Present and Future Perspective of Soybean Cultivation

Abstract

Soybeans have been cultivated as a traditional crop since ancient times in Japan, China, and other parts of Asia. Soybeans, as a source of protein, are rich in essential amino acids, but also contain a variety of functional and nutritional components. Their processed and fermented products support the maintenance of human health. Recently, new soybeans varieties containing superior nutritional components have been cultivated, and growing interest in plant-based foods has led to the establishment of new food products including dairy products such as butter and cream.

1. Introduction: the origin of soybeans and their global production

Soybean (Glycine max (L.) Merr.) is a traditional crop that contributes significantly in Asia’s food culture. Several theories regarding the origin and spread of soybean have been suggested. Nevertheless, it has been reported that soybean was being cultivated in north-eastern China even 3000 years ago [1]. Currently, soybean is one of the most essential plant resources cultivated in various regions worldwide. The global production of soybeans in 2019 was approximately 340 million tons (Figure 1), making it one of the most commonly produced plant resources globally [2]. Approximately 80% of cultivated soybeans are used in livestock feed, and its majority is used for non-dietary purposes (i.e., printing ink, binder and resin dispersion, etc).

Figure 1.

World Soybean production.

Soybean is known as the “miracle crop” as its actual cultivation initiated after 1930, mainly in the United States, and its production increased rapidly and significantly [3]. Soybean is widely cultivated in regions extending from the equatorial tropics up to southern regions of Sweden and Canada in the north, and Argentina and southern Australia in the south, with a substantial number of soybean varieties appropriate for cultivation in different environments and uses appropriate for each region.

2. Functional components present in soybeans

The composition of soybeans varies marginally depending on the variety; however, it commonly consists of protein (35–45%), fat (18–20%), and carbohydrates (22–28%), an insoluble fiber called okara, a low percentage of starch and sucrose, and some oligosaccharides such as stachyose and raffinose [4]. The physiologically functional components of soybeans include vitamins such as K, A, B₂, C, and D with a high content of vitamin E and B₁ while they also contain glycosides in the form of isoflavones and saponins as well as functional lipids including lecithin and sterols.

Besides lipid-associated proteins, two types of globulins, 11S globulin and 7S globulin represent the two major components of soybean proteins (Figure 2) [4, 5]. With respect to the physiological function of soybean proteins, Carroll and Hamilton [6] reported in 1975 that consumption of soybean proteins reduces plasma low-density lipoprotein concentration. Sugano et al. [7] and Kohno [8] further elucidated the mechanism through which the indigestible high-molecular-weight fraction of soybeans binds to excess bile acids in the intestinal tract and is excreted from the body. The U.S. Food and Drug Administration has approved the labeling of foods containing 25 g of soy protein per day as foods that reduce the risk of heart disease development due to the cholesterol-reducing effects of soy protein [9]. In Japan, soy protein represents a functional ingredient in food for special health-related use, and several correlated products are being commonly used [10]. It has been demonstrated that beta-conglycinin detected in 7S globulin reduces the visceral fat content and has been authorized for use in food for specific health-related use [11, 12].

Figure 2.

Soybean composition.

Isoflavones, one of the essential components of soybeans, represent approximately 0.2–0.5% of seed weight, and their composition differ depending on the part of the soybean [13]. The isoflavone content per gram of protein in traditional soybean foods such as tofu is approximately 3.5 mg in aglycon units [14]. Isoflavones consist of genistein (~50%), daidzein (~40%), and glycitin (~10%). The physiological function (nonhormonal agent) of equal, a metabolite of daidzein ingested by the body and produced by intestinal microflora, is of particular scientific interest [15]. Isoflavones have been demonstrated to improve the blood lipid profile when consumed in combination with soy protein as a component of soy foods for a period of one to 3 months [16], and have shown to exert an anti-estrogenic effect on breast and prostate cancer that are hormone-related disorders [17, 18].

