Microwave-Assisted Air-Dehydration of Fruits and Vegetables Processing Conditions and Product Quality
This thesis is focused on aspects of microwave-assisted hot-air drying of fruits and vegetables. The two material coefficients, the relative permittivity (ε) and the loss factor (ε), determine the interaction of microwave energy with foods. These coefficients were measured with a cavity perturbation method, and the results showed a significant peak for ε at 0.8-2 kg water/kg dry matter for five foods (45-70 % on wet basis).
We designed and built a microwave and hot-air oven. Experiments with microwave-assisted air-drying on a pilot scale were carried out for apple, mushroom, thyme, and potato. The experiments showed that the drying time could be reduced by 25-75 % with the aid of microwave energy. Microwave-assisted dehydration with both applicators and a multimode cavity did not improve the physical properties of foods compared to hot air drying. It was necessary to use pre-treatments to be able to modify the physical properties, which were e.g. firmness, shrinkage, and the rehydration capacity of microwave dehydrated products. Heating with high-intensity (20 W/g) microwave energy as a pre-treatment before hot-air dehydration of apple, induced superheating up to 110°C in the apples. This caused punctured cell walls and increased shrinkage, but also increased rehydration capacity compared to hot air dehydrated apples. Pre-treatment of apple and potato by immersion in CaCl2 solution before dehydration in a microwave-assisted air drying process at 50, 60, or 70°C increased the firmness of rehydrated apples, compared to hot air drying. Blanching in a calcium solution at 70°C before dehydration had a similar effect for potato but not for apple. Apples were also pre-treated by immersion in 95 % ethanol and freezing at 18°C, before dehydration at 50, 60, and 70°C. Ethanol was an effective pre-treatment with little shrinkage and high rehydration capacity of the dried apples, compared to other pre-treatments. Freezing before dehydration was very detrimental to the texture of dried and rehydrated apples. Dehydration of thyme with microwave energy and hot air showed that the air temperature was most important to achieve an attractive aroma in sensory analysis. Increased air temperature during dehydration of thyme also influenced positively the intensity of aroma in gas chromatography analysis.