Effects of combined coating and microwave assisted hot-air drying on the texture, microstructure and rehydration characteristics of apple slices

The effect of coating, hot-air and microwave drying on the texture, microstructure and rehydration characteristics of Golden Delicious apple slices (22mm diameter, 4mm thickness) was investigated. Blanching was carried out in hot water (80 degrees C, 1min) to prevent undesirable enzymatic reactions. To induce a porous structure during microwave treatment, coating with a starch (suspension), pectin and carboxymethylcellulose (CMC) solution (20g/L) with and without CaCl2 (10g/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 degrees C, 1.5m/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.