The effect of cold plasma pretreatment on drying efficiency of beetroot by intermittent microwave-hot air (IMHA) hybrid dryer method: Assessing drying kinetic, physical properties, and microstructure of the product

This study examined the effects of cold plasma (CP) pretreatment with durations of 1, 3, and 5 min on drying kinetics, microstructure, and physical properties, including shrinkage, rehydration ratio (RR), and bulk density of beetroot dried samples using intermittent microwave-hot air (IMHA) drying at 40 °C and pulse ratios (PR) of 1 and 2. While increasing PR decreased drying rate (DR) and effective moisture diffusivity (Deff), increasing microwave power from 180 to 600 W increased both, reducing drying time and specific energy consumption (SEC). Extended CP-pretreatment doubled DR, 66% increased Deff, and 24% reduced SEC. Both increasing CP-pretreatment time and microwave power had similar effects on shrinkage, RR, and bulk density, but CP-pretreatment was more prominent; reducing shrinkage by 18%, bulk density by 23%, and increasing RR by approximately 36% in 5-min. X-ray imaging showed that 600 W of microwave power increased porosity by 87%, while 5-min CP-pretreatment increased it by 27%. As the optimum condition, CP-pretreatment for 5 min, continuous microwave application, and 600 W of MW power produced the best kinetics and physical properties. Overall, the results demonstrated that the appropriate CP-pretreatment duration could produce high-quality dried samples.