Abstract
Effect of osmotic-ultrasound dehydration pretreatment on the moisture adsorption isotherms of microwave-assisted vacuum fried purple-fleshed sweet potato (PSP) was determined at 30, 45 and 60 °C and fitted with six models. Absorbed water state of microwave-assisted vacuum fried purple-fleshed sweet potato was measured by low-field nuclear magnetic resonance (LF-NMR). Results indicated that the optimized conditions of osmotic-ultrasound dehydration pretreatment were 11.21 min for ultrasound time, 56.99% sucrose concentration and 74.84 min for osmotic dehydration time. Moisture adsorption isotherms showed type II sigmoid shape. GAB model had the best fit evaluated by the higher values of R2 (>0.9868) and the lower values of RMSE (<0.0074) and χ2 (<9.4893 × 10−5) for untreated and pretreated fried purple-fleshed sweet potato slices. Monolayer moisture content from GAB model decreased from 0.0598 at 30 °C to 0.0452 at 60 °C. The net isosteric heat of adsorption for untreated and pretreated fried PSP was calculated by the Clausius-Clapeyron equation and decreased with increasing moisture contents. The correlation between the bound water population (T21) peak areas of absorbed water and equilibrium moisture content was respectively high for untreated PSP (R2 > 0.9595) and pretreated fried PSP (R2 > 0.9845) by using LF-NMR.
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