LF-NMR online detection of water dynamics in apple cubes during microwave vacuum drying

In this study, changes in various states of water in apple cubes were assessed using low-field nuclear magnetic resonance (LF-NMR) during microwave vacuum drying. Apple cubes were dried at different microwave power levels (100, 150, and 200 W). Indicators including moisture content and water activity were measured. The Carr-Purcell-Meiboom-Gill (CPMG) sequence was used to measure transverse relaxation times (T-2). The results showed that three water fractions with different T-2 relaxation times (around 11, 126, and 1335 ms) were detected in fresh apples, which corresponded to different cell compartments. The transverse relaxation time of bulk water (T-23) and the signal per mass of the bulk water (A(23)/g) decreased significantly with increasing drying time at different microwave power levels. The signal per mass of the total water (A(Total)/g) had significant correlation with the total moisture content (R-2 = 0.9919). Furthermore, good correlation (R-2 = 0.9799) between water activity and NMR parameters based on partial least square regression model were observed. This research revealed that LF-NMR spectroscopy, a nondestructive technique, can detect the changes of water in different populations in the matrix