Influence of sugars on the characteristics of glucono-δ-lactone-induced soy protein isolate gels

The effects of the reducing sugars (glucose and lactose) and the non-reducing sugar (sucrose), heated in combination with soy protein isolate (SPI) at neutral pH, on the physicochemical and rheological properties of SPI were determined. After formation of gels induced by glucono-δ-lactone (GDL), the textural profile and physicochemical bonds of the non-heated and heated SPI gels were investigated. The gelation of SPI was induced in three stages of processing that is similar to some tofu-making procedures. First, SPI was heated in the presence of sugars at neutral pH above the denaturation temperature of SPI; then gelation was induced by GDL at iso-electric pH and finally the acidic gels were heat treated again. Heat treatment with glucose at neutral pH resulted in SPI with higher glycation degree than with lactose, whereas SPI heat treated in the presence of sucrose was not glycated. GDL-induced gels of SPI glycated with glucose was more soluble in water than gels of SPI reacted with lactose, which in turn was more soluble than the control and gels of SPI heated in the presence of sucrose. This indicates a change in the net charge of proteins caused by the glycation reaction. Glucose and lactose had a protective effect on protein denaturation at neutral pH, albeit less than sucrose, resulting in GDL-induced gels with increased water holding capacity and reduced gel hardness than sucrose. Chemical analysis indicated that disulphide bonds were involved in maintaining the structure of the gels, and solubility profiles of gels in different buffers indicate that other types of covalent bonds besides disulphide bonds were formed in gels of glycated SPI, resulting in reduced gel elasticity.