| Starch is one of the most important functional food biopolymers. When starch-containing foods were subjected to repeated freeze-thaw cycles(FTCs), their textures and other physicochemical properties might be changed extensively. Thus, this research aimed to study the physical properties of gels during repeated FTCs. The effect of Na Cl on the freeze-thaw stability was also investigated to increase the freeze-thaw stability. The subject performed the following experiment:Firstly, tapioca starch, sweet potato starch, potato starch, corn starch, waxy corn starch, rice starch, indica rice starch, wheat starch and pea starch were chosen to research the differences in freeze-thaw stability. Syneresis measurements, texture analysis, thermal analysis, micrograph images and resistant starch(RS) content were investigated during FTCs. Syneresis percentage of tapioca starch, rice starch and indica rice starch increased along with FTCs. However, syneresis percentage of pea starch decreased and other starches increased and then decreased from 1 FTC to 5 FTCs. The higher degree of retrogradation(R%) meant the worse freeze-thaw stability of starch; sweet potato starch and pea starch showed high R% and rice starch and indica rice starch showed low R%, while R% of other starches increased significantly along with FTCs. Sweet potato, potato, corn and pea starch gels formed a dense sponge-like structure after 5 FTCs, while other starch gels formed a network with less linkage. An increase in hardness of gels was observed after 5 FTCs. The variation of hardness of potato starch, sweet potato starch, corn starch and pea starch was higher than others. RS content increased gradually along with FTCs. The starch with high R% showed a high RS content.Secondly, the freeze-thaw stability evaluated in relation to the structure feature of starch molecule. The syneresis percentage after 1 FTC as well as R% and RS content after 1 FTC, 3 FTCs, 5 FTCs was positively correlated with the amylose content. Proportional ratio of the amylopectin branch chains DP 6-12 to those of DP 6-24 was determined, and designated as the APC ratio. R% and RS content after 1 FTC, 3 FTCs, 5 FTCs was negatively correlated with APC ratio. Furthermore, since the degree of crystallinity was negatively correlated with amylose content, X-ray diffraction could be used as a convenient method to evaluate the freeze-thaw stability of starch gels.Finally, since 0.5 mol/L Na Cl could increase the freeze-thaw stability of all starches, tapioca starch(TS) was chosen to research the influence of Na Cl(0-1.0 mol/L) on the freezethaw stability. Syneresis measurements, texture analysis and rheology analysis were investigated up to 5 FTCs. Furthermore, thermal analysis, rapid-viscosity analysis, SEM images, the spin-spin relaxation time(T2) and freezable water on TS during FTCs and zeta potential of TS gels were compared to research the possible reasons of the effect. Na Cl reduced the syneresis percentage and hardness of TS gels and increased tanδ, which meant Na Cl effectively preserved the quality of freeze-thawed TS gels. DSC experiments demonstrated that starch retrogradation was retarded by Na Cl addition. Na Cl addition increased the pasting temperature and final viscosity of gels, which implied the starch-salt interaction and the hydration of Na Cl. The zeta potential of the starch gels decreased with increasing Na Cl concentrations; it was due to the starch-salt interaction. Micrograph images revealed that increased Na Cl concentrations decreased the size of ice crystal in the gels and led to fewer links within the starch matrix. The amount of freezable water decreased with increasing Na Cl concentrations which retarded the association of starch molecules during FTCs. The T2 of the system with Na Cl decreased more quickly than that without Na Cl during FTCs. Thus, Na Cl effectively increased the stability of starch by the starch-salt interaction and the destruction of ice crystal formation. |
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