Water Changes of Purple Sweet Potato Slices Using Low-field NMR during Vacuum Freeze Drying

In order to explore the dynamic law of water and the drying quality of purple sweet potato slices during the vacuum freeze drying, the existential form of internal water and the change of water content in purple sweet potato slices were studied by low-field nuclear magnetic resonance and imaging techniques. Additionally, during the drying, stereoscopic microscope and color difference meter were used to observe the microstructure and the color change of purple sweet potato slices respectively. As a result, purple sweet potato slices removed most of free water and partial bound water and semi-bound water, the volume of semi-bound water tended to increase and then decrease, and the whole of inversion spectrum of transverse relaxation shifted to T₂₂ peak. With the drying process, the water content in purple sweet potato slices continued to decline, and the brightness and the clarity of the pseudo-color images accordingly decreased. In terms of drying effect, the observation of microscopic structure showed that the sublimation of water first diffused to both sides of purple sweet potato slices where the purple sweet potato slices formedloose porous structure after water removed. Prolonging the drying time can improve the brightness of purple sweet potato slices, but had little effect on the color. Hence vacuum freeze drying can be used to develop purple sweet potato slices as leisure food.