Drying kinetics and quality characteristics of beetroot slices under hot air followed by microwave finish drying

Abstract

The experiments were conducted to evaluate the impact of air temperature and microwave power level on drying behavior and quality of beetroot. The osmotically pretreated samples in sugar-salt mixed solution were further dehydrated using convective tray drier upto a moisture content of 40 to 60% db (the point where drying rate became slow) followed by microwave drying to a moisture content of 6% (bone dry conditions) to overcome the slow removal of moisture content in the end. Three levels of air temperature 55, 65 and 75°C and microwave power 540,810 and 1080 W each were selected and readings on moisture content, rehydration ratio, color, total soluble solid, water activity and hardness were taken. The results showed that osmotic pretreatment prior to hot air microwave finish drying improved the final quality of dehydrated sample. The drying took place in the falling rate drying period regardless of the drying conditions. Drying at high air temperature (75°C) followed by high microwave power (1080 W) witnessed increased drying rates and substantial shortening of the drying time. Two term and logarithmic model successfully described the drying kinetics of convectively dehydrated and microwave finish dehydrated beetroot slices respectively. Rehydration characteristics of beetroot were improved for samples dehydrated at 55°C and 1080 W, which helped in reduction of shrinkage and case hardening. A greater change in color resulted due to decreased L and b values and increased ‘a’ value with increase in both air temperature and microwave power. Sample dehydrated at high air temperature followed by high microwave power exhibited high water activity, high total soluble solids (TSS) and improved hardness as compared to fresh sample. Key words: Beetroot, air drying, microwave finish drying, quality.