Mechanism of freeze-thaw treatment influencing quality of puffing product for sweet potato using low-field nuclear magnetic resonance

The water content is generally 70% to 90% in fresh fruits and vegetable, which usually includes free water, immobilized water, and closely combined water. The water content and its distribution in the raw materials of fruit and vegetable may affect the drying process, physicochemical, and sensory characteristics of dry products dried by different drying technologies. There are a lot of drying methods reported, such as natural sun drying, vacuum drying, heat pump drying, forced-air drying, microwave drying, convective drying, freeze-drying, and explosion puffing drying. Nuclear magnetic resonance (NMR) is a physical phenomenon generated from the interaction between alternating magnetic field and substance. A pulse NMR system has been widely used in the investigation of bound water in a biological system in recent years. Among them, low-field nuclear magnetic resonance (LF-NMR) is a rapid, non-destructive analysis technique, in investigation of the whole process of samples proton transition after an attack by radio-frequency pulse to interpret the variation of water in samples from the microscopic point. The LF-NMR technology has been successfully applied to study the water content and distribution of the process of repeated freeze-thaw of meat, carrot drying process, banana ripening process, and potato heating and refrigeration process.