| Soybean protein isolate has some surface properties (emulsifying, foaming), but not available. Proteolytic modification were used to improve surface properties. As different protease is specificity for different peptide bonds, the improvement of surface properties is different. The changes of surface properties are mently because of intrinsic structure characteristics of protein. So it is very important to know the relationship between structure characteristics and surface properties. This subject choose four proteases which were special for peptide bonds to limited hydrolyze SPI and determination the surface properties and structure characteristics.with the increasing of hydrolysis degree then discuss the reltionships between the surface properties and structure characteristics.Firstly this subject discussed the different surface properties of different proteases hydrolysis with increasing hydrolysis degree according to the different peptide bonds of proteases. The experiment showed that the emulsifying activity of SPI that limited hydrolysis by trypsin was higher than native SPI, there was highest at DH=3%, increased 5.26%. The emulsifying activity of SPI hydrolyzed with subtilis increased then decreased with the hydrolysis degree increasing, and emulsifying activity at DH=5% was 14.27% higher than other degrees. The emulsifying activity of SPI hydrolyzed by papain and aspergillus oryzae proteinase had no increased. Emulsion stability Modification by all four proteases were reduced.Foam stability of SPI hydrolyzed by four proteases was higher than raw SPI, foam stablility increased with degree and the largest increases to 22.03%,there was max at DH=3% of SPI hydrolyzed by papain, which was 25% higher than raw SPI. Foaming of SPI modified by aspergillus oryzae increased in the minimum and maximum rate of increase was only 13.56%. SPI hydrolyzed by Bacillus subtilis in the foaming of the DH =8%had highest increase that was 14.52%. Foam stability in addition to the trypsin with an increase with hydrolysis increase,other proteases modified SPI had significantly decreased with the hydrolysis increased.SPI hydrylzed by different proteases had different surface properties, which connect with adsorption properties.The rapid adsorption of protein is necessary to facilitate this reduction in surface tension, this subject tested the surface tension in 3 hours.The result was: surface tension of SPI in different hydrolysis degree the rate of decline in different. The lower rate of raw SPI was 7.90%, surface tension of SPI which hydrolyzed with papain at DH = 9% was lowest that is 11.18%, SPI hydrolyzed by other three proteases had higher surface tension rate,the biggest is DH=9.5% by subtilis and DH = 5% by aspergillus oryzae, thus they had better surface properties. The order of protein adsorption at interface hydrolyzed by four proteases was trypsinï¹¥papainï¹¥subtilisï¹¥aspergillus oryzae, and also the emulfying activity by trypsin was higher than others. That indicated that there was certain relationship between emulsion and protein adsorption. There were some relatives betweenn the viscosity of protein and emulsioning or foaming stability. Limited hydrolysis of SPI with trypsin increased the viscosity others decreased according the degree increasing. The viscosity changes show different in the surface properties of its protein.The surface properties also had relationship with structure characteristics of SPI. according to studying the change of structure characteristics of SPI which hydrolyzed by four proteases get following conclusion:The surface hydrophobicity can reflect the hydrophobic group and patches. The subject discuss the surface hydrophobicity of hydrolyzed SPI with four proteases. The record showed that the surface hydrophobicity increased with trypsin, surface hydrophobicity of SPI which hydrolyzed with papain,aspergillus oryzae,subtilis decreased with the degree increasing. That affected the emulsion properties of SPI.Then determined the Zeta-potential, the results showed that: Zeta-potential decreased after hydrolysis with papain,Aspergillus oryzae proteinase and subtilis, but the degree was different. and it is the same as the change of foaming. but for SPI hydrolyzed by trypsin..That may be from the change of protein structure,if the structure become more fit to adsorption that would reduce stable foam with the Zeta-potential reducing properly.The subject also determined the size of hydrolyzed SPI,the size were different of SPI which hydrolyzed by different proteases. From high to low were: trypsin, papain, Aspergillus oryzae protease, Bacillus subtilis neutral protease.The smallest was SPI hydrolyzed by subtilis, and which had higher protein adsorption, would be the reason why the SPI had better emulsion but worse stability.Furthermore the DSC curves of SPI hydrolyzed with trypsin showed that the structure become more Loose and disorder, which may help interface adsorption. The result of SPI adsorption at emulsion also show that SPI which had loop and disorder structure always show good surface-active properties.On the basis of different surface properties and structure characteristics of SPI which hydrolyzed with single proetases, the subject selected trypsin and papain to multiple hydrolysis to continue to explore multiple hydrolysis of the structure characteristics of SPI and the impact of surface properties. The result showed that multiple proteolysis (trypsin:papain=2:1) led to better surface properties than that of single hydrolysis, The surface hydrophobicity, viscosity, cohesiveness of emulsion were lower than hydrolyzed by trypsin, but higher than papain. The size of SPI hydrolyzed by multiple hydrolysis was bigger than hydrolyzed with single proetases, the protein adsorption in the interface also higher than single hydrolysis. These results further information that SPI which owned high surface hydrophobicity, viscosity, help the protein adsorption and form loose and disorder structure to produce good surface properties. |
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