Mass transfer in osmotically dehydrated apple stored at temperatures above zero

Abstract

Material dehydrated in sucrose solution by osmosis for 3 h at 30 °C and for 1 h at 70 °C was stored for 24, 72 and 144 h at 5, 9, 15 and 20 °C. Internal gradients of dry matter and sucrose concentration caused mass transfer in the material. Dry matter content in the surface layers was close to 40%, while sucrose concentration was between 20% and 23%. At the distance of 10 mm from the mass transfer surface dry matter content was still higher than that in raw apple, but sucrose concentration was that of raw apple. Concentration profiles of sucrose changed with time and storage temperature. The higher was the temperature the faster was the diffusion of sucrose. In material osmosed at 30 °C for 3 h and stored for 72 h at 5, 9 and 15 °C sucrose concentration gradients were still present. However, after 144 h at 9 °C the concentration of sucrose became independent on the distance from the mass exchange surface. Diffusion of sucrose in material osmosed at 70 °C for 1 h was much faster than that observed in samples osmosed at lower temperature. The effective diffusion coefficient of sucrose was dependent on temperature and time of storage, and distance from the mass transfer surface as well. Since distance was related to concentration a relationship between sucrose concentration and effective diffusion coefficient was found. The effective diffusion coefficient was of the order of 10 −9–10 −12 m 2/s.