Study of the Drying Kinetics of “Granny Smith” Apple in Tray Drier

Th e drying characteristics of “Granny Smith” apples were analysed using a tunnel drier at diff erent temperatures and diff erent pre-treatments. „Granny Smith“ apples were cut into rectangle-shaped samples 20x20x5 mm in size. Th e drying temperatures for non-treated samples varied from 50oC to 80oC at airfl ow 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). Th e aim of the study was to get dried apples with approximately 10% water content, with good texture, rehydration capability and colour quality. Th e eff ect 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. Th e kinetic equations were estimated using exponential mathematical model. Th e results of the estimation exhibited correspondence to the experimental results. Th e 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 airfl ow 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. Th e given methods signifi cantly improved rehydration properties and accelerated the drying kinetics in comparison to drying of non-treated apple samples. Th e values of rehydration ratio decreased with the increase of the drying temperature. Th e values of eff ective diff usion coeffi cient 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.