Hot air and microwave-assisted hot air drying of apples (Golden delicious), strawberries (Fragaria vesca cv. Elsanta), tomatoes (Lycopersicon esculentom var. Roma), and mushrooms (A. bisporus) were performed in a commercial microwave oven that had been modified to incorporate a hot air dryer. The same microwave power intensity was used in all experiments. We investigated the effect of the above-stated methods on the structural, optical, mechanical, and rehydration characteristics of dried samples. Scanning electron microscopy (SEM) was used to evaluate the structural damage associated with two drying methods. The drying rate was significantly affected by microwave energy; therefore, drying time was reduced by 80, 70, 74, and 66%, respectively for apples, strawberries, tomatoes, and mushrooms, respectively. Specific volume, particle density, and porosity variation were estimated using models proposed in the literature. Some structural damage occurred, as observed in SEM micrographs. Optical, mechanical, and rehydration parameters were enhanced by microwave treatment for apples, strawberries, and tomatoes. However, negative effects were observed in the case of mushrooms.

The effect of coating, hot-air and microwave drying on the texture, microstructure and rehydration characteristics of Golden Deliciousapple slices (22mm diameter, 4mm thickness) was investigated. Blanching was carried out in hot water (80°C, 1 min) to prevent undesirable enzymatic reactions. To induce a porous structure during microwave treatment, coating with a starch (suspension), pectin and carboxymethylcellulose (CMC) solution (20 g/L) with and without CaCl₂ (10 g/L) was carried out. A puncture test and a scanning electron microscopy (SEM) technique were used to analyse the effects of these processes on texture and microstructure. Rehydration in distilled water was also studied. Coating, air-drying (70°C, 1.5 m/s) and microwave treatment (300W, 10s) resulted in the production of puffed and porous products. Microstructure of samples coated by starch and pectin with CaCl₂ improved. Coating had a beneficial effect on the microstructure of samples after microwave treatment. In addition, microstructural studies showed that coating with pectin had a significant effect on the thickness of the cell wall whereas those treated with starch or CMC did not exhibit such an effect. Adding CaCl₂ to the coating solution improved the microstructural properties and prevented central heating phenomena.

The objective of this work was to study the changes of optical properties of apple (Golden Delicious) slices during drying. The optical parameters compared on the basis of Hunter values (L, a, b) changes as well as total color difference (Δ E) and browning index (BI). The effect of coating materials including carboxymethylcellulose (CMC), starch, and pectin as well as microwave treatment on optical properties and microstructure of dried samples were investigated. To analyze the effects of these processes on microstructure, scanning electron microscopy (SEM) was employed. Results showed that optical properties as well as microstructure of apple were affected by coating material and drying condition. Coated samples by CMC had a lower L and higher BI, but different results in the presence of starch were obtained. Microwave treatment in the presence of appropriate coating materials could improve the optical properties of dried apple slices.