| The structure and physicochemical properties of soybean protein in the process of processing and manufacturing have changed correspondingly, which limits its application in food industry. At present, the research on the effects of processing conditions (such as, pH, ionic strength and temperature), denaturing agents, biological agents, and inorganic salts on the structure and physicochemical properties of soybean proteins have been studied. But the effects of sugar alcohols on the structure and properties of soy protein were not reported. Sugar alcohols are a kind of compounds with different physiological functions, which are similar to sucrose, and it is internationally recognized as a safe sweet agent. Previous studies showed that the structure and stability of macromolecules in solution could be protected and improved by sugar alcohols, and the physiochemical properties of macromolecules can be influenced in a certain degree. Therefore, the objective of this study was to evaluate protein solubility, surface properties(hydrophobicity,total sulfhydryl groups and disulfide bond, tryptophan, and tyrosine residues), secondary structures (circular dichroism), and functional properties (including emulsifying properties and foaming properties) of soy protein at different kinds and concentrations of sugar alcohols and processing conditions (such as, pH, ionic strength,temperature and freeze-thaw (F-T) cycles). Furthermore, this paper analyzed the relationship between structural properties and functional properties of modified soy protein. This research should provide a theoretical basis for the application of sugar alcohols in the development of soybean products. The results are as follows:(1)As polyols added, the surface hydrophobicity of soy protein significantly (P<0.05) decreased. UV-Vis spectra and intrinsic fluorescence spectroscopy of soy protein confirmed that the "r" value of second derivative UV-Vis spectra and the maximum absorption wavelength (?max) of intrinsic fluorescence spectroscopy was decreased, and intrinsic tryptophan fluorescence intensity was reduced. These results implied the hydrophobic side-chain groups, which were originally exposed to the exterior protein were occluded. Moreover, CD spectra spectra confirmed that the amounts of a-helix and ordered structure content (?-helical+?-sheet) of soy protein was increased with the addition of polyols. Since the ordered structure content increased, polyols might lead to more compact protein molecules, and decrease the flexibility of protein. With the addition of polyols, the physicochemical properties of soy protein have changed. Solubility, emulsion stability index (ESI) and foam stability significantly (P<0.05) increased in presence of sugar alcohols, but emulsifying activity index (EAI) and foam capacity significantly (P<0.05) decreased. All the soy protein solution and the emulsion were shear thinning fluid. With the increase of the concentration of polyols, the apparent viscosity increased. And significant shear thinning effects are presented.(2)With the addition of polyols, the surface hydrophobicity, maximum absorption wavelength (?max) and tryptophan fluorescence intensity of soy protein was increased at pH 5.0,but was decreased at pH 3.0 and 8.0. Furthermore, CD spectra spectra confirmed that the amounts of a-helix and the amounts of a-helix and ordered structure content (?-helical+ ?-sheet) of soy protein at pH 3.0,5.0 and 8.0 were increased with the addition of polyols. Solubility of soy protein added with polyols was significantly (P<0.05) increased at different pH. The effect of polyols on soy protein solubility significantly (P<0.05) increases with pH going towards the protein isoelectric point, i.e. when protein net charge decreases. Emulsifying activity index (EAI) and foam capacity of soy protein significantly (P<0.05) decreased in presence of polyols at pH 3.0 and 8.0, but emulsion stability index (ESI) and foam stability significantly (P<0.05) increased. With the addition of polyols, emulsifying properties and foaming properties of soy protein significantly (P<0.05) increases at pH 5.0.(3)With the addition of NaCl, the surface hydrophobicity, maximum absorption wavelength (Amax) and tryptophan fluorescence intensity of soy protein was reduced. Moreover, CD spectra spectra confirmed that the amounts of a-helix and ordered structure content (a-helical+??-sheet) of soy protein was decreased. As polyols added, the surface hydrophobicity, the maximum absorption wavelength (Amax) and tryptophan fluorescence intensity of soy protein at different concentration of NaCl solution was increased. Furthermore, CD spectra spectra confirmed that the amounts of a-helix and ordered structure content (a-helical+?-sheet) of soy protein added with salt was increased with the addition of polyols. Solubility, emulsifying properties and foaming properties of soy protein significantly (P<0.05) was decreased with the addition of salt. However, solubility and functional properties of soy protein at different concentration of NaCl solution significantly (P<0.05) was increased with the addition of polyols.(4)As soy protein preheated (80 and 100?), the surface hydrophobicity, maximum absorption wavelength (?max) and tryptophan fluorescence intensity was increased. Furthermore, CD spectra spectra confirmed that the amounts of a-helix and ordered structure content (a-helical+?-sheet) of preheated soy protein was decreased, whose protein structure was flexible. As polyols added, the surface hydrophobicity, maximum absorption wavelength (?max) and tryptophan fluorescence intensity of preheated soy protein was reduced. Moreover, CD spectra spectra confirmed that the amounts of a-helix and ordered structure content (a-helical+?-sheet) of preheated soy protein was increased with the addition of polyols. Heat treatment resulted in a significant decrease in soy protein solubility, but EAI and ESI was significantly (P<0.05) increased. Foam capacity and foam stability of preheated (80?) soy protein significantly (P<0.05) increased. However increasing the temperature on the contrary led to significantly (P<0.05) decline in foam capacity and foam stability of preheated soy protein. Solubility, ESI and foam stability of preheated (80 and 100?) soy protein significantly (P,0.05) increased with the addition of polyols. EAI and foam capacity of preheated (80?) soy protein significantly (P<0.05) decreased with the addition of polyols, but that of preheated (100?) soy protein significantly (P<0.05) increased.(5)Total sulfhydryl group content of soy protein generally decreased with increasing number of freeze-thaw cycle, but disulfide bond content generally increased. The increase in disulfide bond content was generally coincidental with the decrease in total sulfhydryl group content. The result suggests that freeze-thaw process possibly induced the formation of disulphide bonds between polypeptides or within polypeptides. The surface hydrophobicity, the maximum absorption wavelength (?max) and tryptophan fluorescence intensity of intrinsic fluorescence spectroscopy was increased, when soy protein was subjected to freeze-thawing. Moreover, CD spectra spectra confirmed that the amounts of a-helix and ordered structure content (?-helical+?-sheet) of soy protein with subjected freeze-thaw cycle was decreased, whose protein structure was flexible. As polyols added, total sulfhydryl group content of soy protein with subjected 3 freeze-thaw cycles significantly (P<0.05) increased, but disulfide bond content significantly (P<0.05) decreased. And, the surface hydrophobicity, the maximum absorption wavelength (Amax) and tryptophan fluorescence intensity of intrinsic fluorescence spectroscopy was decreased, when soy protein was subjected to freeze-thawing. Furthermore, CD spectra spectra confirmed that the amounts of a-helix and ordered structure content (?-helical+?-sheet) of soy protein with subjected freeze-thawing was increased with the addition of polyols. As soy protein subjected to freeze-thawing, the solubility was significantly (P<0.05) decreased, but emulsifying properties and foaming properties was significantly (P<0.05) increased. The solubility, EAI and foam stability of soy protein with subjected freeze-thawing was significantly (P<0.05) increased with the addition of polyols. However, EAI and foam capacity of soy protein with subjected freeze-thawing was significantly (P<0.05) increased with the addition of polyols. |
PDF Full Text Request