Performance and mechanism of an innovative humidity-controlled hot-air drying method for concentrated starch gels: A case of sweet potato starch noodles

The effects of humidity control on dried starch gels were investigated using starch noodles as a model. A two-stage innovative hot-air-drying regime was developed with the first stage humidity-controlled (70 degrees C, 60% RH) and the second at high temperature (100 degrees C). The proposed drying method is comparable to natural-air-drying in product quality and to conventional hot-air-drying (70 degrees C) in production efficiency. The operating humidity of the first stage predominated the swelling index and rehydration ratio of dry noodles as well as the hardness and chewiness of cooked noodles. The results from XRD, DSC, SEM, digital microscopy and low field TD H-1 NMR evidenced that these outcomes were largely ascribed to the higher shrinkage, lower porosity, smoother surface, lesser shape deformation and higher starch retrogradation resulting from increased humidity. The results reported herein are valuable for regulating the physicochemical properties of dried starch gels and glimpsing the underlying mechanisms of related operations.