The effect of repeated versus continuous annealing on structural, physicochemical, and digestive properties of potato starch

Native potato starch was suspended in distilled water at starch: water ratio of 1:3 (w/v). The starch-water suspensions were then subjected to repeated annealing treatments (RANN) at 55 °C for 12 h, repeated for 8 cycles or continuous annealing treatments (CANN) at 55 °C for 24, 48, 72 and 96 h. The structural, physiochemical and digestive properties of the annealed starch samples were studied and compared with those of the native starch. The scanning electron microscopy and light microscopy analysis showed that the repeated and continuous annealing treatments could keep the integrity and surface perfection of the starch granules. The growth rings of the annealed starch granules were more distinct than those of the native starch granules as revealed by confocal laser scanning microscopy. The crystallinity degree increased and the crystalline retained the B-type pattern after the annealing treatments. There were no chemical bonds and functional groups produced or disappeared during the applied annealing treatments. Infrared absorption peak intensity of starch decreased and short-range ordered structures increased after treatments. The swelling power and solubility decreased at low temperature (50 to 60 °C) and increased at relatively high temperature (70 to 90 °C). The rapid visco-analyzer and differential scanning calorimeter analysis revealed an increase in the setback, final viscosity, pasting temperature and gelatinization transition temperature, and a decrease in breakdown value of the starch after the annealing treatments. On the other hand, the repeated and continuous annealing treatments resulted in starch with low in vitro digestibility degree, indicating formation of resistant starch. Generally, the repeated annealing treatment resulted in starch with high improved properties compared with the starch resulted from the continuous annealing treatments. Therefore, the repeated annealing treatments can be suggested as an effective method for producing of modified starch for food industrial applications.