Auditorio de la ciudad de Huancayo (Tesis nacional)

Coagulation of soy milk is the most important step and the most difficult to master in the tofu-making process because it depends on soybean characteristics, soy milk processing variables, coagulant characteristics, and coagulating variables (Shih et al., 1997). Soybean characteristics include the soybean cultivar, the growing environment, and storage conditions. Soy milk processing variables include, but are not limited to, soybean soaking conditions, the water-to-bean ratio, grinding conditions, and soy milk cooking conditions. Coagulant characteristics include coagulant types and coagulant solution concentrations. Coagulating conditions include methods of mixing coagulant solution into soy milk, soy milk temperature, and amount of coagulant added into soy milk. The amount of coagulant added into soy milk, or the concentration of coagulant in soy milk, is a critical variable during soy milk coagulation. It profoundly affects the yield and textural properties of the resulting tofu. It is therefore desirable to use coagulants at optimal concentration during coagulation (Liu and Chang, 2004).

Insufficient amounts of coagulant may result in incomplete precipitation of soy protein and make the subsequent filtration difficult, whereas excess amounts of coagulant make the texture of soybean curd hard and unpalatable. It appears that calcium sulfate is the most commonly employed salt in soybean precipitation; however, this salt is practically insoluble. The addition of this salt therefore needs skill; otherwise, the quality of soybean curd may vary from time to time.

Optimal coagulant concentrations (OCC) are, however, affected by soybean characteristics, soy milk processing conditions, and coagulating conditions. Automating the mechanical operation of coagulation intelligently, especially when quick-acting coagulants such as CaCl2 and MgCl2 are used, is challenging. Under fixed mechanical

conditions for coagulation, variance in OCC results from soy milk characteristics (Liu and Chang, 2003).

Information about the relationships between OCC and soy milk characteristics is limited (Skurray et al., 1980). Soy milk cooking is a prerequisite for tofu-making, and cooking conditions affect soy milk properties. However, the effect of soy milk cooking conditions on OCC has only recently begun to be investigated. The difficulties have led to lack of results of measuring OCC. A rapid titration method to determine the critical point of coagulant concentration (CPCC) was developed by Liu and Chang (2003). The availability of this convenient and reproducible method has made it possible to make an extensive study of coagulant requirements for a number of soybean materials. The study shows that the CPCC is a characteristic parameter of soy milk and could be used as an effective indicator of OCC for making filled tofu. Depending on methods of preparation, textural properties, and moisture content, commercial tofus are generally classified as dry tofu (Doufugan), firm tofu (Momen), soft tofu, silken tofu (Kinu), and filled (packed) tofu. In filled tofu preparation, a coagulant is normally added to cooled soy milk, followed by heating without mixing to initiate and finish the coagulation process of the proteins to form curd in the package. This unique processing feature makes filled tofu preparation the most easily controlled. In addition, curd does not need to be broken and transferred. Therefore,

filled tofu is a good product for monitoring of changes in tofu texture resulting from soy milk or coagulant (Liu et al., 2004).

Soy milk characteristics including solid, protein, phytate, pH, titratable acidity, mineral content, and 11S/7S protein are affected by the heating rate, heating time, and sequence of dilution and heating (Liu et al., 2004). The CPCC was significantly (p < 0.05) positively correlated with phytate content (grams per gram of protein), pH, and 7S protein content but negatively correlated with protein content, 11S protein content, 11S/7S ratio, titratable acidity, and original calcium content. Within the same soybean material, more protein required more coagulant, but higher protein concentration during cooking resulted in less coagulant required by each gram of protein during coagulation. The CPCC decreased with increasing soy milk heating time or a decreasing heating rate. The sequence of heating and diluting to prepare soy milk also had an effect on CPCC (Liu et al., 2004). This type of information is helpful for the understanding of the mechanisms of tofu- making. It will ultimately enable tofu manufacturers to control the coagulation process to improve end-product quality.

Shen et al. (1991) used nine soybean cultivars to study characteristics that affect yield and quality of tofu coagulated with GDL. The yield of tofu was not affected by the size of beans. Protein and total solids in soymilk increased when protein and moisture increased in soymilk. The yield of pressed GDL tofu increased with the protein content of soybeans (or soymilk) plus decreased calcium content. The yield of pressed GDL tofu was 20% higher than CaSO4 tofu.

Tofu-curd is made by the flocculation of proteins in soybean milk with an addition of calcium. The proteins consist of soluble and particulate fractions. The influences of calcium and pH on the protein solubility of these fractions were investigated. The protein particles precipitated at lower calcium concentrations than those of soluble proteins. This precipitation of proteins took place at higher pH with calcium than that without calcium. It is therefore considered, that the first step to tofu-curd is the formation of a network by the protein particles at low calcium concentration. The next step was deemed to be the binding of the soluble proteins to the network by further addition of calcium and decreasing pH (Ono et al., 1993).

Tofu manufacturers use a variety of tofu grade soybeans, identity preserved, blended, and flaked, for soymilk and tofu. Some important manufacturing variables are soaking and grinding characteristics, water to soybean ratio for soymilk; solids in the soymilk; and time and temperature of heating the soymilk. Also important are the temperature and extent of stirring during coagulation and type and concentration of coagulant. The amount of coagulant added in the manufacture of tofu is one of the critical control points, which helps determine the product’s texture, taste, flavour and yield. Past laboratory and pilot-scale tofu research has focused on determining and standardizing optimum levels of coagulant to be used for tofu processes (Johnson and Wilson, 1984).

Moizuddin et al. (1999) indicated that good soymilk coagulation occurs when the curd has separated and has moved away from the edges of the coagulation effect. Existing methods to determine optimum coagulant concentration such as light transmittance (%T) of whey, whey volume and tofu yield (Sun and Breene, 1991) are measured after coagulation is complete. The transparency method also requires the whey to be free of

coagulated particles for reproducibility. The time required to cool the samples for measurement allows the soymilk to cool below the critical coagulation temperature. This also limits the measure of pH to indicate the optimum amount of coagulant during processing. The whey volume, tofu yield and composition could be used for prescreening. However, they cannot serve as indicators to adjust the batch being coagulated. Rapid methods for measuring the optimum coagulant concentration during tofu manufacture have been reported by Moizuddin et al. (1999). Their research showed that measuring conductivity to determine optimum coagulation was fast and reproducible. Unlike pH, their probe contained no chemical compounds that could leak and it could tolerate a high heat environment. Thus it could be used directly in the coagulation vessel or in-line in a continuous processing system.

Cai and Chang (1998) researched characteristics in the production of tofu as affected by a soymilk coagulation method. Their results showed that increasing the propeller size of a stirrer can reduce stirring time and the amount of coagulant required for coagulation. Different soybean varieties may require different coagulation conditions to maximize their tofu yield and quality. For large-scale tofu production, the coagulation variables can be adjusted to compensate for the varietal physicochemical differences to improve tofu yield and quality. The results of the varietal study also imply the potential limitation in developing a set of experimental conditions for the evaluation of the suitability of various soybean varieties for tofu making.