Effect of microwave-assisted vacuum drying on the drying kinetics and quality parameters of Moringa oleifera leaves

Microwave-assisted vacuum drying (MAVD) has been studied to improve drying kinetics and physical and antioxidant properties of dried moringa leaves. Microwave power and vacuum level produced a significant reduction (p < .05) in drying time and improvement of drying rate. It was further confirmed by an increase in effective diffusivity value (D-eff) by an increase in power and vacuum levels. The reliance of D-eff on microwave power was further confirmed by an increase in activation energy from 13.564 to 17.31 W/g with an increased power level. Fitting experimental moisture ratio to the thin layer drying models revealed that the Midilli et al. model could better describe the MAVD of moringa leaves with the highest R-2 values. Physical characteristics such as color and rehydration ratio were better for MAVD samples than hot air-dried samples. Increased drying rate and rehydration ratio were due to the open porous structure of the moringa leaves dried by MAVD, as revealed from the SEM imaging. The vacuum level and power level were significant (p < .05) to the total color difference but the change in color at power levels of 270 and 450 W were insignificant (p > .05). Also, the vacuum level was significant (p < .05), and the microwave power level was insignificant (p > .05) to the rehydration ratio of the MAVD moringa leaves. Phenolic composition and antioxidant properties of MAVD moringa leaves showed increased phenolic content and improved antioxidant activity. Microwave power showed a significant effect (p < .05) on antioxidative properties, whereas vacuum level (p > .5) showed an insignificant effect. The highest power level of 450 W and vacuum level of 500 mmHg produced better quality moringa leaves with good physical and antioxidative properties. Practical Applications Drying is the most common and oldest preservation method for storing food for a longer time. It is very much important to dry moringa leaves that have high moisture content and very less shelf-life. But conventional drying methods cause great nutritional and textural changes. Microwave-assisted vacuum dryer combines the advantage of quick and shorter drying of microwave system and lower temperature process as in vacuum system. This eventually reduces the drying time and temperature stress on the food product. This study aids in scale-up and design a MAVD system for moringa leaves based on its drying kinetics. MAVD of moringa leaves was best modeled by Midilli et al. model. Evaluation of physical characteristics of leaves after drying and its comparison with hot air-dried one revealed better color and rehydration property that is considered as the key character of dried samples. MAVD also produced increased antioxidant properties with good retention of phenolic components. So, the method of Microwave Assisted Vacuum Drying (MAVD) has got tremendous importance that could be industrially exploited in Moringa leaf processing plants.

Automated summary

Microwave-assisted vacuum drying (MAVD) can improve drying kinetics and physical and antioxidant properties of dried moringa leaves. MAVD reduces drying time and improves drying rate by increasing microwave power and vacuum level. It also enhances physical characteristics and antioxidant properties of moringa leaves.