**4.1. Soy milk curdling**

**3. Food processing**

86 Food Production and Industry

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**3.1. Formation of** *Tofu* **curdling**

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**Figure 2.** Formation of *Tofu* curdling [15].

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forming system of *Tofu* curdling is shown in Fig. 2.

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soybeans immobilizes the protein in it [17].

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Soy proteins have properties that produce curd to add specific metal ions. The property is applied for *Tofu* production. Tofu, soybean curd from soybean milk, is consumed throughout Asia. It was eaten in the tenth century in China and Japan. *Tofu* is traditionally consumed after it is produced with a combination of magnesium dichloride (MgCl2) as *Nigari* and calcium dichloride (CaCl2) as *Sumashi-ko*. More recently, glucono-δ-lactone (GDL) has been added to it for commercial production. Tofu resembles cheese or yogurt made from milk curd of cows or other mammals. It is made from soy milk. It curdles by *Nigari* or a coagulant agent [15]. The

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Soy milk is an emulsion composing lipid and soy protein, mainly glycinin and β-conglycinin. Actually, 60% of the protein in soy milk protein is composed of these two proteins, and a large size of the protein body was constructed with their proteins and others. Lipid is a triacylgly‐ ceride composed of linoleic acid, oleic acid, and phospholipids. Before treating, triacylglycer‐ ide is stored in oil bodies in soybeans. During soy milk production, and of course during *Tofu* production, oil drops are suspended, forming an emulsion after crushing of soybeans. The oil drop is formed and held stably by lecithin, phospholipid, and oleosin, all proteins forming oil bodies in soybeans. Furthermore, an outside layer is covered by soy protein. Phytic acid (*myo*inositol-1,2,3,4,5,6-hexaphosphate) and mineral and oligosaccharide are in the soluble com‐ ponent of soy milk [16]. When some bivalent ions such as calcium ion and magnesium ion are added to soy milk, the ions combine with phytic acid. As a result, decreasing the pH of the

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As discussed above, soybean proteins provide rich nutrition [4, 19]. In fact, soymilk consump‐ tion is increasing quickly throughout the world because of its health benefits. Moreover, differently from bovine milk, it contains no cholesterol. Yogurt-like foods and cheese-like foods made from soybeans can be consumed by people who are concerned about health issues or allergies related to bovine milk. Tofu, soybean curd from soybean milk, resembles cheese or yogurt made from milk curd of cows or other mammals. However, the cheese mouthful sense and physical properties are not identical to those of casein protein. Unlike the protein casein in bovine milk, enzymatic curdling of soybean milk produces poor flavor and texture.

It is not yet a viable alternative to dairy foods. For that reason, the commercial use of enzymes such as bromelain, ficin, and papain for the curdling of soybean milk has been unsuccessful [20–22]. The authors have reported that physical properties of soy protein modified by enzyme reactions such as germinated proteolysis in soybeans. Therefore, in this section, along with a report of yeast containing soybean curdling enzyme [23], this investigation was undertaken to screen and identify specific food yeast strains (*Saccharomyces* sp.) that produce a soybean milk curdling enzyme and to purify the enzyme using chromatographic procedures.
