Soybean

Soybean

The soybean, Glycine max, belongs to the family Leguminosae, which refers to the fruits of the flowering plants, legumes. Historical and geographical evidence indicates that the soybean originated from East Asia, specifically Northern China. Soybean has been cultivated and incorporated as food and medicine into the daily lives of the Chinese for the past 5,000 years. In China, the soybean was regarded as one of the sacred grains, including rice, wheat, barley, and millet.

During the process of cultivation, the Chinese has transformed soybean into various types of soy foods. Soy paste (Jiang or Chiang in China and Miso in Japan), soy sauce, stinky tofu, natto, and tempeh are to name a few. These cultivation methods and soy food preparation were progressively introduced to other Asian countries, such as Japan, Vietnam, and Korea. Each country slightly modified the soybean product.

Soybeans were first introduced to the United States in the 18th century. Only until the 1900s did soybean become an important fruit. Currently globally, production of soybean is estimated at 150 million metric tons, with major producers being the Unites States, Brazil, China, India and Argentina. Demands of soybeans have been significantly increasing due to its nutritious and medicinal values.

Classification

High Order Taxa

Plantae (Kingdom); Magnoliophyta (Phylum); Magnoliopsida (Class); Fabales (Order); Leguminosae or Fabaceae (Family); Glycine (Genus)

Species

Glycine max

Description and physical characteristics of soybean seed

Soybean is a hearty plant that can be easily grown. Soybean seeds are spherical to long ovals in shape. Although most soy seeds are yellow, soy seeds come in other various seed coat colors, such as: blue, green, dark brown, purplish black, or black. Soy seed varies in size, too. The seed coat of a mature soy seed is extremely hard and water resistant so that germ that is encased within the soy seed is protected. Damages to the seed coat inhibit the seed from germinating.

Chemical composition of the soybean seed

Soybean seeds are extremely high in protein content. On average, dry soybean contains roughly 40% protein, 35% carbohydrate, 20% soybean oil, and 5% ash (non-aqueous, metal oxides). Therefore, soybean has the highest protein content among legume species. Soy protein is a heat-stable protein, thus allowing soy seeds to undergo high temperature cooking and fermentation, without destroying the entire chemical composition of the soybean.

The soluble carbohydrates in soybeans are made up of various saccharine: disaccharide sucrose, trisaccharide raffinose, and tetrasaccharide stachyose. These soluble carbohydrates can easily be broken by microbes down during fermentation to create a distinct flavor, odor and texture in soy products.

Other valuable components that are found in soybean include phospholipids, vitamins, minerals, and isoflavones. Asia has referred to soybeans as the “miracle beans” and the “yellow jewel.”

Cultivation of Soybean

Soybeans are successfully grown in regions with high temperature. Since the soy protein is a heat-stable protein, high temperatures cannot easily destroy the seed itself. The optimum temperatures to cultivate soybeans are 20°C to 30°C (68°F to 86°F). Temperatures below 20°C and above 40°C can inhibit growth of the soybean plants. Soybeans can be grown in soil or sand, however, soil with high content of clay is not an optimal environment to grow soybeans. Soil with high organic contents allows soybeans to perform nitrogen fixation. By establishing a symbiotic relationship with the bacterium, Rhizobium japonicum, soybeans and R. japonicum, an aerobic microbe, can break down nitrogen gas from the atmosphere into ammonia, which is a nitrogen product that is usually low in the soil

Fermentation of Soybean

Stinky Tofu

Stinky tofu, or chaotofu, is a traditional Chinese dish where tofu is fermented in a stinky brine to create a specialized flavor, color and odor. Stinky tofu, along with Chinese soy cheese called sufu, is block-type fermented soy foods. During fermentation of stinky tofu and sufu, sufu is made by utilizing molds, whereas bacteria aid in the fermentation process in stinky tofu.

A bacterium is smaller than mold and their functions are significantly different. Molds are capable of multiplying under aerobic conditions. In these aerobic environments, molds produce amylases, proteases, and other hydrolases. Bacteria, on the other hand, utilize and forms proteolytic enzymes made by microorganisms under anaerobic conditions. The proteolytic enzymes that are produced in the anaerobic, stinky brine partially hydrolyze the proteins in the tofu to make the soy proteins more digestible. Intermediate metabolites, such as ammonia, are produced from the proteolytic enzymes, thus yielding an odorous brine or stinky brine.

The stinky brine that is used to make this distinct tofu come in different variations. The stinky brine’s ingredients can compose of: various vegetables such as (a) amaranth leaves, bamboo shoots, and winter melon, (b) salted mustard brine with shrimp and salted egg brine, (c) fish, shrimp, and animal organs or (d) strong ammonia, which speeds up the overall fermentation process of tofu. Carried out in an open-fermentation process, the stinky brine allows the tofu that soaked in the brine to undergo a natural zymotic growth by producing a strong, stinky odor. The main bacterium that is involved in the fermentation of stinky tofu is the Bacillus sphaericus.

Bacillus sphaericus is an aerobic, mesophillic, spore-forming bacterium that is naturally found in soil. B. sphaericus is part of the Bacillus family. B. sphaericus has a circular chromosome made up of 4,639,821 base pairs, with a 37% GC content and a two-copy plasmid (pBsph) of 177,642 base pairs, with a 33% GC content. There are 85 tRNA genes representing all twenty of the amino acids and 10 rRNA operons in the chromosome. B. sphaericus is a gram-positive bacterium, which contains a thick cell wall composed of peptidoglycan. The cell is rod-shaped and can form endospores. B. sphaericus is incapable of directly breaking down polysaccharide, thus requires an exclusive metabolic pathway that can utilize a wide variety of organic compounds and amino acids. B. sphaericus is capable of growing in the presence of oxygen, therefore utilizing oxygen as part of its aerobic cellular respiration.

