Monitoring of Water Content of Apple Slices Using Low-Field Nuclear Magnetic Resonance During Drying Process

In an aim to examine water content, distribution, and state of apple slices during hot air drying, this study determined the inversion spectra of transverse relaxation time (T-2) of apple slices during the drying process at different temperatures using low-field nuclear magnetic resonance (LF-NMR) and magnetic resonance imaging (MRI). The results showed that drying temperatures was negatively correlated with water content and positively correlated with drying rate. The relaxation peak area of free water decreased, while those of semi-bound and bound water increased and subsequently decreased. The water content of apple slices had quadratic polynomial correlations with signal amplitudes of LF-NMR (peak areas of total, free, and semi-bound), R-2 of these regression fitting equations were basically above 0.8 and 0.9. The models of total and free water had good prediction performance, suggesting that LF-NMR is suitable for rapid non-destructive testing of water in apple slices during hot air drying.