Measurement of water mobility and distribution in vacuum microwave-dried barley grass using Low-Field-NMR

摘要

Water mobility is an indicator of product quality of dried barley grass. To investigate the water mobility and distribution of dried barley grass, microwave vacuum drying (MVD) was conducted at different powers (100, 200, and 300 W) until water content of dried product reached 3% (w.b). Water profile was measured by Low-field nuclear magnetic resonance (LF-NMR), including free water, surface water (molecular attraction), and immobilized water (attraction of polar clusters). Drying time at 100, 200, and 300 W presented about 90, 70, and 50 min, respectively. Changes of free water and immobilized water and surface water were affected differently by variant microwave power. After drying for 50 min, the mobility of water is reduced greatly and proportion of surface water predominated. Transverse relaxation time for free water and immobilized water decreased significantly (P <= 0.05) from start and then leveled off. Predomination of surface water and immobilized water assures the safety and quality of products of dried barley grass. Low-field nuclear magnetic resonance (LF-NMR) is outstanding to measure the water state of barley grass during MVD.