| This study focused on sweet potato starch, and three different methods including debranching-heat-moisture treatment, repeated heat-moisture treatment, and xylitol-heat-moisture treatment were used to prepare slowly digestible starch (SDS) from sweet potato starch, and their structure, properties, and formation mechanism were investigated.Firstly, debranching-heat-moisture treatment was used to prepare SDS from sweet potato starch, and the effects of pullulanase concentration, moisture content of heat-moisture treatment, and temperature of heat-moisture treatment on the formation of SDS were investigated, and their structural, properties and formation mechanism were also explored. Under the conditions of pullulanase concentration of 25ASPU/g, moisture content of 30%, and temperature of 100 ℃, the content of SDS reached up to 31.60%. Compared with native sweet potato starch, debranching treatment (DT) and debranching-heat-moisture treatment (D-HMT) decreased the percentage of starch fraction with degree of polymerisation (DP)≤ 13, increased the percentages of starch fractions with DP 13-30 and DP>30, and decreased the molecular weight of starch sample. Meanwhile the pasting temperatures of DT and D-HMT starch samples were higher, while the peak viscosities, swelling power and solubility values were lower than that of native starch. Compared with the DT starch sample, the To, Tp, Tc, Tc-To and △H of D-HMT starch samples for the second endothermal were increased significantly, the surface became more smooth, the ratio of 1047 cm-1 and 1022 cm-1 and relative crystallinity were increased, and the crystalline pattern was altered from Ca to A type.Secondly, repeated heat-moisture treatment was used to prepare SDS from sweet potato starch, and the effects of moisture content, temperature, and cycling times on the formation of SDS were investigated, and their structure, properties and formation mechanism were also explored. Under the conditions of moisture content of 30%, temperature of 100 ℃, and cycling times of 3, the content of SDS reached up to 19.61%. Compared with the native starch sample, the viscosity, swelling power and solubility of repeated heat-moisture treatment starch sample decreased, while the pasting temperatures and gelatinization transition temperatures increased, and the agglomerations of granules were found in them. Moreover, the △H, absorbance ratio of 1047 cm-1 and 1022 cm-1 and relative crystallinity decreased, and the crystalline pattern altered from Ca to A type. Furthermore, as the cycling times increased, the in vitro digestibility, physicochemical and structural properties of modified starch samples exhibited regular change.Thirdly, xylitol-heat-moisture treatment was used to prepare SDS from sweet potato starch, and the effects of moisture content, temperature, addition content of xylitol on the formation of SDS were investigated, and their structure, properties and formation mechanism were also explored. Under the conditions of moisture content of 30%, temperature of 110 ℃, and xylitol addition content of 0.4g, the content of SDS reached up to 37.44%. Compared with the heat-moisture starch sample, the swelling power and viscosity of xylitol-heat-moisture treatment starch sample decreased, while the pasting temperatures and gelatinization transition temperatures increased, and the agglomerations of granules were found in them. Moreover, the AH and absorbance ratio of 1047 cm-1 and 1022 cm-1 decreased.In conclusion, the effects of the three different combined heat-moisture treatments on the content of SDS were investigated, their structure and properties were also explored. The results showed that repeated heat-moisture treatment was the best one among these three methods to prepare SDS of sweet potato starch. Although the SDS content by repeated heat-moisture treatment was lower than that by other two methods, it was easy to be applied, non-addition and no by-products of chemical reagent, and it was a cost-effective and environmentally-friendly method, which was fitted for mass preparation of SDS from sweet potato starch. |
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