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Physicochemical properties and in vitro starch digestibility of potato starch/protein blends
Q. LIU (1), Z. Lu (2), R. Yada (3), E. Donner (2) (1) Agriculture and Agri-Food Canada, Guelph, ON, Canada; (2) Agriculture and Agri-Food Canada, , Canada; (3) Food, Nutrition and Health Program, Faculty of Land and Food Systems, The University of British Columbia, , Canada.

In this study, starch-protein interaction, thermal properties, rheological behaviour, gel structure, texture profile and <i>in vitro</i> starch digestibility during processing (cooking, cooling and reheating) were investigated in a composite of potato starch/protein blends (0, 5, 10, or 15% potato protein isolate). The effect of protein on starch re-crystallization and short-range ordering in starch were further studied by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). DSC and dynamic viscoelasticity showed that protein proportionally restricted starch granule swelling in the blends during cooking, while facilitating amylopectin retrogradation during cold-storage by heterogeneous nucleation and electrostatic complexing, resulting in a more complete overall crystallinity but a less perfect crystallization than in pure starch gel. However, the facilitating effect diminished with the increasing blend ratio of protein, attributed to its high molecular weight and more extended phase separation. FTIR readily detected the short-range ordering from slow retrogradation of amylopectin in processed samples, which proportionally increased with increasing ratio of protein within the concentration used in this study. Cooling of freshly cooked starch/protein blends built new short-range ordering in starch, which could not be completely reversed by reheating. Regardless of processing method (cooking, cooling or reheating), slowly digestible starch (SDS) and resistant starch (RS) contents were positively correlated to the increasing ratio of protein in the blend (P < 0.05). It is concluded that heterogeneous nucleation and electrostatic complexing contributed to the enhanced re-crystallization of amylopectin in the blends during starch gelation and retrogradation.