Effect Of Salt Ions On The Gel Properties And Microstructure Of Soy Protein Isolate

Gel property is one of the important functional properties of soy protein isolate(SPI), the unique spatial structure of soy protein gel gives it the ability to hold water and nutrients so that the foods with soy protein addition have good organization structure and sensory quality. There are two kinds of soy protein gels namely heat-induced gel and cold gel according to their different preparation methods. Because of the widely application of soy protein gel in food industry, the studies of soy protein gel are always going on. While the cold gels of soy protein with mild formation conditions provide new ways for the development of new foods and gradually arouse attention. The formation and structure properties of soy protein gel have a close relationship with gelling conditions, so it has important theoretical and practical significance to study the effect of conditions on the heat-induced gels and cold gels.This work studied the difference of properties and structures of heat-induced gels and cold gels combined with the most commonly used two kinds of salt Na Cl and Ca Cl2 in food industry, and then analyzed the effects of spry-drying and lyophilization on heat-induced aggregates prepared under different preheating temperatures.Firstly, this work studied the structure and gel properties of heat-induced gels prepared with different salt concentrations under various conditions, results showed that the salt tolerance of gels containing Na Cl increased with p H and protein concentration, while heating time had minor effect on the salt tolerance. With temperature increasing from 75°C to 95°C, the salt tolerance had a lowest value at 85°C. Water holding capacity of gels with Na Cl addition is higher than the gels containing Ca Cl2, and both decreased with salt concentration increasing, but the former gel strength is weaker than the later. Increasing heating temperature from 75°C to 95°C resulted in the increase of WHC of gels containing Ca Cl2, while the WHC of gels containing Na Cl showed a minimum at 85°C. Gel hardness increased with heating temperature for both salts. Heating time had no effect on WHC, but gel hardness increased with heating time. With p H increasing from 6.0 to 9.0, the WHC decreased and gel hardness increased for both Na Cl and Ca Cl2, but the hardness of gel containing Ca Cl2 at p H 9.0 showed a different trend compared with gels formed at p H6.0 and p H7.0. Protein concentration has a positive effect on the WHC and gel hardness. Gels containing Ca Cl2 had coarser structure than gels containing Na Cl.Secondly, this work studied the structure and gel properties of salt-induced cold gels prepared under different environmental conditions, results showed that a much lower Ca Cl2 concentration was required to cause gelation than Na Cl. Na Cl couldn’t cause gelation of aggregates formed at 75°C. The WHC of gels containing Na Cl was much higher than that of gels containing Ca Cl2 and gel hardness of the former was lower. The preheating temperature, salt concentration and protein concentration had minor effect on the WHC of gels containing Na Cl, while gels containing Ca Cl2 showed an increase in WHC with the increase of protein concentration and decrease of Ca Cl2 concentration. With preheating temperature increasing form 75°C to 95°C, the WHC of Ca Cl2-induced gels increased firstly and then decreased. The hardness of gels prepared with Na Cl and Ca Cl2 increased with the the increase of preheating temperature, salt concentration and protein concentration. Ca Cl2-induced gels were much coarser than Na Cl-induced gels.Finally, the effect of freeze frying and lyophilization on heat-induced aggregates were studied. Results indicated that drying had great effect on mesoscopic properties of aggregates. The aggregates after freeze drying couldn’t form gels with salt addition, while part of the aggregates after spraying could form gels with Ca Cl2 addition. Spray drying had different effects on the gelling ability of aggregates prepared at different temperatures. Increasing in preheating temperature resulted in the increase of hardness of Ca Cl2-induced gels and the decrease of WHC. The Confocal Laser Scanning Microscopy(CLSM) images showed that the homogeneity of cold gels changed with preheating temperature and salt concentration.