Comparison of drying characteristics of Thompson seedless grapes using combined microwave oven and hot air drying

Comparison of drying characteristics of Thompson seedless grapes using combined microwave oven and hot air cabinet dryer was investigated. The drying rate curves indicated the absence of a constant-rate drying period in all drying methods. Within a certain microwave power range (75–900 W in the current study), increasing microwave power speeds up the drying process, thus shortening the drying time. No benefits were seen when increasing drying time from 30 to 120 min when grapes drying started in hot air cabinet dryer and finished in microwave oven for 1 min at any power level. The higher value of energy consumption during grapes drying belonged to hot air cabinet dryer alone as drying method with value of 564.5 MJ/kgwater evaporated. The average total soluble solids was 90.4° Brix when drying was achieved by microwave oven followed by hot air cabinet dryer, meanwhile, it was 90.2° Brix when drying was achieved by hot air cabinet dryer followed by microwave oven. The total soluble solid was 92° Brix when drying process started and finished in hot air cabinet dryer alone. The average drying ratio was 4.21 when drying was achieved by microwave oven followed by hot air cabinet dryer; meanwhile, it was 4.19 when drying was achieved by hot air cabinet dryer followed by microwave oven. The hot air cabinet drying method had higher drying constant ‘k’ compared to the other two methods. The microwave oven followed by hot air cabinet dryer as a drying method achieved 78% of the optimum selection percentage. However, the optimum drying method has a selection percentage of 100%.