Potato starch retrogradation in tuber: Structural changes and gastro-small intestinal digestion in vitro

Structural changes of potato starch during retrogradation in tuber and its resulting digestibility were studied. Freshly cooked (FC) tubers were stored at 4 degrees C for 1,3 and 7 days (FCR) and then reheated at 50, 70, and 90 degrees C (FCR-r). The starch retrogradation enthalpy (Delta H-r) and crystallinity were both higher for retrograded tubers than for freshly cooked or for retrograded + reheated tubers. Different water populations in tuber were detected by a low-field NMR (LF-NMR), having relaxation times T-21 (< 15 ms), T-22 (70-200ms), and T-23 (> 400 ms). The relaxation time of each water population decreased during refrigerated storage. The relaxation time T-22 of 1, 3 and 7-day retrograded tuber increased during reheating but not to the level of the freshly cooked tuber. Ease of starch hydrolysis of the samples was studied by using an in vitro gastro-small intestinal digestion model. The 7-day retrograded + reheated sample showed a significantly lower starch hydrolysis (%), similar to those of the 1-day retrograded sample without reheating. The relaxation time of a water population indicates mobility - the water with low relaxation time is more mobile and less restricted which could facilitate enzyme diffusion leading to greater starch hydrolysis (%): in this study low relaxation time T-22 was positively correlated to greater starch hydrolysis of the treated tubers (p < 0.05).