Determination of optimum conditions for convectional and microwave drying of osmo-dehydrated pear slices

The objective of the present study was to investigate the effect of osmotic dehydration as a pretreatment on physical and chemical quality characteristics of "Deveci" pears dried by convectional and microwave drying methods. The independent variables for osmotic dehydration were solution temperature (40-70⁰C), solution concentration (40-70⁰Bx) and dehydration time (3-9 hours). The analysis results were evaluated to determine the optimum osmotic dehydration conditions by Response Surface Methodology (RSM). Solution temperature of 57⁰C and 70⁰C; solution concentration of 70⁰Bx and dehydration time of 9 and 5.4 hours were found out as optimal dehydration conditions prior to convectional and microwave dryings, respectively. Pear slices dried without applying osmotic dehydration were determined as the control sample. Dry matter, water loss, weight reduction and solid gain rates of pear slices increased depending on dehydration conditions during osmotic dehydration. Moreover, color changes (5.71-25.87) and browning index (0.022-0.058) of osmotically dehydrated samples were found lower than those of control samples (color changes: 15.48-25.21; browning index: 0.058-0.080). Effective diffusion coefficient decreased depending on dehydration conditions during convectional drying of osmo-dehydrated pear slices. Rehydration rates of osmo-dehydrated samples (%232-383) was lower as compared to non-treated pear slices (%374-376). Water activity decreased with the increase of solution temperature, solution concentration and dehydration time of pear slices during osmotic dehydration. In this study; osmotic dehydration as a pretreatment process prior to convectional and microwave drying maintained bioactive substances including total phenolics and antioxidant activity. Otherwise, only osmotic dehydration did not found as a sufficient process to prevent microbial growth. The storage stability of pear slices which were dehydrated under optimum conditions was investigated by storing them at 21ºC for 6 months after drying with convectional and microwave. In conclusion, osmotic dehydration application had positive effects on the quality properties of pear slices for both drying methods. In addition, microwave drying of pear slices under optimum osmotic dehydration conditions is considered more effective and favorable compared to convectional drying.