Effect of hot-air oven dehydration process on water dynamics and microstructure of apple (Fuji) cultivar slices assessed by LF-NMR and MRI

Non-destructive analysis of water dynamics during drying is of importance for quality control of food products. In this study, different water dynamics and migration in Fuji apple slices dried at various hot-air oven temperatures, i.e. 50, 60, 70, and 80 °C and air velocity at 0.2 m/s were monitored using low-field nuclear magnetic resonance (LF-NMR) and magnetic resonance imaging (MRI). Multi-exponential fitting of the transverse relaxation T2 parameter demonstrated four distinct water peaks in all samples corresponding to strongly bound, lightly bound, entrapped/immobilized, and free water as follows: T21 in the range of 0.01–1 ms, T22 in the range of 1–10 ms, T23 in the range of 10–100 ms, and T24 in the range of 100–1000 ms, respectively. The water content was measured and analyzed by the traditional technique using the oven drying method. The overall results were highly significant, depicting that the transverse relaxation times T24, signal per mass of the free water A24, and water content significantly decreased (p< 0.05), while the color (L*, a*, b*) and shear force (SF) curves increased with extended drying. Furthermore, good correlations were observed between the LF-NMR parameters and color, water content, and SF in differently processed samples during the dehydration process. Scanning electron microscopy (SEM) and MRI provided the structural changes and spatial water distributions during the drying process. LF-NMR exhibited great potential in evaluating the various water dynamics and quality of Fuji cultivar apples during the drying process.Abbreviations: AA: ascorbic acid; ANOVA: analysis of variance; CPMG: Carr-Purcell-Meiboom-Gill; DW: distilled water; F: Fuji; LF-NMR: low-field nuclear magnetic resonance; MRI: magnetic resonance imaging; SE: spin-echo; SEM: scanning electron microscopy; SF: shear force; SIRT: simultaneous iterative reconstruction technique; TE: echo time; TR: repetition time.