| Soy protein is widely used as food ingredients in food industry, as it has good nutritional value and superior functional properties. Heat treatment of soy protein isolate(SPI) can change its structure and functional properties and generate fibrillar aggregates. In this paper, firstly, it established a method to separate the fibrillar aggregates and polypeptides from acid heated SPI with the use of ultrafiltration in order to manipulate the properties of heated SPI. Secondly, it studied the influencing factors of interaction mechanism and structure evolution of heated SPI-SBP complexes, in order to further improve the properties and application value of acid heated SPI. The main conclusions of this paper are as follows:1. The SPI fibrils and polypeptides were separated effectively by ultrafiltration method, in order to improve the properties of acid heated SPI. 100 k Da ultrafiltration membrane was chosen in the experiments, and it turns out that twice ultrafiltration operation can get effective separation of fibrillar aggregates and polypeptides. After heat treatment to SPI, the solubility of the protein at the isoelectric point increased, but solubility in the neutral p H conditions became smaller leading to the increase of the amount of electrical charges. Fibrillar aggregate from heated SPI had similar amino acid composition and zeta-potential to heated SPI, but its solubility in the isoelectric point and neutral p H was less than heated SPI’s. Polypeptides from heated SPI contained less hydrophobic amino acids and more negatively charged amino acids. Polypeptides had preferable solubility in the p H of 2.0 ~ 9.0. Polypeptides formed amorphous aggregates in the acidic p H, but dissociated in the neutral p H.2. In this paper, the influence of protein-polysaccharide complex ratio(r) and salt ionic strength on the interaction of SPI fibrils and SBP was studied. The mechanism of interaction and structure evolution between heated SPI and SBP were also studied. The results showed that SPI-SBP and heated SPI-SBP complexes were influenced by the ratio of protein and polysaccharide(r) and the salt ionic strength. When SBP content rise, the complexes system’s turbidity became smaller, and the isoelectric point moved to low p H. The optimal salt concentration of SPI-SBP complexes was 50 m M. The optimal salt concentration of heated SPI-SBP complexes was 10 m M. and Heated SPI-SBP and SPI-SBP complexes had different phase diagrams, and both of them could be divided into four regions, including co-solvent complex, the intramolecular soluble complex, intermolecular soluble complex and intermolecular insoluble complex. However, the intermolecular soluble complex region of heated SPI-SBP is larger than the SPI-SBP complex’s, whose range of p H was from 2.5 to 9. Heated SPI and SBP complexes could also form by hydrophobic interactions in the neutral p H. In heated SPI, it was the fibrils that played the important role in forming complexes with SBP.3. In this paper, the interfacial and emulsifying properties of SPI-SBP and SPI fibrils-SBP complexes were studied. The results showed that heat treatment of SPI will help improve its complexes’ interfacial activity. Increasing the content of SBP raised the surface pressure of SPI-SBP complexes, but it was opposite for heated SPI-SBP complexes. Heat treatment of SPI could significantly improve its complexes’ emulsifying activity and emulsion stability at p H 4.5, but not at p H 7.0. With the increase of the heating time of the protein at p H 4.5, the composite emulsion’s properties was changed, particle size smaller, absolute value of the potential becomes larger, interfacial protein content lower, stratification stability and storage stability better. Increasing the content of SBP at p H 4.5 could improve the stability of SPI-SBP and SPI fibrils-SBP complexes, while composite emulsions at p H 7.0 had worse stability. |
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