Study On The Storage Stability Of Rice Starch System Based On Water Activity And Glass Transition Theory

Based on the water activity(aw)and glass transition theory,this paper discussed the influence of the two theories on the storage stability of rice starch(IRS and WRS)and soluble dietary fiber-rice starch composite system(IRS-SDF and WRS-SDF),briefly analyzed the moisture adsorption isotherm and thermodynamic properties of one starch-based foods(instant rice noodle).The results can provide theoretical basis and technical approach for quick frozen,transportation,freeze drying of starch-based foods and foods rich in dietary fiber.The main research contents and conclusions are as follows:(1).Desorption and adsorption isotherms of IRS and WRS were determined by gravimetric method and water activity ranging from 0.11 to 0.90 at 25 ?.The results showed that desorption and adsorption isotherms of IRS and WRS were belonged to type II behavior(S-curve).The hysteresis loop was found to exist in moisture adsorption-desorption isotherm of IRS and WRS,and was of type H3 according to the classification of IUPAC.Eight models were used to fit experimental data by linear regression analysis method to ascertain the best of fit.Comparisons were evaluated with the R2,RSS,MRD and RMSE.Results indicated that GAB model were suitable for fitting the moisture sorption isotherm of rice starch.The parameters X0(monolayer moisture content)of GAB fitting adsorption isotherm of IRS and WRS were 7.43%and 8.87%,respectively.This can provide theoretical basis and technical approach for the choice of storage conditions and drying process of rice starch.(2).Differential scanning calorimetry(DSC)was used to determine the Tg and freezing point(TF)of rice starch(IRS and WRS)and soluble dietary fiber-rice starch composite system(IRS-SDF and WRS-SDF).According to the understanding of state diagram,as the water activity increased from 0.11 to 0.90,the glass transition temperatures decreased from 18.68 to-29.72? for WRS-SDF.In the range of high moisture content,with the increase of water content,there is a certain degree of increase of the freezing point temperature of all samples.When in the same water activity,the Tg of IRS and WRS is significantly greater than IRS-SDF and WRS-SDF composite system,respectively.This can explain that soluble dietary fiber-rice starch composite system are less easy to store compared to rice starch.According to the understanding of state diagram,the shelf life prediction model of IRS,WRS,IRS-SDF and WRS-SDF were established by the Williams-Landel-Ferry(WLF)equation.(3).The moisture sorption isotherms of instant rice noodles were determined by gravimetric method and water activity ranging from 0.110 to 0.909 at 15?,25?and 35 ? based on adsorption theory.Seven models were used to fit experimental data by linear regression analysis method to ascertain the best of fit.Comparisons were evaluated with the coefficient of determination(R2),residual sum of squares(RSS),mean relative deviation modulus(P)and the standard error of estimate(SE).Thermodynamic properties,namely net isosteric heat of sorption,differential entropy and enthalpy-entropy compensation,were determined from sorption isotherm data of instant rice noodles.Results indicated that the moisture sorption isotherms were belonged to type ? behavior in different temperature.The parameters X0 of GAB fitting sorption isotherms in 15?,25 ? and 35? were 9.23%,8.34%and 7.65%,respectively.The net isosteric heat of sorption and differential entropy were calculated by using the moisture adsorption isotherm and decreased as moisture content increased.The enthalpy-entropy compensation theory was applied to sorption isotherms and the isokinetic temperature for instant rice noodles was calculated by plotting the differential enthalpy versus differential entropy.It was found that the adsorption process investigated was entropy-controlled and spontaneous.