The Study Of The Relationship Between Protein Oxidation And Lipid Oxidation In The Heat-treated Soy Protein Isolate Stabilized Emulsions

Emulsion is a good nutrient transport system, while lipid oxidation is a major factor that will destruct the stability of emulsions. At the same time, proteins in the food system are also susceptible to the attacking of free radicals, lipid hydroperoxides and secondary oxidation products, such as reactive aldehydes, which resulting the changes of structure and functional properties of proteins, that is, lipid oxidation and protein oxidation often occur together and lead to the "joint oxidation". To study this problem, base on the properties of soy protein isolate, this paper firstly studied the differences in the secondary structural properties of natural and 70°C, 90°C and 120°C treated SPI, then prepared emulsions with these four types proteins, and analyzed the differences of the physical chemistry stabilities of them, and during the storage at 37°C for 25 days, the particle sizes, microstructures and lipid oxidation products of these four emulsions were analyzed.The results showed that with the increasing of treatment temperature, the carbonyl values, β-folded content, free radical scavenging capabilities of SPI were gradually increased, while the content of free amino groups and α-helix were decreased gradually, however, the surface hydrophobicity index, sulfhydryl content and the abilities to chelate metal ions showed a rise at the beginning and then declined.In addition, compared with the natural and 70°C treated SPI stabilized emulsions,the 90°C and 120°C treated SPI stabilized emulsions showed better stabilities, they had better acid and freeze-thaw stabilities, and during the storage time, they had small changes in particle size, no apparent flocculation and coalescence, and the degree of lipid oxidation of them were also relatively low.Then this paper studied the influence of the oil oxidation products on the protein oxidation in emulsions. Results showed that the lipid oxidation products could react with amino acid side chains in protein and caused oxidative damage to proteins, which lead to the degradation of amino acids, decreasing of solubility, significantly increasing of carbonyl groups and formation of fat- protein polymer.Meanwhile, during the oxidation process of SPI, the β-conglycinin(7S) was more sensitive to lipid oxidation, and among the three subunits of 7S, α’ subunit was the most obviously, followed by the α and β subunits, while in globulin(11S) components, acid subunit was more sensitive to oxidation when compared to the basic subunit. And with the increasing of free radicals accumulating, they might attack protein and lead to the rupturing of peptide chains.Finally, the difference of the modification degree of proteins in the continue phase and the interfacial of these four emulsions were evaluated, the results showed that there was a dynamic exchanging process of proteins between the oil phase and the continuous phase, the distribution in the emulsion was a key factor that effected the modification degree of proteins. Protein that were adsorbed on oil droplets interfaces had been significantly modified during storage at 37°C, while proteins that were not adsorbed, their modification degree was relatively weak.