Physicochemical properties of different-sized fractions of sweet potato starch and their contributions to the quality of sweet potato starch

Sweet potato starch consists of granules varying in size. However, the size-dependence of its physicochemical properties and the contribution of different-sized fractions to the functionality of unfractionated starch are not well recognized. These were addressed in the present study by comparing the whole starch with its small-, medium- and large-sized fractions. In composition, the contents of protein, lipid and ash decreased but the apparent amylose content increased by an increase in granule size. The large-sized fraction had significant higher relative crystallinity than its counterpart small- and medium-sized fractions. The degree of short-range order calculated from deconvoluted FTIR spectra for large-sized fraction was identical to the whole starch and showed the lowest degree of order. On the molecular level, larger-sized granules had amylose populations with greater molecular weight. Small-sized fraction had lower average chain length of amylopectin, more DP 6–12 chains but less DP > 25 chains. Insignificant difference in thermal properties was observed among the unfractionated starch and its different-sized fractions. Breakdown, setback and consistency coefficients as well as resistance to digestion showed significantly positive correlation with granule size. However, the pasting temperature negatively correlated with granule size (r = -0.994, p < 0.05). An additive effect on pasting behavior but a non-additive effect on rheological properties was estimated by the weighted summation of individual-sized fraction. Results suggested that the significant impact of granule size on the functionalities of sweet potato starch, which could provide useful information for the utilization of these different-sized fractions.