Gel formation by β-conglycinin and glycinin and their mixtures

Abstract

Gel formation and gel properties of β-conglycinin, glycinin and their mixtures were studied as a function of pH using small and large deformation rheology and differential scanning calorimetry. We conclude that heat denaturation is a prerequisite for gel formation. Gelation temperatures of β-conglycinin were lower than those of glycinin and more dependent on protein concentration. At pH 7.6, protein solutions gelled at a higher temperature than at pH 3.8. Glycinin gels were stiffer than β-conglycinin gels at the same pH and protein concentration, and fractured at a higher strain and stress. At pH 7.6, G′ is lower than at pH 3.8 for both proteins and the gels could be deformed to a larger extent. Based on the appearance of the gels (turbid at pH 7.6, white at pH 3.8) and the fracture properties, we conclude that different network structures are formed as a function of pH. The reason why glycinin gives a better gel than β-conglycinin is believed to be due to a difference in network structure as well as in strength of interaction between the protein molecules. Mixing of both soy proteins resulted in improved gelling properties at pH 3.8. The elastic modulus of the mixture was larger than the weighed sum of the separate contributions. Furthermore, mixing reduced the protein dispersability at pH 7.6. This strongly indicates the presence of an interaction between the proteins. Gels of the 1:1 mixture (pH 3.8) had a fracture stress and strain in between those of the gels of the separate proteins.