بررسي تغييرات برخي ويژگي هاي فيزيكي برگه هاي سيب خشك شده با روش تركيبي خشك كن هواي گرم و مايكروويو

nvestigation on Some Physical Properties of Dried Apple Slices Using a Combination of Hot Air and Microwave Driers

In this study structural and color changes of thin slice (22 mm diameter and 4 mm thickness) of Golden Delicious dried apple were investigated. Drying carried out using combined method including: coating (1 min in room temperature), hot air drying (70 °C and air velocity 1 m/s) and microwave treatment (300 W, 10 s). To prevent undesirable enzymatic reactions and to improve porous structure, blanching pre-treatment was carried out by hot water (80°C, 1 min). To induce a porous structure during microwave treatment, coating method with starch, pectin and carboxy methyl cellulose solutions (2% w/w) was used. To promote loss factor of samples and improve structural strength CaCl2 solution (1% w/w) during coating treatment was used. Ultimately produced samples by this method were compared with other products from air and freeze-dried ones. To study these methods and pretreatments effects on structural properties, density and porosity of dried products were investigated. Color changes were also studied using a Hunter Lab colorimeter. Total porosity was 53.26 and 90% for hot air and freeze dried products respectively, however it was about 63.59% for coated samples. Results showed that using CaC12 in coating solution could also increased porosity of samples up to 75%. Studying the color change showed that in all cases using CaCl2 could increase L* values. Applying microwave treatment in all cases had the same effect. However after microwave treatment, in samples coated with starch and CMC, browning index decreased.