Acid induced gelation of heated soymilks: Protein structure, molecular interactions and gel properties

Heating conditions of soymilks have tremendous effects on the gelation and gel properties of glucono-deltalactone induced tofu. The defatted soymilk was heated for 5 min at 65-115 degrees C in a controlled way. Protein structure of heated soymilk were characterized by non-reducing electrophoresis, far ultraviolet circular dichroism and ANS fluorescence spectrum and it was found that surface hydrophobicity and the maximum structural shift was occurred at the temperature of 90 degrees C. An isothermal titration calorimetry (ITC) based method for the evaluation of protein interactions was developed. Upon acidifying of the soymilks, the experimentally determined exothermic effects were observed but were smaller than the calculated protonation heat. An endothermic effect was proposed to compensate the difference which was most probably resulted from the hydrophobic interactions. The magnitude of the endothermic effect which reflected the strength of the hydrophobic interactions, exhibiting the similar temperature dependence as that of surface hydrophobicity. Gelling behavior and gel properties of the acidified soymilks were studied by rheometer and confocal laser scanning microscopy. The soymilk heated at 90 degrees C showed the shortest gelation time, the highest storage modulus and the densest gel networks. This study will further deepen the understanding of tofu processing mechanism.