| Tofu, as a traditional Chinese food, is known to be beneficial to human health and is very popular in East and Southeast Asian countries. Because of the functional properties and health benefits reported in recent years, it is gaining acceptance increasingly in the world. However, the development of soybean products continue to be hampered by the problems of the wastewater generated during the process of tofu making and the short shelf life. To improve the processing technology and ingredient of traditional tofu products and promote their storability, environmental friendliness and convenience has always been one of the main objectives of soybean product industry. In recent years, the making of coagulant-indued tofu-type gels using SPI as the basic material has become the research hotspot in this field. Hence, the aim of this dissertation is to investigate the structural properties of SPI available for the making of favorable tofu-type gels and the interactions between SPI and oligosaccharides, polysaccharides and oil; attempt to provide certain theoretical basis for the making of innovative soybean-based tofu-type gels and solve the practical problems during the production process of traditional tofu.The content of moisture, protein, total sugar and oil in the commercial tofu products induced by GDL, MgCl2 and CaSO4 and their textural properties were investigated. The results showed the orders of the content of protein, total sugar and oil in the tofu sample were MgCl2-tofu> CaSO4-tofu> GDL-tofu and the moisture content was just the opposite. As regards to the value of gel hardensss, CaSO4-tofu and MgCl2-tofu were twice and thrice the value of GDL-tofu, respectively. The difference of the abovementioned parameters among the didderent tofu varieties was due to the various processing technology, including ratio of soybean to water, curd breaking and pressing. In addition, the gel stuctures of the three types of tofu varied from each other. The structure of MgCl2-tofu was coarse with large porosity. This could be resulted from the rapid release of Mg2+ during the coagulation process. But the network structures of CaSO4 tofu and GDL-tofu were more compact and homogeneous because of the relatively slow reactions of CaSO4 and GDL with soymilk. All the aspects considered, we chose CaSCO4 as the coagulant in the follow-up studies.SPI aggregates with different denaturation degree of 7S and 11S and various particle size were produced under different heating temperature and holding times and the properties were characterized. The results indicated that the number-mean diameter of SPI aggregates subjected to heat treatment at 65 and 75 ? was in the range of 43.58?53.77 nm with 7S denatured mostly; while the number-mean diameter of SPI aggregates subjected to heat treatment at 85 and 90 ? was in the range of 64.85-77.60 nm with 11S denatured mostly. The increase of the particle size of SPI could be due to the the formation of aggregates of large molecular weight (MW ?1000kDa) which was closely related to the denaturation of 11S. In addition, influences of the degree of denaturation of SPI and the particle size of heat-induced aggregates on the gelation properties of SPI were investigated. The results revealed that gel hardness and water-holding capacity correlated positively with the degree of denaturation of glycinin (11S) and the particle size of the SPI aggregates. The formed gels showed more uniform and denser network structures with increasing degrees of denaturation and particle size of SPI. Hydrophobic interaction might be dominant for the gels produced by SPI with relatively lower degrees of denaturation, whereas for the gels prepared by SPI with higher denaturation degrees of 11S, disulfide bond probably played a more important role. The SPI dispersions were preheated at 90 ? for 15 min to denature the soybean protein completely in the following chapters.The influnences of addition of oligosaccharides (sucrose, raffinose and stachyose) and soy soluble polysaccharide (SSPS) on the rheological and textural properties of CaSO4-induced gels formed by soy protein were investigated. The results showed that the hardness and water-holding capacity of the gels were significantly strengthened (P< 0.05) by the incorporation of oligosaccharides at the 1%,3% and 5% (w/v) levels and soy polysaccharide at the 0.3% and 0.5% (w/v) levels. The SPI gels containing 5% (w/v) stachyose exhibited the greatest value of hardness and the increment was 29.9% when compared to control gel. The storage modulus after the temperature cycle, frequency sweep, creep-recovery tests and large deformation properties demonstrated the affixion of above cosolutes enhanced the rigidity (elastic properties) of gels. Confocal scanning laser microscopy (CLSM) analysis indicated the cosolutes also improved the microstructures of the gels and herein the gels containing stachyose exhibited the greatest compactness. The volume excluded effects might play a major role in the cosolutes system. The gel solubility in different buffers verified the speculation and suggested that the protein-protein interactions were enhanced by the incorporation of cosolutes and hence accounted for the improvement of the gel properties.Soy protein isolate (SPI) dispersions were subjected to the preparation of emulsions by incorporation of various concentrations (1%?5%, v/v) of soy oil via different emulsifying process (A:Native SPI?emulsification?heat denaturation, B:Denatured SPI?emulsification). Rheological and textural properties of CaSO4-induced gels formed with the generated SPI emulsions were investigated and compared. The results showed that the viscoelastic properties during gelation process were affected by the addition level of soy oil and the emulsification before or after denaturation of SPI. The gel rigidity (elastic modulus) after gelation, harness and water holding capacity of CaSO4-induced gels were all gradually improved with the increasing oil concentrations and the gels prepared by the latter emulsions exhibited greater performance of gelling properties than that formed with former emulsions at the same oil concentration. The CLSM images demonstrated the gels formed by the latter emulsions, with smaller oil droplets dispersed within the networks, featured more homogeneous and compact network structures than the gels formed by the former emulsions.On account of the improvement of properties of CaSO4-induced SPI gel attributed to the incorporation of SSPS at relatively low concentrations (0.3% and 0.5%, w/v), the effect of addition of konjac glucomannan, gellan gum and curdlan and soy soluble polysaccharide (SSPS) on the rheological and textural properties of CaSO4-induced SPI gels was investigated. The results suggested the gellation temperature was reduced, typically, the decreases of which were 16 ??29 ? and 17 ??39 ? for the SPI-konjac glucomannan gel and SPI-curdlan gel, respectively. The values of final G'of gels were enhanced and the increments were 4%-22%? 16%?90%and 61%?131% for the SPI-curdlan gel, SPI-gellan gel and SPI-konjac glucomannan respectively when compared to the control gel. Moreover, the hardness, water-holding capacity and microstructures of gels were improved by the incorporation of the three types of polysaccharides. |
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