Effects Of Protein Oxidation On Structure And Gel Properties Of Soy Protein

Soy protein is one of the most important food ingredients which widely used in the food industry because of unique nutrition and excellent gel properties. The gel properties of soy protein are affected by many factors, one of which is protein oxidation. Soybeans are rich in lipoxygenase (LOX) and polyunsaturated fatty acids. Cell structure of soybean is damaged during the course of soybean processing, LOX-mediated polyunsaturated fatty acids peroxidation is accompanied by release of free radicals and reactive oxidation products which resulted in oxidative modification of soy protein structure, leading to changing the gel properties of soy protein. The dissertation is focus on the influence of lipid peroxidation products on the structure and gel properties of soy protein.In this study, peroxyl radical derived from thermal decomposition of 2, 2'-azobis (2-amidinopropane) dihydrochloride (AAPH) under aerobic condition, 13-hydroperoxy-9Z, 11E-octadecadienoic acid (HPODE) which generated LOX-mediated linoleic acid peroxidation, malondialdehyde and acrolein was selected as representatives of LOX-mediated lipid peroxidation-derived free radicals, lipid hydroperoxides and reactive aldehydes, respectively. Effects of peroxyl radical, HPODE, malondialdehyde, and acrolein on the structure of soy protein were characterized by solubility, protein carbonyl content, free sulphydryl and total sulphydryl content, free amine and available lysine content, circular dichroism spectroscopy, surface hydrophobicity, intrinsic fluorescence, size exclusion chromatogram, and sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Incubation of soy protein with increasing concentration of AAPH, HPODE, malondialdehyde, and acrolein resulted in gradual generation of protein carbonyl derivatives, loss of free sulphydryl groups, total sulphydryl groups, free amine, available lysine,α-helix structure, surface hydrophobicity, and intrinsic fluorescence intensity, blue shift of wavelength of the maximum emission was accompanied by formation of oxidation aggregates and covalent cross-link by non-disulphide.Two-state model was applied to study the effects of protein oxidation on the properties of soy protein unfolding and refolding in urea. The results indicated that oxidatively modified soy protein was easy to unfold, but difficult to refold. As extent of protein oxidation increased,ΔGH2O, m, and [urea]1/2 of soy protein gradually decreased. The phenomena indicated that oxidative modification lead to a decrease in protein structural stability. In addition, oxidative modification also resulted in an increase in the rate of unfolding of soy protein in urea, but a decrease in the rate of soy protein refolding.Oxidatively modified soy protein was more susceptible to thermal denaturation. In the process of thermal-induced soy protein denaturation, particle size of soy protein increased as heating temperature increased from 50°C to 90°C. As extent of protein oxidation increased, thermal denaturation of soy protein in 90°C water-bath for 30min resulted in a decrease in particle size of peroxy radical modified soy protein, the same denaturing conditions resulted in an initial increase in particle size of malondialdehyde modified soy protein with increasing malondialdehyde concentration, and then decreased with further increasing malondialdehyde concentration. Particle size and thermal aggregates content of cooling of thermal-induced denaturation of peroxyl radical, HPODE, malondialdehyde and acrolein modified soy protein under conditions of incubation soy protein in 100°C water-bath for 30min steadily decreased as extent of protein oxidation increased.Protein oxidation resulted in a decrease in hardness and water-holding capacity of soy protein gel. An increase in coarseness and interstice of the gel network was accompanied by uneven distribution of interstice as extent of soy protein oxidation increased. A decrease in disulfide content and surface hydrophobicity as well as formation of oxidation aggregate in the process of oxidative modification were contributed to the decline of soy protein structural stability, which resulted in a decrease in content and particle size of thermal aggregates, leading to a decrease in hardness of soy protein gel. Effects of oxidative modification by lipid free radicals and reactive aldehydes on structure and gel properties of soy protein were the most significant.