| Lotus seed had a high nutritional value and health effect.Starch was one of the main components of lotus seed.In the process of starch food processing,starch mainly occured gelatinization and retrogradation.In the process of processing,starch was affected by its own properties and processing conditions.Different food systems had different potential of hydrogen(pH values),and the pH value had a certain impact on the physical-chemical,properties and structural properties of starch.The previous study of our research group found that the content of amylose in lotus seed starch was about 40%,which was a kind of specific starch with high content of amylose and was easy to retrograde.The previous study also found that under the optimum retrograded temperature was 4 C.And the storage time had a certain impact on the retrogradation of lotus seed starch.The physicochemical properties of starch paste could reflect the retrogradation process.What’t more,the physicochemical properties of starch were closely related to the structural properties.Therefore,in this study,the effect of pH values on the gelatinization characteristics,structural and physical-chemical properties of lotus seed starch were investigated.The non isothermal kinetics and retrogradation kinetics of lotus seed starch under different pH conditions were established.It aimed to clarify the relationship between pH and gelatinization characteristics of lotus seed starch,as well as the structural and physical-chemical properties of lotus seed starch after cold storage.The results were as follows:(1)Study on the effect of pH on the gelatinization characteristics of lotus seed starchThe viscosity of lotus seed starch pastry firstly increased and then decreased with the increase of pH values.The thermal stability of lotus seed starch was stronger under acidic condition compared to alkaline condition.The starch pastes with different pH values were non-Newtonian fluids with pseudo plastic fluid characteristics.The fluid type of starch paste could not be changed under different pH conditions.The rheological properties of the starch paste could be well fitted by Power law equation.The thinning phenomenon had obvious thixotropy,and the gel formed was weak gel,as well as the system was mainly viscous.The lotus seed starch paste at pH 5 had obvious thixotropy.Moreover,the thermal enthalpy value,peak temperature,termination temperature and gelatinization temperature of lotus seed starch firstly increased and then decreased with the increase of pH value.In strong acid system,the gelatinization and retrogradation of lotus seed starch paste became slow because starch was acidified.According to non-isothermal kinetics,pH5 was the great condition of gelatinization.The results of correlation analysis showed that lotus seed starch had strong retrogradation ability under pH5 and pH7 conditions.(2)Study on the effect of pH on the structural characteristics of lotus seed starch after cold storageThe results of molecular weight showed that the molecular weight of lotus seed starch after cold storage at pH 5 and pH 7 was significantly higher than those of other pH conditions,which might be related to starch regeneration.Under pH 5 and pH 7,the microstructure of lotus seed starch was denser,while under other pH conditions,it was dispersed.The results of XRD and solid-state NMR showed that the crystal form of lotus seed starch was C-type under different pH conditions,and the crystallinity of lotus seed starch under pH5 and pH7 conditions was significantly higher than other pH levels.The infrared spectroscopy results showed that the order degree of lotus seed starch under pH5 and pH7 conditions was significantly higher than those under other pH conditions.The results of water distribution showed that the lotus seed starch was mainly free water under the condition of pH5 and pH7.Avrami kinetic equation fitting showed that under the condition of pH3,the nucleation index was less than 0,indicating that lotus seed starch was acidolyzed under strong acid conditions and was not prone to aging.Whereas the retrogradation of lotus seed starch after cold storage at other different pH values was one-time nucleation.The order of nucleation and growth rate of lotus seed starch after cold storage at different pH were as follows: kpH5>kpH7>kpH11>kpH9.(3)Study on the effect of pH on the physicochemical properties of lotus seed starch after cold storageThe RVA results showed that it was more difficult for lotus seed starch to undergo secondary gelatinization under the condition of strong acid and alkaline than other pH conditions.The retrogradation value of lotus seed starch under pH5 was significantly lower than that under other pH conditions.The rheological results showed that the apparent viscosity of lotus seed starch was the lowest at pH3,which might be due to the modification of the starch under strong acid conditions.The lotus seed starch under pH9 and pH11 conditions were not easy to retrograde,which was consistent with the results from the structural properties.DSC results showed that the retrogradation enthalpy of lotus seed starch refrigerated at pH5 was significantly higher than those refrigerated at other pH values.Furthermore,the correlation results showed that under different pH and different refrigeration time,the structural properties of lotus seed starch had a certain correlation with the physical and chemical properties.However,the correlation between the physical and chemical properties of short-term retrograde and long-term retrograded lotus seed starch was inconsistent with the structural characteristic parameters.The viscosity of short-term regenerating lotus seed starch was related to its polydispersity coefficient,and the viscosity of long-term regenerating lotus seed starch was related to its crystallinity and ordered structure. |
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