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 (T2). The results showed that three water fractions with different T2 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 (T23) and the signal per mass of the bulk water (A23/g) decreased significantly with increasing drying time at different microwave power levels. The signal per mass of the total water (ATotal/g) had significant correlation with the total moisture content (R2 = 0.9919). Furthermore, good correlation (R2 = 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.