The drying characteristics of “Granny Smith” apples were analysed using a tunnel drier at different temperatures and different pre-treatments. „Granny Smith“ apples were cut into rectangle-shaped samples 20x20x5 mm in size. The drying temperatures for non-treated samples varied from 50oC to 80oC at airflow velocity of 2.8 m s-1. Prior to drying at 60oC, the samples were pretreated either thermally or chemically, or both (blanching in hot water, steam blanching, blanching in hot 0.6% CaCl2 solution, freezing to temperature of –18oC and dipping in 1% ascorbic acid solution). The aim of the study was to get dried apples with approximately 10% water content, with good texture, rehydration capability and colour quality. The effect of the temperatures and pre-treatments on the quality of dried apple samples was determined on the basis of colour and volume changes and reconstitution characteristics. The kinetic equations were estimated using exponential mathematical model. The results of the estimation exhibited correspondence to the experimental results. The best results for non-treated apple samples: shorter drying time, better rehydration properties and small colour changes were achieved at the drying temperature of 60°C and airflow velocity of 2.8 m s-1. Blanching in hot water, steam blanching and blanching in hot 0.6% CaCl2 solution resulted in higher values of rehydration ratio in comparison to other tested pre-drying methods. The given methods significantly improved rehydration properties and accelerated the drying kinetics in comparison to drying of non-treated apple samples. The values of rehydration ratio decreased with the increase of the drying temperature. The values of effective diffusion coefficient were higher in the case of drying of blanched samples in hot water at temperature of 85°C for three minutes in comparison to drying of non-treated apple samples.

tunnel dryer; apples; Granny Smith apple; pre-treatment; blanching; drying; rehydration; effective diffusion coefficient

Published by University of Zagreb, Faculty of Agriculture, Zagreb, CROATIA
Sponsored by Ministry Science and Education of the Republic of Croatia