Processing optimization for improving soybean protein's emulsifying properties after freeze-thaw

Abstract: Soybean protein has become an important raw material in food processing industries due to its high nutritional value and good functional properties. One of the most important properties of (soybean protein isolate) SPI is emulsifying in food. However, native soy globulins function poorly because of their compact globular structures, which makes it difficult to be utilized directly in food processing. Protein stabilized emulsions are highly sensitive to environmental stresses such as low temperature, and therefore coalescence and creaming occur, which limit their utilization in frozen food. There are no soy protein products specialized for freeze-thaw foods in China. The quality of freeze-thaw foods will be destroyed by the low temperature because the functionalities of soy protein are difficult to maintain. Maillard reaction, a condensation reaction between the reducing end of carbohydrates and the primary amine of proteins, is a well accepted and safe method. This method is the modification of soy protein by maillard reaction for improved functionalities, its reaction processes are conducted without adding any catalyst by heating. In this paper, glycosylation reaction products were obtained under wet-heating conditions in order to improve their freeze-thaw stabilities. Special soy proteins were prepared to meet the demand of soybean proteins in the application of freeze-thaw food system. In order to prepare a kind of soy proteins that can maintain high emulsion after freeze-thaw, the soy protein was modified by wet glycosylation with the dextran as glycosylation donor. Box-behnken model optimizaiton processes were established. The resulting model can be used for analysis and prediction for its goodness of fit. The experiments were designed to explore the effects of three factors including protein concentration, mass ratio of protein and sugar, reaction time on freeze-thaw stability of modified products with response surface methodology. The optimum conditions were obtained with the protein concentration of 40 mg/mL, the mass ratio of protein and sugar of 1:3, and the reaction time of 4 h. The freeze-thaw stability of modified products obtained in this condition was significantly higher than unmodified samples. The emulsifying activity index (EAI) before and after freezing and thawing were 1.687 and 1.780 times of the control, 1.367 and 1.274 times of the control, respectively. The emulsifying properties of the SPI-D mixture were investigated in the study. The results indicated that the EAI and ESI of SPI-D conjugate were significantly higher than non-heated mixtures. The addition of dextran without heating has little effect on the freeze-thaw property of soy proteins, because the EAI and ESI of non-heated mixtures are not significantly different compared with the SPI.The results of the infrared spectrum showed that SPI-D conjugate in 3 700-3 200 cm-1 stretching vibration, compared to SPI appears a wider absorption, at 1 260-1 000 cm-1 also appears strong absorption. The results proved that the complexity of soy protein and dextran was caursed by the reaction. Compared to SPI, the superior performance of biopolymer conjugates in emulsions was attributed to the polysaccharide moiety of the hybrid, following adsorption of the protein part at the oil-water interface. This highly solvated layer near the interface, enhances steric repulsion forces between neighboring oil droplets and retards the creaming process. Therefore, using wet glycosylation to improve the freeze-thaw stability of SPI has applicable potential. This method provides a theoretical basis for special soy protein which is more suitable for producing frozen foods.