Comparing different processing methods in apple slice drying. Part 2 solid-state Fast Field Cycling 1H-NMR relaxation properties, shrinkage and changes in volatile compounds

The objective of this paper was to study the effects of different drying methods that are: microwave (MW), hot air (HA) and a combination of both (HY), on the 1H-NMR relaxation properties, shrinkage and volatile compounds of Golden Delicious apple slices. Fast field cycling NMR relaxometry reveals that the HA samples dried at different temperatures (65 and 75 °C) appear to contain less restrained water as compared to the sample obtained by MW heating at the same temperatures. In fact, the longitudinal relaxation rates (T1) of the water molecules in the HA dried slices resulted shorter than those measured for the MW dried samples, thereby revealing that in the MW slices, water molecules behave more like a pseudo-ice layer. For this reason, shrinkage effect was much more accentuated in apples dried by the HA and combined methods, compared to the MW thermal procedure. Likewise, the effect of HA drying was larger than MW and HY in changing the aromatic profile of the apples slices, thus causing a significant decrease in esters while ketones increased.