Cross-linking in soy proteins is hypothesised to have an impact on the texture of tofu. In vitro incubation showed soy proteins and its two fractions, glycinin and β-conglycinin, were cross-linked using glutaraldehyde, formaldehyde, glyceraldehyde and transglutaminase (TGA). Increasing concentration of these carbonyl compounds and TGA, and temperature of the carbonyl compounds treatment, increased the reactivity of cross-linking. Glutaraldehyde was the most reactive in forming aggregated proteins, followed by formaldehyde and glyceraldehyde. Both carbonyl moieties of glutaraldehyde are believed to be essential for the rapid cross-linking reaction. In the unfractionated soy proteins, β-conglycinin had a higher reactivity than glycinin. In in vitro incubation using TGA, soy proteins served as good substrates for TGA, in which β-conglycinin was more susceptible to TGA than glycinin in the unfractionated soy proteins. The addition of TGA, and 1 and 2 mM glutaraldehyde prior to soymilk boiling in situ resulted in a small number of cross-linked proteins, which correspond to an increase in fracture force. The addition of glutaraldehyde after soymilk boiling resulted in a slight decrease in fracture force compared to the control. At higher concentrations of glutaraldehyde (15 and 30 mM), soy proteins were mostly cross-linked, regardless of addition before or after soymilk boiling. Highly cross-linked proteins resulted in a significant decrease in the fracture force. For TGA treatment, the fracture force was increased with increasing TGA concentration from 1000 to 5000 ppm, added either before or after soymilk boiling. However, the TGA treatment showed only a small quantity of cross-linking. It is hypothesised that TGA hydrolysed glutamine of proteins to glutamate and changed the functional properties of proteins. Upon examination of the microstructure, it was found that the TGA treatment resulted in a fine-stranded network, compact structure and less porosity. These characteristics resulted in a higher fracture force. In contrast, in the glutaraldehyde treatment, the network consisted of a higher porosity, loose network and diffuse structure, which gave lower fracture force. Thus, it appears that substrate modification to the structure of the soy proteins may have a greater impact than the number of cross-links. These findings are likely to have implications for production of soy products with a wide range of textures by manipulating the soy protein properties.

Cross-linking in soy proteins is hypothesised to have an impact on the texture of tofu. In vitro incubation showed soy proteins and its two fractions, glycinin and β-conglycinin, were cross-linked using glutaraldehyde, formaldehyde, glyceraldehyde and transglutaminase (TGA). Increasing concentration of these carbonyl compounds and TGA, and temperature of the carbonyl compounds treatment, increased the reactivity of cross-linking. Glutaraldehyde was the most reactive in forming aggregated proteins, followed by formaldehyde and glyceraldehyde. Both carbonyl moieties of glutaraldehyde are believed to be essential for the rapid cross-linking reaction. In the unfractionated soy proteins, β-conglycinin had a higher reactivity than glycinin. In in vitro incubation using TGA, soy proteins served as good substrates for TGA, in which β-conglycinin was more susceptible to TGA than glycinin in the unfractionated soy proteins. The addition of TGA, and 1 and 2 mM glutaraldehyde prior to soymilk boiling in situ resulted in a small number of cross-linked proteins, which correspond to an increase in fracture force. The addition of glutaraldehyde after soymilk boiling resulted in a slight decrease in fracture force compared to the control. At higher concentrations of glutaraldehyde (15 and 30 mM), soy proteins were mostly cross-linked, regardless of addition before or after soymilk boiling. Highly cross-linked proteins resulted in a significant decrease in the fracture force. For TGA treatment, the fracture force was increased with increasing TGA concentration from 1000 to 5000 ppm, added either before or after soymilk boiling. However, the TGA treatment showed only a small quantity of cross-linking. It is hypothesised that TGA hydrolysed glutamine of proteins to glutamate and changed the functional properties of proteins. Upon examination of the microstructure, it was found that the TGA treatment resulted in a fine-stranded network, compact structure and less porosity. These characteristics resulted in a higher fracture force. In contrast, in the glutaraldehyde treatment, the network consisted of a higher porosity, loose network and diffuse structure, which gave lower fracture force. Thus, it appears that substrate modification to the structure of the soy proteins may have a greater impact than the number of cross-links. These findings are likely to have implications for production of soy products with a wide range of textures by manipulating the soy protein properties.

Master of Science / Department of Grain Science and Industry / X. Susan Sun / Soybean protein modified with sodium bisulfite behaves like latex adhesives, with adhesive strength comparable to formaldehyde-based adhesives. β-conglycinin and glycinin are two major protein components of the adhesive system. The objective of this research was to investigate the effect of sodium bisulfite on the physicochemical, morphological, and adhesion properties of glycinin and β-conglycinin in order to better understand the function of glycinin and β-conglycinin in the formation of the soy latex adhesive. Sodium bisulfite broke the disulfide bonds that linked acidic and basic polypeptides of glycinin, and the reducing effect was enhanced with increasing sodium bisulfite concentration. Although cleavage of disulfide bonds was expected to destabilize proteins, the thermal stability of glycinin increased as the sodium bisulfite concentration increased. Sodium bisulfite modified glycinin had higher surface hydrophobicity, which facilitated hydrophobic interations between molecules and aggregation of glycinin. The balance between hydrophobic interactions and electrostatic forces makes glycinin form unique chain-like structures. Adhesive performance of glycinin dropped significantly at lower sodium bisulfite concentration and then increased as sodium bisulfite concentration increased up to 24 g/L. Excess sodium bisulfite was detrimental to adhesive strength and water resistance. High-molecular-weight aggregates were observed in unmodified β-conglycinin, but these aggregates were dissociated by sodium bisulfite treatment. Similar to glycinin, the thermal stability of β-conglycinin was improved by the modification. However, the denaturation enthalpy of β-conglycinin decreased significantly at high level of sodium bisulfite (36 g/L). The turbidity at pH 4.8 also dropped extensively at the concentration of 36 g/L. The contact angle of β-conglycinin reached its minimum at 6 g/L sodium bisulfite on cherry wood and 24 g/L on glass. Morphology study proved that sodium bisulfite modification made the β-conglycinin solution more dispersed. At pH 9.5, water resistance of β-conglycinin was improved to a small extent by 6 g/L sodium bisulfite. At pH 4.8, adhesive performance was enhanced by 3 g/L and 6 g/L sodium bisulfite. High level of sodium bisulfite at 36 g/L reduced the adhesive performance of β-conglycinin drastically.