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

Prunes is one of the favorite fruits in the genus Prunus of the family Rosaceae, mainly planted in the Xinjiang Province of China. Fresh prunes are prone to water loss, softening, rotting, mold, and a series of quality degradations, due to the high moisture content. Drying prunes can extend the storage period for a long shelf life, particularly for the added value of the product. In this study, a series of experiments were carried out to investigate the moisture migration during drying. The drying characteristics of prunes were obtained at different temperatures (50, 65, and 80 ℃) and wind speeds (1, 2, and 3 m/s). Five kinetic models were then selected to fit the drying curve using the Biot number. Among them, the Bi-G model accurately represented the drying, according to the processing parameters and the fitted curves. Overall, the drying rate decreased gradually, while the moisture diffusion was first controlled externally, and then controlled by internal diffusion; The moisture diffusion efficiency increased with the increase of drying temperature and wind speed. The more significant effect of drying temperature was observed at the same time. The low-field nuclear magnetic resonance (LF-NMR) imaging technique was used to collect the relaxation spectrum and proton density images of prunes. Magnetic resonance imaging (MRI) images showed an uneven distribution of water in the fresh prune fruits. Particularly, the epidermal region shared a significantly higher water density than the pulp one. There was a variation in the water transfer potential inside the prune, as the drying process progressed. Water flowed to form a new equilibrium following the difference in transfer potential. The internal moisture reached the equilibrium point at 60% dry mass. The internal water existed in three forms: free, semi-bound, and combined water, which were 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, accounting for 86% of the total; and combined water was the highest in prunes at the end of the drying, accounting for 93% of the total. There was an interconversion relationship among the three forms of water during the whole drying. However, the overall trend of conversion was attributed that the water with a weaker binding force was converted to the water with a stronger binding force. There were two main directions of water migration: water diffused from the skin of the prunes to the outside air by evaporation; and water diffused to the inside of the prunes following the difference of water gradient. The surface of the prunes was crusted to block the moisture migration pathway during drying. As such, the drying rate was reduced to obtain the high drying quality of prune. The step-down temperature drying was allowed for the prune skin moisture to maintain a moderate drying rate, in order to prevent the surface from drying too fast and crust formation. The drying quality of prunes was enhanced significantly, compared with the constant temperature drying. The finding can provide the theoretical basis to optimize the prunes drying.