After the stinky brine is inoculate with B. sphaericus, the ammonia content increases from 100mg/L to 3400 mg/L due the protein in the tofu is hydrolyzed by microbial proteases that forms amino acids, followed by deamination processes to form ammonia. Throughout the fermentation process, the ammonia gradually increases due to the growth of more alkali-tolerant bacteria, which is more favorable over lactic acid bacteria. Due to the favorable growth of alkali-tolerant bacteria B. sphaericus, the cell count of the overall lactic acid bacteria decreases. With initial introduction of the bacterium, the pH drops from 6.5 to about 4.7 due to the production of lactic acid and to the growth of lactic acid bacteria. During the rest of the fermentation process, the pH increases slowly until it has reached pH of 7.5.

Natto

Miso comes in a variation of colors

Miso

Miso is Japanese fermented soy bean paste or semisolid that can be served as a soup or used as a seasoning to heighten the flavor of meat and poultry. Miso is mainly made from soybean with addition of enzymes from rice, wheat, or soybean koji and salt. Koji processing utilizes a filamentous mold called Aspergillus oryzae as part of the fermentation process.

Miso comes in a variety of colors. Based on their colors, miso can be classified as white miso (butter color), red miso (reddish brown color), and light-color miso (light yellowish/ golden). The different colors of the miso are differentiated by their fermentation process.

The main components of miso are soybean, rice, or wheat, salt and water. The secondary components are koji, seasoning and nutritional enrichment ingredients, preservatives and ethanol. The soybean is rich in protein and lipids, which is suitable for miso processing. Rice is can be used miso processing because it has a high moisture uptake, low viscosity, and high rice koji enzymatic activity, which is needed to yield a strong sweet taste and aroma after the saccharization of saccharides that are abundant in soybean. Wheat is another alternative ingredient in miso processing because wheat is rich on glutamic acid, which results in stronger umami flavor, aroma and bolder miso color. Koji, or Aspergillus oryzae, is used as a starter mold, which contains medium-length hyphae with sporangiophores. Short hyphae produce stronger proteases while short hyphae produce stronger amylases. As a result, sweeter miso uses A. oryzae with high amylase activity, while salty miso uses A. oryzae with high protease activity. The optimum temperature for A. oryzae growth is 30°C to 35°C, with a relative humidity of 95%. However, the optimal temperature for the digestion of protein and saccharides by the enzymes from miso koji are 45°C to 50°C for protease and 55°C to 60°C for amylase. The optimal pH for A. oryzae is pH 6.0. As the pH decreases, the protease activity increases.

Steaming the soybean is required to prevent the growth of Bacillus subtilis contamination. B. subtilis can inhibit the growth of A. oryzae. After the soybean has been streamed to remove microorganisms that adhere to the surface of the soybean, salt and a brine, which is composed with yeast (Saccharomyces rouzii and Torulopsis versatilis) and a bacterium called Pediococcus halophilus, is mixed with the streamed soybean. P. halophilus is a essential in the miso processing. Pediococcus halophilus is an anaerobic, coccus-shaped, salt-tolerant bacterium.

P. halophilus has a circular chromosome made up of 29,924 base pairs, with a 35% GC content. P. halophilus is gram-positive, non-motile, and non-spore forming. It is categorized as lactic acid bacteria, which are a group of bacteria that produces lactic acid as a metabolic end product of saccharide fermentation. P. halophilus can break down sugars, which are abundant in soybeans, by utilizing the enzyme glucose dehydrogenase. P. halophilus cannot grow in the presence of oxygen.

During the fermentation, the environment becomes suitable for salt-tolerant organisms, such as p. halophilus. The optimal temperature for bacterial growth is at 30°C. P. halophilus stops growing at 40°C. With introduction of the yeast and P. halophilus, the pH drops from 5.7 to 4.9-5.1. Throughout the fermentation process, the nitrogen concentration also increases. The desirable nitrogen concentration should be 1.51. It has been reported that yeast and P. halopohilus produced non-volatile amines, such as tyramine, histamine, and phenethlamine, which are not detected from the ingredients used to make miso. The interactions between the various molds, yeast and bacteria results in acids reacting with alcohols to produce esters, which contributes to the miso’s aroma. Another interaction that occurs is that the color is produced by the interaction of the amino acids and sugars. Amino acids play the dual role of enhancing flavor and darkening the color of miso.

References

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2. Lui, Keshun. “Fermented Soy Foods: Overview.” Handbook of Food and Beverage Fermentation Technology. New York: Marcel Dekker, Inc., 2004.

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7. Messina, M., C. Gardber, and S. Barnes. Gaining insight into the health effects of soy but a long way still to go: Commentary on the Fourth International Symposium on the Role of Soy in Preventing and Treating Chronic Disease. J. Nutri. 132(3): 547S – 551S, 2002.

8. Teng, D.F., C.S. Lin, and P.C Hseih. Fermented Tofu: Sufu and Stinky Tofu. Handbook of Food and Beverage Fermentation Technology. New York: Marcel Dekker, Inc., 2004.

9. Hu, Xiaomin et al. “Complete Genome Sequence of the Mosquitocidal Bacterium Bacillus sphaericus C3-41 and Comparison of Those Closely related to Bacillus Species.” Journal of Bacteriology 190.8 (2008): 2892 – 2902.

10. Teng, D.F., C.S. Lin, and P.C. Hseih. Fermented Whole Soybeans and Soybean Paste. . Handbook of Food and Beverage Fermentation Technology. New York: Marcel Dekker, Inc., 2004.

Photo Credits

Edited by [Amelia Cline and Mimi Van Dang], students of Rachel Larsen