Use of low‐field nuclear magnetic resonance for moisture variation analysis of fresh in‐shell peanut during different drying methods

Abstract In exploring the law of heat transfer and water diffusion of peanut, low‐field nuclear magnetic resonance (LF‐NMR) was used to analyze moisture migration in peanuts subjected to hot air drying (HAD), infrared drying (IRD), and sequential hot air‐infrared drying (HAD‐IRD) at 40, 50, 60, and 70°C. In HAD and IRD, free water was removed first, and some of free water was converted into loosely bound water. In HAD‐IRD, free water and loosely bound water were primarily removed. Regression models were used to predict changes of free water in peanuts. Based on MRI, the binding force on water decreases with the increase of temperature, and water migration from the inside to the outside is accelerated. HAD‐IRD can promote the removal of free water and the conversion of bound water into more easily removable water. This study revealed that HAD‐IRD can effectively reduce the drying time compared with HAD and IRD. Moreover, LF‐NMR can be an effective tool for detecting moisture changes in peanuts. The research results provide guidance for the selection of peanut processing technique. Practical applications Drying is an important food processing and preservation method based on the principle of reducing the moisture content of materials. Our study shows that HAD‐IRD is a superior, feasible drying method, as compared with other published works. This drying method removes a large amount of free water by hot air in the early stage and removes the remaining water by radiant heating in the later stage, which achieves rapid dehydration and greatly reduces the drying time. By identifying the moisture migration mechanism of the three drying methods, this study improves the drying theory and provides a theoretical basis for drying in the peanut processing industry.