Analysis of moisture transfer of prunes during drying using low-field NMR

Prunes is one of the plants in the genus Prunus of the family Rosaceae, which is rich in nutrients and has high economic value and mainly planted in the Xinjiang Province of China. However, due to the high moisture content, fresh prunes are prone to water loss, softening, rotting, mold, and a series of quality degradation problems that affect consumption. Drying prunes can extend the storage period and increase the added value of the product. An experiment has been designed to investigate the moisture migration during the drying process of prunes and the drying characteristics of prunes at different temperatures (50, 65, 80 ℃) and wind speeds (1, 2, 3 m/s). Biot number is an important parameter with a wide range of applications in heat transfer science. In this study, five drying kinetic models based on the Biot number were selected to fit the drying curve. By comparing the parameters and the fitted curves, it was found that the Bi-G model can accurately depict the drying process. Overall drying rate decreases gradually, moisture diffusion was first controlled externally, and then controlled by internal diffusion; the moisture diffusion efficiency increased with the increase of drying temperature and drying wind speed, and the effect of drying temperature is more significant. The low-field nuclear magnetic resonance (LF-NMR) and imaging technique were used to collect the relaxation spectrum and proton density images of prunes. Magnetic resonance imaging (MRI) images showed an uneven distribution of water in fresh prunes fruit, with the epidermal region having a significantly higher water density than the pulp region. As the drying process progressed, the water transfer potential inside the prune changed, and water flowed following the difference in transfer potential to form a new equilibrium. The internal moisture reached the equilibrium point at 60% dry mass. The internal water existed in three water forms: free water, semi-bound water, and combined water, which dominated at different drying points: free water was the highest in fresh prunes, accounting for 93% of the total moisture; semi-bound water was the highest in prunes at the middle stage of the drying process, accounting for 86% of the total moisture; and combined water was the highest in prunes at the end of the drying process, accounting for 93% of the total moisture. During the whole drying process, there was an interconversion relationship between the three forms of water, but the overall trend of conversion was that the water with a weaker binding force was converted to the water with a stronger binding force. There are two main migration directions of water: water diffuses from the skin of the prunes to the outside air by evaporation; and water diffuses to the inside of the prunes following the difference of water gradient. During the drying process, it was also found that the surface of prunes would be crusted, blocking the moisture migration pathway, reducing the drying rate、 and affecting the drying quality of prunes. The step-down temperature drying on the one hand can make the prune skin moisture to maintain a moderate drying rate, to prevent the surface from drying too fast and crust formation, on the other hand can enhance the drying quality of prunes, compared with the constant temperature drying. The results of this study can provide theoretical basis for the optimization of prunes drying process and process design.