Structural changes in apple rings during convection air-drying with controlled temperature and humidity.

The structure of heat pump dried apple slices, developed as a function of air temperature and constant humidity, was studied by measuring porosity and using electron microscopy. The porosity of the apple rings increased linearly when the moisture content decreased during drying and then reached a constant value. In all dried apple slices, a degree of cellular collapse occurred. Case hardening occurred in the surface of the dried tissue when the apple slices were dried at 40-45 and 60-65 degrees C, and in the extreme case (at 60-65 degrees C) cracks were formed on the surface.