Effect of pre-treatments on drying, density and shrinkage of apple slices

The effects of different pre-treatments on convective drying of apple slices were studied and compared to drying without pre-treatments. An impregnation with starch, an HTST process, and a combination of the two were used. When HTST was applied, air drying at mild (conventional) temperatures was used to finish the drying process. The HTST process had high mass transfer rates, however, when it ceased, the drying rate returned to the behavior observed on mild temperature curves, at all air velocities studied. The apparent density was also investigated and showed lower values for several conditions applied. ANOVA indicated which factors are significant to the observed decrease in apparent density. The Duncan test highlights experimental situations where these variables have an influence. Apparent density is almost constant as dimensionless moisture content diminishes, but it decreases when values are below around 0.2. Volume variations showed a linear behavior with the moisture content changes at the studied conditions. (c) 2006 Published by Elsevier Ltd.