Water Distribution and Water-Binding Mechanism of Fresh Sweet Potato Residue

To investigate the water-binding mechanism of fresh sweet potato residue, the water distribution in starch processing residues from 10 varieties of sweet potato was determined by low-field nuclear magnetic resonance (LF-NMR), and the correlation between water states and major organic components was analyzed. Then, the causal relationship was verified through enzymatic hydrolysis experiments. The results showed that three or four states of water were found in the 10 varieties of fresh sweet potato residues, namely compound (T21, 0.658–2.310 ms), immobilized (T22, 7.055–21.550 ms), entrapped (T23, 377.410–705.480 ms) and free flowing water (T24, 705.480–3 764.936 ms). The percentage of entrapped water in sweet potato residues was the highest (93.59% on average), followed by immobilized water (5.15% on average). The percentages of compound water and free flowing water were only 0.85% and 0.41%, respectively. The percentage of entrapped water was significantly positively correlated with the contents of pectin, dietary fiber, and insoluble dietary fiber. The amount of water discharged from sweet potato residues significantly increase after being hydrolyzed by pectinase, but did not significantly change after being hydrolyzed by cellulase, hemicellulase or lignin peroxidase. Under scanning electron microscopy (SEM), it was observed that most of starch particles were encapsulated or bound to cell wall debris. This study can provide a theoretical basis for the development of an efficient dehydration technology for sweet potato residues.