Characterization of moisture transfer during intermittent drying process for broccoli from LF-NMR experiments

Abstract

Intermittent drying is considered as one of the alternative drying methods to save energy and improve product quality. In this study, low-field nuclear magnetic resonance (LF-NMR) has been used to evaluate the dynamics of the water mobility and distribution during the intermittent drying process. The results have shown that the first drying period of 1 hour is not effective to induce the moisture self-diffusion in the tempering period. After drying of 2 hours, the moisture transport in the tempering period became rapid and the changes reflected on T2 curve became visible clearly. The T2 values decreased gradually during drying, which indicated the reduced water mobility. As the tempering period progresses, the T2 continuously reduced and varied, which indicated the redistribution of water molecules during the tempering period. With the prolonged tempering time, the changes in T2 tended to be more stable, but it cannot reach equilibrium when there was concentration gradient. It was worth noting that A21 and A22 had an opposite trend during the intermittent drying process, indicating that the mobility of the bound water was increased during the intermittent drying. Effectively also when the samples were dried with the intermittent drying strategies, the vitamin C retention has increased by almost 50% relative to that of the continuous mode.