because they are a concentrated source of isoflavones. The evidence supporting the prevention of chronic diseases by soy foods and isoflavones ranges from very well established benefits, such as the decrease of heart disease and reducing cholesterol levels, to highly speculative effects, such as the relief of menopausal symptoms and osteoporosis. Therefore, it is evident that consuming soy foods has more positive impacts than negative effects, even without conclusively proving that doing so lowers the risk of developing chronic diseases.