3. Improvement of soybean characteristics via breeding

Cultivation of soybean varieties suitable for different environments in various regions has been previously investigated, using high yield and pest resistance as the relevant development parameters. Development of soybeans with improved nutritional and functional characteristics has been pursued after utilizing wild species of soybean and employing genetic resources such as genetic mutations that are naturally generated during the cultivation process. Wild soybean (Glycine soja Sieb. et Zucc.) for instance, is characterized by its high protein and low-fat content. Yamada [19] reported that protein content (its main component) is adversely associated with its lipid content based on the examination of hybrids among this legume and soybean strains. Development of high protein soybeans by sacrificing a certain amount of the lipid content and agronomic traits would also be possible. Cultivation of soybeans including a higher protein content compared to conventional varieties is anticipated to contribute in the adaptation of soybean use products for protein intake, and for the improvement of productivity. Furthermore, it has been described that the 11S globulin content of soybeans increases when all 7S globulin genes are externally suppressed using microRNA [20]. Since 7S and 11S globulins are the main proteins in soybeans accounting for more than 50% of total protein content, changes on either protein levels result in significant effects on the gel water holding capacity, which is the main property of soybean protein. These proteins are thus expected to lead in the development of products that are completely different from conventional processed products from soybeans based on gelation properties.

Previous studies have attempted to enhance the content of alpha-tocopherol (α-Toc) and isoflavone through cultivation [21, 22]. Toc is a fat-soluble antioxidant commonly known as vitamin E. There are four naturally occurring homologs: α-, β-, γ-, and δ-Toc, with α-Toc being the most bioactive homolog. The total Toc content in soybean oil is relatively high among vegetable oils and fats. Nevertheless, the α-Toc content is low, ranging 5–7% [23] and as a result, vitamin E activity of soybean is not significant. Importantly, increasing the α-Toc content of soybeans has become a goal of cultivation. Dwiyanti et al. [21] demonstrated that gene expression related with α-Toc content was associated with the expression levels of one of the γ-Toc methyltransferase (E.C.2.1.1.95) isozyme (γ-TMT3) by genetic analysis of hybrids containing high α-Toc content using soybean genetic resources. In contrast, certain varieties have been identified in which α-Toc content increases from ×1.5 to ×5-fold during ripening in high temperature regions compared to that in standard regions [24], and an approach involving both genetic and cultivation environment is crucial when cultivating soybeans containing a high vitamin E content [24].

Various studies have also examined the potential of producing high-isoflavone soybeans [22, 25]. In isoflavone-related genetic analysis, it has been discovered that overexpression of the MYB transcription factor gene regulating the flavonoid biosynthetic pathway, increases the isoflavone and flavonol levels [26], suggesting the possibility of increasing isoflavone content via genetic engineering. In addition, a study has suggested that isoflavone content increases by ×3-fold when grown at a relatively lower temperature compared to that of standard growing conditions [26]. Isoflavone content may be thus increased by selecting varieties with genetic variation while considering the culturing conditions associated to the climate of the production area.

4. Traditional Japanese foods prepared from soybeans

Soybeans have been cultivated throughout Japan since the Yayoi period (1700–2300 years ago) [27, 28] and have contributed significantly in the preparation of Japanese food consumed nowadays. Currently, soybean is cultivated all over the country, from Hokkaido in the north to Okinawa in the south and there are varieties suitable for the cultivation conditions of each region. In the fiscal year of 2019, soybean demand in Japan was 3,670,000 tons, with food-related use accounting for 28% or 1,019,000 tons, while the remaining 72% accounted for non-food applications such as livestock feed. Currently, ~80% of soybeans intended for consumption are imported from overseas, primarily from the United States, Canada, and Brazil [29]. The main food-based applications of soybeans include the production of miso (soybean paste), soy sauce, tofu, fried bean curd, natto (fermented soybeans), dried bean curd, and soy milk (Figure 3). As an essential ingredient in Japanese cuisine, these traditional foods not only contribute to the health and nutrition of the Japanese population, but also in Japanese food culture. In December 2013, the “traditional food culture of the Japanese people” was registered as an UNESCO Intangible Cultural Heritage [30], and Japanese food culture is continuously gaining global attention due to the health benefits and taste of tofu, soy sauce, and miso [31].

Figure 3.

The strain of Traditional Soybean Products.

