Optimization of temperature and power density in microwave-assisted hot air oven drying and storage stability of dried apple sticks

Abstract

Apple (Malus Demestica cv. Cortland) is a perishable fruit due to its high moisture content. In this study, apple sticks were dried to increase their shelf life. Osmotic dehydration was used as pretreatment followed by microwave-assisted hot air drying (MAHD) at different temperatures (45 ºC, 55 ºC, 65 ºC) and microwave power densities (0, 0.5, 1 W/g). Optimum process parameters were estimated by the central composite design of response surface methodology (RSM). The results showed that the temperature and power density had a significant effect on response variables. Optimum conditions of moisture content (9.54%), time (100 minutes) and change in color (7.54) were found at 65 ºC and 1 W/g microwave power density (MWPD). Then the samples dried at optimum conditions were stored at ambient temperature and analyzed for color, moisture content, texture and total fungal count at regular intervals for 75 days. The results showed a non-significant change in color and fungal counts from 10.45 to 10.73 and from 0.45 to 0.69 log cfu/g respectively, and a significant decrease in moisture content from 10.11% to 5.69% was observed from 0 to 75 days. While increasing firmness resulted in increased force and energy to the breaking point and micrographs of electron microscope showed shrinkage in cell structure along with storage. The brittleness of apple sticks increased significantly during storage but remained acceptable till the end of 75 storage days.