Effect Of High Temperature On Hydrolysis And Repolymerization Of Starch Molecular Chains And The Application Of Pyroconversion Dextrin

In this paper,pyroconversion dextrin(PD)was prepared from tapioca starch(TS)and high amylose corn starch(HACS)with the catalyst of hydrochloric acid by dry method.The optimal process conditions of PD were got by single factor and orthogonal test based on the indicators of dietary fiber content and whiteness.The results showed that the dietary fiber content of tapioca pyroconversion dextrin(TPD)was 60.79%at the pyroconversion temperature of 170°C,the pyroconversion time of 150 min and the addition amount of hydrochloric acid of 14%.And the dietary fiber content of high amylose corn pyroconversion dextrin(HACPD)was 55.45%at the pyroconversion temperature of 180°C,the pyroconversion time of 210 min and the addition amount of hydrochloric acid of 20%.Compared with the native starch(NS),PD existed in the form of granules.The results of polarized light and X-ray diffraction(XRD)showed that the crystal structure of the granules was destroyed,and this phenomenon also proved by differential scanning calorimetry(DSC).Besises theα-1,4 andα-1,6 glucosidic linkages of NS,theα-1,2、β-1,2、β-1,4 andβ-1,6glucosidic linkages were formed during the pyroconversion process.The weight loss rate of TS and HACS from the thermogravimetric analyzer(TGA)were 88.86%and 84.54%respectively,while the weight loss rates of TPD and HACPD were 85.94%and 83.37%respectively.After acid-thermal treatment,the reducing sugar content and cold water solubility of the both NS were increased.The cold water solubility of TPD and HACPD were93.81%and 98.40%respectively,and their aqueous solutions was pH<4.5.The acid-thermal stability of PD was reflected by the change of reducing sugar content.The both PD had the highest reducing sugar content at pH=1 when heated(85°C/0.5 h),and the reducing sugar content of TPD and HACPD was 13.25%and 12.20%,respectively.In addition,the content of reducing sugars increased by less than 1%when PD was stored(25°C/24 h)at pH 1～7 and heated at pH 2～7.In order to investigate the changes of starch molecular chains during pyroconversion process,we used TS and HACS as material to prepare the pyroconversion dextrin at different temperatures.Scanning electron microscopy(SEM)results displayed that the granule morphology of the both PD were complete,but there was a spalling phenomenon on the some of them.Compared with the native starch,the weight average molecular weight(M_w)and number average molecular weight(M_n)of the both PD at all temperatures decreased significantly.With the increase of temperature,the M_w and M_n of TPD and HACPD dextrins increased only when the temperature was 180°C.Meanwhile,the reducing sugar content of TPD and HACPD gradually decreased from 12.32%and 8.49%to 4.90%and 4.39%,respectively.Samely,the relative crystallinity of TPD and HACPD gradually decreased from25.85%and 18.73%to 2.65%and 5.69%,respectively.This phenomenon revealed that the molecule chains of two pyroconversion dextrin had undergone the polymerization reaction during the pyroconversion process,and the rearrangement of the molecular chains did not occur.The results of Fourier transform infrared spectroscopy(FT-IR)showed that the caramelization reaction may led to the formation of C＝O groups.At the same pyroconversion temperature,HACS was more difficult to hydrolysis and repolymerization than TS.The solubility of TPD and HACPD at all temperature was above 60%,and most of them were even close to 100%.The pH value of aqueous solution was 4～5.There was a significant correlation(P<0.05)among the dietary fiber content,pH value and whiteness of the both PD by the Pearson correlation analysis.The thermodynamic properties of the both PD were determined by DSC and TGA.The results displayed that there was still no endothermic peak of TPD and HACPD even at 100°C.The weight loss rate of the both PD prepared only at this temperature was slightly higher than that of NS,and The weight loss rates of TPD and HACPD were 91.02%and 84.63%,respectively.The fluctuation of reducing sugar content reflected a good stability of the both PD under acid-thermal environment.The effect of the coating prepared by HACPD on the bread was studied.The results showed that there were no significant differences in texture,specific volume and quality of bread core amnong all bread(P>0.05)except the appearance.The skin of the bread after coating was bright and shiny,and this phenomenon was also checked by SEM.The coating could improve the whiteness of bread skin to a certain extent and help to increase the dietary fiber content in the skin of the bread.