Kinetics of the Dehydration of Cucumber Slices with a Refractance Window™ Dryer
Cucumber (Cucumis sativus) slices, 3 mm thick, were dried in a laboratory scale batch Refractance WindowTM dryer. To construct the dryer, a laboratory water bath was modified by covering the bath with a transparent polyethylene terephthalate (PET) plastic film. A water temperature of 60oC was maintained beneath the plastic film. The variation of the moisture content of the cucumber slices was measured during the drying process. Using the experimental data, the drying curve, the drying rate curve, and the Krischer curve, were plotted. The thin-layer mathematical drying model that best describes the drying kinetics of the drying data was determined. The variation of the rehydration ratio of the cucumber slices with rehydration time was determined. The effective moisture diffusivity of the 3 mm thick cucumber slices was also determined. Observations indicate that the cucumber slices dried to a moisture content of less than 10% (dry basis) in about 120 minutes. The regression analysis results showed that from the 17 models studied, the Haghi and Ghanadzadeh thin-layer drying model best describes the drying behaviour of the 3 mm thick slices with a coefficient of determination (R2) value of 0.9994. The rehydration ratio of the cucumber samples varied from 3.47 to 5.25 with rehydration times of 10 to 300 minutes respectively. The moisture content after 300 minutes of rehydration was about 1150% dry basis (92% wet basis). The effective moisture diffusivity of the 3 mm thick cucumber slices was determined to be 7.30×10-10 m2/s. The implications of the work are that the Refractance WindowTM (RW) drying technique will dry cucumber slices at a faster rate than the traditional methods and the drying data obtained can be used in the design of an industrial scale RW dryer.