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Microwave glycation of soy protein isolate

Namlı, Serap
Soy protein is used as a functional ingredient in the food industry since it has good functional properties like gelling, emulsifying, water and oil holding capacity. It contains all the essential amino acids, lowers the cholesterol, and the risk of cardiovascular disease. Therefore, the improvement of the functional properties of soy protein is an important issue for food science and industry. Glycation is known as the non-enzymatic reaction between the carbonyl group of sugars and the free amino group of proteins. It is known to increase solubility, emulsifying activity, and stability of the proteins. In this study, microwave glycation of soy protein isolate in an aqueous medium with fructose, glucose, and D-allulose (rare sugar) were performed. Protein-sugar ratio (1.6 and 7.2) and the reaction pH (7 and 10) were the other factors examined. As the control, microwave glycation was compared to water bath glycation for soy protein isolate glycated with fructose at pH 10. The concentration of free amino groups was measured by OPA method. Reducing sugar concentration of glycated protein was quantified by HPLC experiments. The formation of the final Maillard products was examined by the UV-VIS at 420 nm and the effects of glycation on solubility were measured by the Lowry method. Structural changes of the soy protein isolate after glycation were investigated by Fourier Transform Infrared spectroscopy (FT-IR) and Time Domain Nuclear Magnetic Resonance (TD-NMR) relaxometry. Alkaline pH was found to be more effective for microwave glycation of soy protein isolate. The reactivity of sugars for microwave glycation reaction was ordered as D-allulose > fructose > glucose. Microwave heating at alkaline conditions caused the isomerization reaction of fructose to D-allulose and glucose, D-allulose to fructose, and glucose to fructose. Microwave heating did not have an effect on increasing the browning degree of the protein upon heating and it was interpreted as microwave glycation did not form the final stage Maillard products. Moreover, microwave heating was found to decrease the solubility of protein at pH 10. FT-IR results showed that a higher degree of glycation reaction was provided by microwave heating. T2 relaxation times obtained by TD-NMR provided valuable information about the structural modification of glycated soy protein isolate and the mobility of water in the system before and after glycation. According to results, microwave heating was found to be more effective for glycation of soy protein isolate than water bath heating.