Soymilk proteins

Figure 1, a modification of the method used by Puski and Melnychyn (JO, shows the preparative procedure used to isolate an acid-precipitated fraction of soymilk protein (RDP). 

Figure 7. Densitometer tracings for soymilk proteins and RDF fractions as a function of heat...

Tofu. Tofu is prepared by adding a coagulant such as calcium sulfate to soymilk to precipitate the protein and oil into a gelatinous curd. The curd is then separated from the soluble portion (whey), pressed, and washed to yield a market-ready product. Tofu, a traditional food in Japan (90), was populari2ed in the United States in the late 1970s and is available in many U.S. supermarkets. 

Table II. Protein to Oil Ratio of Tofu and Soymilk as Affected by Protein and Oil Content of Soybeans (14)...

Although not available commercially, morama milk can be consumed as a refreshing and nutritious beverage similar to dairy milk or soymilk. It can be used as an infant supplement providing additional protein, energy, and other nutrients to vulnerable populations where the supply of dairy milk is inadequate. It can also be an intermediate material for other applications such as yoghurt. 

The chemical composition of morama milk is indicated in Table 5.11 and it is compared with soymilk and dairy milk (Mpotokwane et ah, 2007). Morama milk has 6% total solids compared with 10% for soymilk and 12% for dairy milk. The morama milk solids include protein, which is about 1.5%, fat is 3.1%, carbohydrates is 1.1%, and ash is 0.2%. It has high levels of sodium (47.9 mg/100 g) and iron (3.7 mg/100 g) compared with the other two milks but much lower calcium (6.8 mg/1000 g). The proportion of unsaturated fatty acid in morama milk is significantly higher than in soymilk and dairy milk (Jackson et al., 2009). 

Fermented morama milk products such as morama yoghurt have been reported by Phuthego et al. (2009). Morama milk yoghurt like soymilk yoghurt, provide economic and nutritional benefits, because they are likely to have higher protein levels at comparable or lower cost than regular fermented milk products (Karleskind et al., 1991). This is because... 

Irreversible Insolubilization of Soybean Protein During Drying. Soymilk is an economical high-protein food of high nutritive value produced by grinding soaked whole soybeans with water, heat-... 

Figure 1. Scheme for isolation of acid precipitate protein from soymilk, adapted from Ref. 4... 

Table II gives the results of residual trypsin inhibitor levels for the various soymilk preparations. The 90 and 120 sec microwave treatments were the most effective in inactivating the trypsin inhibitor complex while hot water treated and unheated samples showed the highest levels. It is not surprising to find that microwave processing is more efficient than hot water in suppressing trypsin inhibitor considering the rapid penetration of food material by microwaves and the susceptibility of protein action to small heat induced changes in tertiary structure. Hence, Collins and McCarty (12) found microwaves produced a more rapid destruction of endogenous potato enzymes (polyphenol oxidase and peroxidase) than hot water heating.

Effect of heat processing method on emulsion stability of an acid-precipitated protein fraction from soymilk... 

Gennadios, A., and Weller, C.L. (1991). Edible films and coatings from soymilk and soy protein. Cereal Food World. 36, 1004-1009. 

The protein solubility test (KOH) or nitrogen solubility index (NSI) is an indicator of solubility of protein in an alkaline solution of potassium hydroxide (KOH). It indicates the percentage of total nitrogen (protein) that is soluble. Protein solubility is important for tofii and soymilk production and for estimating the level of toasting or damage in soybean meal. Soybean meal should have a protein solubility index when... 

In addition to the products of lipid oxidation, methanethiol and dimethyl trisulfide were shown to contribute to the complex odor characteristic of soy protein products such as SPI and soy protein concentrates (Boatright Lei, 2000 Lei Boatright, 2001) and soymilk (Lozano et al., 2007) at concentrations comparable to hexanal. Since the threshold in water for methanethiol was reported at 0.02 ppb compared to hexanal at 4.5 ppb (MacLeod C Ames, 1988), these sulfur compounds are intense flavor notes in soy protein products. Lei and Boatright (2007) provided evidence that methanethiol is generated in aqueous slurries of SPI or defatted soy flake from methionine by a free radical mechanism involving manganese, sulfite, and... 

