Fermented. Soy Foods and Sauce

Lewis B. Lockwood, Allan K. Smith.

Soybeans, in the form of soy sauce and soybean cheese, paste, sprouts, milk, and curd, have been an important source of protein in the diet of the Chinese and Japanese, for centuries. In Asia the whole bean is not ordinarily eaten. The people favor mostly fermented soy products or other modifications sprouts. curd, or milk in which the characteristic flavor and shape of the beans are lost. Only soy sauce and monosodium glutamate have found much favor in Western countries.

Asiatic people live largely on a vegetable diet. It is estimated that in China meat and eggs make up less than 3 percent of the food of the peasants, compared to 21 percent among Americans. About 95 percent of the protein eaten is of vegetable origin. Much of it comes from soybeans, which form about 20 percent of the basic diet in Northern China.

Soy sauce, shoyu in Japanese, is the most popular use made of the soybean. It is a dark-brown liquid, very salty and sharp in flavor. Its odor suggests cooked beef. It accentuates the flavor of vegetables and meat. Sweetening and thickening agents and spices may be added to give the sauce variety in flavor.

Soybean paste is a semisolid, mushy food. Its flavor is like that of soy sauce. In Japan it is known as miso. It is used as a relish for rice, in soups, and to add flavor to vegetables.

Soybean cheeses of the Orient are solid or semisolid. They are made by fermenting the soybean curd. Soybean cheeses are unlike American and European cheeses in flavor and appearance; it is too bad that the name cheese has been applied to them. Some of the soybean cheeses have a flavor resembling soy sauce. Others are quite different. Many are too salty for the American taste.

The Chinese made soy sauce in ancient times as a household industry. Descriptions of the process are found in books written more than 1,500 years ago. It has remained largely a family art: even now some manufacturers point with pride to the fact that their factories have been operated as family enterprises for five centuries.

Soy sauce is manufactured by two basic processes. One involves a fermentation technique and the other a chemical method. A third procedure, which is thought to have some advantages, is a combination of the two. The third method is still in the developmental stage. The products of the different methods differ somewhat in taste and odor; the fermentation product is the most acceptable. Sometimes the products of two basic processes are blended.

The fermentation method is a mixed fermentation by three micro-organisms: A mold, Aspergillus oryzae; a bacterium, Lactobacillus delbruckii; and a yeast, Zygosaccharomyces soja, Z. major, or a yeast closely related to Hansenula anomala. In the traditional Chinese method, the manufacturer adds a prepared mold culture, a koji, to the soybeans, but relies on chance inoculation for the bacterium and yeast. Modern methods include the use of pure-culture inocula of all the microorganisms needed in the fermentation.

The materials are soybeans, wheat or other starchy grains or flour, and salt. First, the beans are washed and then soaked in water for 12 to 24 hours, depending on the temperature or season of the year. Longer soaking is needed in winter or when the temperature is low. The soaking finished, the beans are drained of excess water and cooked with steam under 10 pounds pressure. The cooking period covers several hours.

The second step includes the addition of soft wheat, which has been cleaned, roasted, and cracked or very coarsely ground. It is mixed with the cooked soybeans in the ratio of about 3 pounds of wheat (initial weight) to 10 pounds of cooked soybeans (initial weight).

In the third step, the mixture of beans and wheat is inoculated with a culture of one or more strains of the mold Aspergillus oryzae. The inoculation cultures are made by growing the mold on steamed polished rice. An ounce of rice will make enough koji to inoculate 10 pounds of the soybean-parched wheat mash. In modern factories, cultures of the necessary yeast and bacteria are added at this point. A good yeast growth before the mold growth becomes apparent is believed to result in a sauce of superior quality. In some modern plants the order of adding the micro-organisms is changed; the yeast is added to the steamed beans about a day before mixing them with the parched wheat, and the yeast starts to grow before the mold gets under way. The procedure is said to give very good results.

After inoculation, the mash is spread in a layer 3 inches deep in wooden trays or baskets about 4 inches deep. The trays are stacked so that air will circulate freely over the beans. During the fermentation stage, the mold grows throughout the mash and gives off considerable heat. The temperature of the material may reach 40 C. (104 F.) or higher, if it is not controlled. This phase of the process lasts about 3 days. A thin white surface growth of mold appears, and turns yellowish as spore formation begins. The brine fermentation, or second phase of the fermentation process, is then started. If it is started too soon, the supply of the mold enzymes, which are responsible for certain necessary chemical reactions, will be inadequate. If the start is delayed too long, many spores will cover the surface of the beans and may contribute undesirable flavors to the sauce.

