The role of protein cross-linking in soy food texture
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
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.