The use of hot air, microwave or microwave+infrared drying combination treatments in drying of black grapes pre-dehydrated by using different osmotic solutions

The osmotic dehydration (OD) is a process that partially removes water from plant tissues such as fruit and vegetables. It is commonly used as a pre-treatment before air-drying. In osmotic dehydration, foods are placed in osmotic solution. Three different mass transfer mechanisms occur in the osmotic dehydration; (i) water migration from food to the solution, (ii) solute migration from solution to food, and (iii) solute concerning product extracted to the solution.In this study, seedless black grapes (Vitis labrusca, Black kismis) were osmotically dehydrated to 30% water soluble solids using sucrose, fructose or maltodextrin solutions at 40, 50, 60% concentrations. Experiments were conducted by a shaking incubator at 35?C, with constant agitation speed of 300 rpm and constant fruit:solution ratio of 1:5 (w/w). Total and water soluble solids, color, total anthocyanin content, water activity, solid gain and water loss analyses at acquired pre-dehydrated grapes were determined.Pre-dehydrated grape samples; which were treated in fructose solution having 60% concentration with the shortest arrival time (150 min.) to 30% water-soluble solids, the highest total anthocyanin amount of 305.8 mg/kg (wet basis), the minimum ?E value (3.89); were selected as the best sample.Pre-dehydrated grapes in fructose solution with 60% concentration were finally dried to moisture content of approximately 20%; i) by using hot air in a tray dryer (70?C, 1.3 m/s air speed), ii) microwave (540 W), iii) microwave+infrared drying combination (180 W microwave+1250 W infrared waves).After final drying, total soluble solids, color, total anthocyanin content, rehydration ratio, loose bulk density, particle density, water activity and shrinkage analyses were carried out. The method using 540 W microwaves, was selected as the optimum final dehydration method.