This molded soybean-wheat mash is placed in deep vessels and barely covered with brine made with 22-percent salt solution. In the old Chinese factories, 50-gallon earthenware vessels are used. These are set in the open and are covered only during rain. Many of the old Chinese manufacturers believe that sunlight, even moonlight, affects the flavor of the sauce.

The beans are stirred daily for the first few weeks, then weekly until the end of the fermentation period. In modern Japanese and Chinese factories the aging process is carried out in concrete vats of about 5,000-gallon capacity. Air is blown through the bean mash to stir and mix it every 2 or 3 ) days in the beginning, but after several weeks the material is aerated for 30 minutes once a week. Gas is given off during the first 2 weeks of the brine fermentation.

After 3 months preferably after a year or longer the mash is pressed to remove the liquid. This is considered the best grade of soy sauce. A second grade is made by suspending the press cake in 18- to 20-percent brine and pressing. A third (occasionally a fourth, or even a fifth) grade is prepared by further extraction of the press cake in like manner. Each extraction gives a product of weaker flavor and hence of less commercial value than the preceding one. The sauces are pasteurized at about 65 C. (149 F.). If higher pasteurization temperatures are used, a cloudy sauce results because of the precipitation of partly degraded proteins. Alum is then added as a flocculating agent, and the sauce is filtered.

The salt content of the sauce, 18 to 20 percent, prevents the growth of most micro-organisms. Sodium salicylate or B-naphthol may be added as preservative, but that is not necessary if the salt content is high enough. Cloves, cinnamon, nutmegs, pepper, ginger, and other spices enhance the flavor and are believed to prevent spoilage. They are added only to the lower-grade products. Caramel may be added to darken the sauce. Licorice or maltose may be added to sweeten it.

The chemical changes in the production of soy sauce are complex and interrelated. Wheat serves as a carbohydrate source for the growth of the micro-organisms. The mold undoubtedly supplies the enzymes necessary to convert the starch to sugar, which, in turn, is acted upon by all three microorganisms. The mold and yeast produce some alcohol from the sugar. The bacteria produce lactic acid and other organic acids. Esters, such as ethyl acetate, are also formed by interaction of the alcohols and organic acid. They account for much of the aroma and flavor of the sauce. Other important flavor constituents are amino acids or salts of amino acids. Monosodium glutamate is the most important of these. The amino acids are produced by enzymatic decomposition of the proteins of the soybeans and wheat. Most of the protein degradation and carbohydrate fermentation occurs during the first 2 weeks of the brine fermentation. After this time, the flavor matures by very slow reactions, which involve the formation of esters and the splitting of dextrins.

The chemical composition of soy sauce with a specific gravity, at 15 C., of 1.19 to 1.20 is (in parts per 1,000 parts of soy sauce) : Total solids, 250; sodium chloride, 130 to 150; total nitrogen, 6 to 13; protein nitrogen, 0.8; amino nitrogen, 3 to 6; volatile acids (as acetic), 8.0 to 40.0; nonvolatile acids (as lactic), 50; sugar (as glucose), 20 to 70; and dextrin, 8.

IN THE MANUFACTURE OF SOY SAUCE by the chemical method, soybean meal is first extracted with water, which has been adjusted with acid to the isoelectric point of the protein in the meal. The isoelectric wash removes soluble carbohydrates and nitrogen compounds and raises the protein content of the meal above 65 percent. The meal is then cooked with 17 percent hydrochloric acid under a steam pressure of 30 pounds to the square inch for 15 to 18 hours to reduce the protein to amino acids. In place of the soybean meal, soybean-protein curd may be used as the starting material, and constant-boiling hydrochloric acid used in the cooking process. After hydrolysis of the protein, the acid is neutralized with sodium hydroxide or sodium carbonate, and the resultant solution is marketed as chemical soy sauce. Sometimes, after incomplete neutralization, a crop of crystals of monosodium glutamate is removed. The remaining amino acids are neutralized, and the concentration is adjusted to a suitable amino-nitrogen content. Such a sauce is poorer in glutamic acid but richer in other amino acids.

In the soy sauce prepared by the chemical method, the protein hydrolysis is more complete than in the fermented product. The chemically produced material is essentially a solution of salt and amino acids. Some of the flavor and odor constituents of the fermented sauce (such as peptides, alcohols, esters, and nonnitrogenous organic acids) are not found in the chemically manufactured sauce.

Recent attempts to combine the convenience, economy, and speed of the chemical method with the desirable flavor characteristics obtained by the fermentation process involve the partial hydrolysis by acid treatment, followed by neutralization of the acid. The material is then fermented for about a month by the organisms used in the fermentation method.