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

Abstract

The objective of this paper was to study the effects of three different drying methods: 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 revealed that the HA samples dried at different temperatures (65 and 75 °C) appeared to contain less restrained water as compared to the sample obtained by MW heating at the same temperatures. In fact, the longitudinal relaxation times (T1) of the water molecules in the HA dried slices were shorter than those measured for the MW dried samples, thereby revealing that in the MW slices, water molecules behave more like as a pseudo-ice layer. For this reason, shrinkage was much more accentuated in apples dried by the HA and hybrid methods, compared to the MW procedure. Likewise, HA drying had a bigger effect than MW and HY on the aromatic profile of the apple slices, causing a significant decrease in esters while ketones increased.