The original form of miso is considered to have been established in Japan from the Chinese continent during the Nara period (710–794). Over the course of its 1400-year history, fermented soy sauce prepared from soybeans and grains mixed with mold-containing koji and salt has spread throughout the country and has been enhanced to fit each region’s preferences. Soy sauce production is thought to have been established independently in each local environment [32]. Currently, various types of miso are available and they can be grouped into four main types: (1) rice miso prepared from soybeans, rice koji (malted rice) and salt; (2) barley miso prepared from soybeans, barley koji and salt; (3) soybean miso prepared from soybean miso balls, koji and salt; and (4) mixed miso prepared by mixing several types of miso. Rice miso is further classified based on different flavors and colors according to the koji ratio and salt concentration used, and is classified into sweet (white, red), sweet (light-colored, red), and dry (light-colored, red) rice miso while barley miso is classified as sweet and dry [33]. Among these, rice miso is the most widely produced miso and is manufactured in many regions of Japan. Rice miso is prepared by adding rice malt and salt to steamed soybeans and fermented. The salt inhibits bacterial growth as well as proteolytic enzymes, saccharolytic enzymes, and lipolytic enzymes produced by koji degradation to its ingredients; the aromatic flavor components that determine the taste of miso and miso-like color are mediated by the action of salt-resistant lactic acid bacteria and salt-resistant yeast during the fermentation and subsequent maturation processes [34]. Aroma extract dilution analysis (AEDA) is conducted to examine the aroma components in fermented soybean foods such as miso [35]. AEDA involves the detection of components eluted from a gas chromatography (GC) column based on the odor and can analyze the aroma of trace amounts of components that cannot be detected by GC [36]. The complex aroma of raw miso is attributed to major components such as 4-hydroxy-2(or 5)-ethyl-5(or 2)-methyl-3(2H)-furanone and 3-hydroxy-4,5-dimethyl-2(5H)-furanone. Furanone and other recently identified substantially low abundant components are considered to be key factors affecting the aroma of miso [37, 38, 39]. Moreover, it has been revealed that aroma alterations due to heat treatment during manufacturing and cooking procedures may be triggered by an increase in methional content, a major component, and a decrease in the levels of three components, 1-octen-3-one, (Z)-1,5-octadien-3-one, and trans-4,5-epoxy-(E)-2-decenal. Recently discovered trace components are important factors affecting soybean characteristics [40]. Miso contains numerous biological regulators, and it has been reported to inhibit melanin production, osteoporosis as well as to reduce cholesterol and blood pressure [41, 42, 43]. Miso consumption has considerably affected health maintenance of Japanese population during its long history, and is expected to improve accordingly the diets of the global population.

5. Development of new products prepared from soybeans

Soybeans contain an optimal balance of essential amino acids as suggested by their amino acid score of 100. Milk has the same amino acid score (100) and is an essential component of the human diet since ancient times. Throughout the history of food, consumption of milk can be summarized by the “dairy milk tree,” that is characterized by the combination of fermentation and separation of protein-rich and fat-rich milk components [44]. A variety of dairy products ranging from butter and powdered milk to cream, condensed milk, ice cream, cheese, yogurt, and Lactobacillus-based beverages are prepared from milk, and they have become closely intertwined with the food culture of each region. Various studies have examined processing of soybeans to yield water-soluble protein components and oily components such as milk. Nevertheless, protein and fat components in soybeans are strongly associated, and effective classification of the two components represents a major challenge [45, 46].

Fuji Oil organization, which has been investigating soybeans for more than half a century, enhanced the process of separating lipophilic and hydrophilic proteins and developed an approach for separating soy milk into two parts: oily soy cream and low-fat soy milk. This new separation technology has been patented as the world’s first Ultra Soy Separation (USS) method (Figure 4). The soymilk cream and low-fat soy milk separated using the USS process has been confirmed to contain various tastes and functions not detected in conventional soy milk. Soymilk cream has a deep richness of soybeans and improves the flavor of the ingredients combined with it and confers a delicious creamy texture to food. Low-fat soy milk is a healthy ingredient that although contains very limited oil amounts, it has a significantly strong soybean flavor and can be utilized as a soup stock. The first premium soy milk products developed after the USS manufacturing method were presented in the Japanese Pavilion of the Milan Expo in 2015, where food was the main theme and attracted particular attention as a food material that contains a new plant-based taste (Figure 5). A variety of desserts and beverages prepared using soy butter, soy cream, and low-fat soy milk that all benefit from plant-based taste of this novel ingredient, are now available in convenience stores in Japan and attract the attention of “flexitarians” who are interested in plant-based foods. In the near future, “dairy milk tree” based on soymilk cream and low-fat soy milk may further develop with the formation of a variety of new traditional foods. In summary, soybeans are anticipated to contribute in the amelioration of the health of the Japanese and global population.

Figure 4.

USS (Ultra Soy Separation) method.

Figure 5.

Cheese-like soy milk products and their applications.

6. Conclusions

Soybean cultivated globally can be an important strategic crop that can help the resolution of the food shortage issue due to the increasing population based on its high protein content and nutritional value in the near future.