Drying Kinetics and Bioactivity of Beetroot Slices Pretreated in Concentrated Chokeberry Juice and Dried with Vacuum Microwaves

Beetroot slices 18mm in diameter and with a thickness of 9.6, 6.3, 3.35, or 2.6mm were pretreated in 40 degrees Bx chokeberry juice at a temperature of 50 degrees C for 2 h and then dried by a vacuum-microwave (VM) method at different microwave power levels, such as 120, 240, 360, 480, and 480/120 W. The drying kinetics were described by a fitted model that incorporated contributions from the surface area of the samples, microwave power, and VM processing time. As the microwave power during VM drying increased, the bioactive potential decreased for untreated samples and increased for pretreated samples. Increasing the samples' specific surface area resulted in shorter drying time, lower temperature of the material during drying, and enhanced quality of the dried product. For osmotically pretreated beetroot slices with a specific surface area of 827 +/- 18 m2 m(-3), a final VM drying step at 240W is recommended to produce high quality vegetable snacks.