Effect of hydrothermal treatment on formation and structural characteristics of slowly digestible non-pasted granular sweet potato starch

The formation and structural characteristics of slowly digestible non-pasted granular starch in sweet potato starch were investigated under various hydrothermal treatment conditions. The moisture content of the sweet potato starch was adjusted to 20, 50 or 90%, and the starch was heated at 40, 55 or 100 degrees C for 12 h in a dry oven. The relative crystallinity of the hydrothermally treated samples was decreased with increasing temperature, and the X-ray diffraction patterns of the samples were altered from C-b-type to A-type. Microscopic observations did not reveal any changes in the starch granules of any samples except those with moisture contents of 50 and 90% that were heated at 100 degrees C. When gelatinization parameters were examined, samples with moisture contents of 50 and 90% that were heated at 55 degrees C and samples of all moisture contents that were heated at 100 degrees C had peak temperatures higher than that of raw starch but gelatinization enthalpies lower than that of raw starch. The swelling factor of the samples heated at 40 degrees C did not change significantly, whereas that of samples heated at 55 and 100 degrees C was decreased at increased moisture levels. The sweet potato starch with 50% moisture content that was heated at 55 degrees C had the highest content of granular slowly digestible starch, about 200% that of raw starch, although our study did not involve further hydrothermal treatment conditions. Further study is required to complete a process for more efficient production of heat stable and slowly digestible starch.