Effects Of Residual Lipids In Defatted Soy Flour On The Fractionation, Structure And Functionality Of Soyean Protein

Soybean proteins are composed of two major components, glycinin and β-conglycinin, which account for around40%and30%of total proteins, respectively. The glycinin and β-conglycinin, or11S and7S fractions, are primarily responsible for the functional properties of soybean proteins. Fractionation of these proteins is beneficial to better understand their separate contribution to functionality, to evaluate their impact on nutrition and structure, and to identify opportunities for new product development.Many methods have been reported in the literature for purifying the glycinin and β-conglycinin fraction. However, few data have been reported on the fractionation of soybean glycinin and β-conglycinin isolated from lower Residual lipids soybean flour (LRLSF). This dissertation focuses on the preparation of soybean glycinin and β-conglycinin, and the effect of oxidation on the structure and functionality of soybean proteins during the fractionation.Processing conditions such as reducing agent, store time and pH on yields and purity of β-conglycinin (7S) were evaluated in the fractionation of soybean β-conglycinin with LRLSF. Compared with sodium bisulfite (SBS) or dithiothreitol (DTT) reducing agent, the yield and purity of β-conglycinin was the highest when P-mercaptoethanol (ME) was added to the protein extract. Prolonging store time in the precipitate stage (protein extract), the yield of β-conglycinin decreased continuously, but the purity of the protein increased by degrees in the experiment. When store time exceeded12hours, the velocity increment of7S purity slowed, while there was no significant difference at P≥0.05for total sulfhydryl and disulfide content. The yield and purity of β-conglycinin was the best when the pH of3#protein extract was adjusted to4.7. Based on the single-factor experiments, the optimized extraction conditions of7S by response surface methoclology (RSM) were determined as ME concentration of9.5mmol/L, cryoprecipitation time of12h, and pH4.86. The yield and purity of β-conglycinin were23.86%and8.64%higher than traditional preparation, respectively.Processing conditions such as pH, reducing agent and store time on yields and purity of glycinin (11S) were also evaluated in the fractionation of soybean glycinin with LRLSF.. Adjusting the pH of protein extract from6.2to6.6, the yield of glycinin decreased by16.71%, while the purity of the protein increased by4.60%. Sulfhydryl and disulfide content of proteins increased by degrees with intensifying pH. Compared with dithiothreitol (DTT) or β-mercaptoethanol (ME) reducing agent, the yield of glycinin was the highest when sodium bisulfite (SBS) was added to the protein extract at pH6.4. The effect of DTT on yields of glycinin was the lowest of three kinds of reducing agent. It was similar to the purity of glycinin when three kinds of reducing agent were used. Prolonging store time in the precipitate stage,10h was the best for yields and purity of glycinin in the experiment, while there was no significant difference at P>0.05for total sulfhydryl and disulfide content. The decreased free sulfhydryl content of glycinin indicated that the oxidation of free sulfhydryl and the formation of disulfide bond while extract time prolonged. Based on the single-factor experiments, the optimized extraction conditions of11S by response surface methoclology (RSM) were determined as SBS concentration of8.89mmol/L, cryoprecipitation time of10h, and pH6.32. The yield and purity of glycinin were28.48%and5.58%higher than traditional preparation, respectively.Based the optimum condition, soybean glycinin and P-conglycinin were prepared from thee kinds of material, namely, lowly oxidative soybean flour, fresh low temperature defatted soybean flour and low temperature defatted soybean flour treated with high temperature. And the effects of interaction between lipid oxidation and protein on oxidation extent, structural characteristics, and aggregation morphology of P-conglycinin and glycinin were investigated during the fractionation of the protein. The results showed that oxidation resulted in an increase of protein carbonyl content of β-conglycinin and glycinin, a decrease of free sulfhydryl, total sulfhydryl, and the α-helix content. Compared to soybean glycinin, β-conglycinin was more susceptible to oxidation in the same oxidation condition. Oxidation led to a decrease of surface hydrophobicity of β-conglycinin and glycinin as well as maximum tryptophan fluorescence intensity, and blue-shifted wavelength of the maximum emission, indicating that protein oxidation resulted in aggregation of β-conglycinin and glycinin. β-conglycinin was more sensitive to oxidation than glycinin because β-conglycinin contains more amino acid residues with aliphatic side chain groups which could be easily oxidative attacked. Oxidation also caused formation of soluble oxidative aggregates, aggregation extent of β-conglycinin was higher than glycinin in the same oxidative stress environment. In addition, oxidation resulted in backbone fragmentation of β-conglycinin molecule as well as dissociation of glycinin.Effect of protein oxidation on solubility, water holding capacity, oil-absorbing capacity, emulsifying properties, foaming properties, and gelation was also investigated in the paper. Oxidation resulted in a decrease of β-conglycinin and glycinin solubility. Decline of β-conglycinin solubility was more significant than glycinin in the same oxidation conditions. Water holding capacity and oil-absorbing capacity of β-conglycinin and glycinin decreased as oxidation extent of protein increased, and decrease extent of β-conglycinin water holding capacity and oil-absorbing capacity was more significant. Oxidation led to a decrease of emulsifying properties, emulsifying stability, foaming properties, foaming stability, gel hardness, and gel strength of β-conglycinin and glycinin. Compared to glycinin, decrease extent of β-conglycinin functional properties was more obvious. Protein oxidation resulted in an increase of initial of G’ value in heating period in the process of gel formation. In the holding temperature and cooling period, G’of β-conglycinin and glycinin decreased as protein oxidation extent increased. The gap between curves of G’ and G" gradually decreased as extent of oxidation of β-conglycinin and glycinin increased, indicating that protein oxidation led to a decrease of gel strength of β-conglycinin and glycinin. In the same oxidation conditions, effect of oxidation on rheological properties of β-conglycinin was much more significant than glycinin.Based on the obtained facts, we can conclude that structure and functionality of7S and11S will be affected by different degree oxidation of the lipid remained in material, and high quality7S and11S can be prepared with lower lipid-remaied and lowly denaturalized soybean flour.