Guo, S.T. T. Ono. The role of composition and content of protein particles in soymilk on tofu curding by glucono-5-lactone or calcium sulfate./. FoodSci. 2005, 70, C258-C262. 

Liu, Z.S. S.C.K. Chang L.T. Li E. Tatsumi. Effect of selective thermal denaturation of soybean proteins on soymilk viscosity and tofu s physical properties. Food Res. Int. 2004,37, 815-822. 

Soymilks in North American markets nowadays are mostly chilled products, made by modern food-processing techniques. The starting material can be whole soybeans, soy flour, soy protein isolates, or combinations of the former. 

A steam-infusion cooking process, known as hydrothermal cooking (HTC), was developed to produce soymilk continuously from ground full-fat soy flour (Johnson et al., 1981). It was claimed that soymilk processed by the HTC process had less beany flavors because of the much shorter time for lipoxygenase to be active and because steam flashing stripped volatiles. The process also increased recovery of dry matter and protein in the soymilk. 

Typically, soymilk contains 8-12% total solids, depending on the water bean ratio used during processing. Protein content is about 3.6% fat, 2.0—3.2% carbohydrates, 2.9—3.9% and ash, -0.5%. The composition of soymilk compares favorably with those of cow s milk and human milk (Chen, 1989). In addition, soymilk is lactose-free and contains higher levels of protein, iron, unsaturated fatty acids, and niacin however, it contains lower amounts of fat, carbohydrates, calcium, riboflavin, thiamine, methionine, and lysine (Kosikowski, 1971). Therefore, many commercial soymilks are fortified with vitamins, minerals, and in some cases, amino acids. The most widely used nutrients for fortifying soymilk are vitamin calcium, and methionine. 

The major reason for this difficulty arises from the fact that there are complex interactions of many factors that are involved in the making of tofu (Saio, 1979 Skurray et al., 1980 deMan et ah, 1986 Beddows Wong, 1987a, 1987b Ohara et ah, 1992 Shi Ren, 1993 Shih et ah, 1997 Liu, 1999 Kim Wicker, 2005 Mine et al., 2005 Poysa et al., 2006 Yoon Kim, 2007). These factors (or variants) mostly center around three key areas (i.) the way soymilk is prepared, (ii.) the way soy protein is coagulated, and (iii.) the way tofu is pressed and packaged. 

During yuba production, protein and lipid contents in the film successively formed decrease, while the carbohydrate and ash contents gradually increase. Therefore, the first several pieces of yuba to be lifted off the heated soymilk are considered premium products. They have creamy white color, mild flavor, and less sweet taste. They stay relatively soft and flexible even when dried. The later formed products are regarded as second rate, because they become sweeter with a faintly reddish tinge, lack internal cohesiveness, tear more easily, and become brittle upon drying. 

Chen, S. Preparation of Fluid Soymilk. In Proceedings of the World Congress on Vegetable Protein Utilization in Human Foods and Animal Feedstuffs. Applewhite, T.H. Ed. AOCS Press Champaign,... 

Other soy protein products include dried soy milk and tofu (see the Chapter Food Use of Whole Soybeans), as well as mixtures of SF, SPC, or SPI with milk or egg protein, gelatin, or other components for specific functional applications. Extruder-texturized fburs and concentrates and spun fiber isolates, which resemble meat in appearance, may be made at the point of use but usually are supplied in bulk from strategically located production facilities. Edible co-products of soy protein ingredient manufacture include the hulls and the cotyledon fiber (cell walls) from SPI or soymilk production. The extract by-product from SPC may be used as the feedstock for isolating isoflavones. 

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