| In this experiment, the defatted soy flour were taken to prepare soy protein isolate. Modification of SPI was hydrolyzed by alkaline protease and catalysted by transglutaminase (TGase). The crosslinking conditions were optimized to obtain soy protein isolate with higher foaming properties and emulsion stability. Determine the relationship between the change of soy protein isolate function properties and its internal structure was by the method of correlation, then studied on the mechanism of the TGase modified soy protein isolate polymerization.The experiment used alkaline protease to hydrolysis SPI. The separation and purification of soy protein isolate hydrolysates was done by hollow fiber ultrafiltration, which the polymer molecular weight were20KDa,6KDa respectively. Then cross-linking the different molecular weight ranges>20KDa,6-20KDa and<6KDa with transglutaminase respectively. The results showed that the foaming properties, foam stability, emulsion stability of the molecular weight range>20KDa soy protein isolate hydrolysates increased most significantly, increased by3.90,32.76and61.87respectively. However the emulsifying properties of the three different components were decreased.Using TGase crosslinked the molecular weight>20KDa soy protein isolate hydrolysates as the research object, studied the effects of reaction time, enzyme added, pH and reaction temperature on the foaming and emulsifying properties. The optimal parameters of foaming properties and foam stability were as follows:reaction time was2h, enzyme added20U/g, pH was6.0, reaction temperature was40℃. Under these conditions the foaming properties of soy protein isolate hydrolysates was149.54, compared with the previous increase of46.6%. Foam stability was98.52, compared with the previous unmodified SPI increase201%in this condition. The optimal parameters of emulsion stability were as follows:time was1.2h, enzyme added20.6U/g, pH was7.5, temperature was40℃. Under these conditions the emulsification of soy protein isolate hydrolysates was0.696, compared to unmodified soy protein isolate decrease slightly. The emulsifying stability of soy protein isolate hydrolysates increased to16.25, compared to unmodified soy protein isolate increase33.20%.The comparison analysis between unmodified soy protein isolate and soy protein isolate hydrolysates showed that:the crosslinking reaction of TGase could improve surface hydrophobicity of SPI. The surface hydrosulfuryl content and disulfide bond content showed opposite trend after modified. The surface hydrosulfuryl content was reduced with disulfide bond content increasing. The surface protein molecules mercapto (-SH) was exposed to the protein surface and form disulfide bond(S-S) due to TGase crosslinking. The content of free amino groups of SPI was reduced with crosslinking degree increasing after modified. These result indicated that TGase can catalyze acyl transfer and promote protein molecules deaminase in the process of crosslinking.The correlation analysis demonstrated the relationship between the change of foaming properties, emulsification and the surface hydrophobicity, surface sulfhydryl content, disulfide bond content, free amino content of modified and unmodified molecular weight>20KDa SPI. There were significant positive correlation between emulsification and surface hydrophobicity, emulsification and disulfide bond content had negtive correlation. A significant positive correlation was observed between emulsifying stability and surface hydrophobicity. The foamability had significant positive correlated with surface hydrophobicity and disulfide bond content, which had significant negtive correlated with surface sulfhydryl content. The increase of the foam stability not only showed a significant positive correlation with the above factors, but also showed a significant negative correlation with the free amino content.Using modernization analysis apparatus analysised that:TGase polymerization SPI changed its side chain structure in the polypeptide chain. The content of the α-helix, random coil increased in the modified soy protein isolate, but the content of β-sheet structure decreased. In the secondary structure, β-sheet transformed into a-helix under crosslinking by TGase, which suggested that the modified soy protein isolate’s protein conformation lead to change by TGase crosslinking. |
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