Changes of rheological properties of apple tissue undergoing convective drying

The aim of this work was to investigate rheological properties of apple cub es undergoing convective drying. Compression-relaxation test was used to fo llow changes of properties under investigation. Raw apple appeared to be very heterogeneous material from the rheological p oint of view. The relationship between stress and strain was concave downwa rds and the concavity increased until water content of 2.5 g/g d.m. was rea ched. Then the relationship begun to straighten and for dry material develo ped stress was linearly dependent on strain. The resistance of deformation decreased with decreasing water content. Analysis of relaxation showed that the rate of relaxation increased with decreasing water content and the unr elaxed stress was smaller the lower was the water content. Apple cubes undergoing drying are pictured as material composed of three co mpartments with different rheological properties. The outer layer is dry, i nelastic and difficult to deform. Underneath is moist material with no turg or and relatively easy to compress, and the core with properties of raw app les. During drying the second layer grows in expense of the core and porous structure with many voids filled with air is formed. Share of each compart ment in the volume of apple cube undergoing drying affects average rheologi cal properties of the material.