Soybeans contain around 40% of high quality protein and 20 % of oil. Soy protein has long been used as ingredients for its emulsification and texturizing properties in a variety of foods, soymilk and tofu being the most popular. Soymilk is essentially a water extract of soybeans and there are many variations on the basic soymilk processing steps. Tofu, or bean curd, is made by coagulating soy milk, and then pressing the resulting curds into blocks. This thesis was mainly devoted to thermal denaturation and coagulation of soy proteins and targeted several selected important factors as they relate to the functional properties.
The effects of different chemical coagulants as well as proteases on yield and quality of tofu from soybeans were studied. Eight tested chemical coagulants were able to coagulate the soymilk and the results showed that the concentration of soymilk and type of coagulant had a great influence on the properties of the tofu gel. The results also confirmed that the use of a suitable concentration of the quick-acting coagulants is more critical than that of the slow-acting coagulants in tofu making. In general, the extent of soymilk gelation is not determined by a single characteristic but rather results from a combination of factors.
The gelation ability of various most common commercially available proteases to coagulate non-defatted soymilk was surveyed and the thermal stabilities of selected protease systems were compared. The difference in the temperature where the enzyme shows its highest activity seemed to be the most significant indicator when choosing a suitable enzyme for a certain industrial application. The three most effective and versatile soymilk coagulants were identified. The presence of small amounts of ficin in the system increased the protein recovery when calcium chloride was used as a coagulant.
The most commonly used techniques of analysis of degree of hydrolysis (TNBS, OPA and pH-stat) of soy protein were compared. It was concluded that the pH-stat technique was useful for evaluating the progress of an enzyme-catalyzed protein hydrolysis process on an industrial scale while the OPA method seemed to be the most suitable method to be used for determining DH during the proteolysis of soymilk in laboratory conditions.
The roles of soybean proteins, protein fractions and subunits to differences in gelling properties of different soybean varieties were examined. The variability and the interrelationship between soybean seed traits were established and the seed characteristics related to soymilk yield and tofu quality were identified. The results suggested that it is useful to predict the quality of tofu from a combination of characteristics of the soybean seed. It was concluded that large differences exist in soybean seed characteristics and their contributions towards the properties of the final product and implications were made towards the relative importance of individual soybean seed traits to the functional and textural properties of soy products.
The SDS gel capillary electrophoresis was applied to characterize soybean storage proteins. The lab-on-a-chip technology was compared with capillary electrophoresis and these two methods were used to quantify the relative amount of 7S and 11S fractions in various soybean cultivars. It was concluded that both lab-on-a-chip instrument and a traditional CGE were adequate for analysis of soy-based products. Both systems were able to reliably quantify the relative amount of protein fractions in samples and thus demonstrate their different genetic origin. The great advantage of the lab-on-a-chip technology is its time-efficiency while the traditional CGE is a preferred instrument for method development. The usefulness of the chemometrical analysis of electrophoretic profiles as a method for objective evaluation, data reduction and interpretation was shown.
The possibility of improvement of the protein extraction from soybeans in order to provide a basis for the optimization of soymilk production was studied. The enzyme-assisted extraction using the hydrolytic enzyme treatment to disrupt the soybean cell wall components was expected to improve the protein extraction yield. The results confirmed that the right selection of operational variables led to an increased yield of soymilk as well as its protein concentration. It was also shown that the addition of selected enzyme preparations into the soymilk process design resulted in an increased extraction yield of proteins from seeds into soymilk. The protein quality did not deteriorate during the enzyme-assisted extraction process and a small amount of microbial transglutaminase added together with a coagulant produced tofu with a significantly increased yield while maintaing satisfactory textural properties.