Kinetics of nutrient change and color retention during low‐temperature microwave‐assisted drying of bitter melon ( Momordica charantia L.)

The changes in nutrient (phenolics and vitamin C) and color of bitter melon slices, during microwave‐assisted drying (MWAD) at different microwave power values (1.5, 3.0, and 4.5 W/g) and air temperatures (20, 25, and 30°C) are reported. Fits of the experimental data to a semi‐empirical mathematical model were used to determine the half‐lives of the nutrients. Color change during drying was similarly characterized using CIELAB color space parameters. The rates of nutrient degradation and color change increased with an increase in microwave power, and drying temperature, but the drying times decreased significantly. The total electrical energy consumed under various drying conditions was measured. Although a microwave power of 1.5 W/g and an air temperature of 25°C resulted in the highest levels of nutrient retention, the drying time and process energy consumption were relatively high. In comparison, the drying conditions of 4.5 W/g and 30°C provided an acceptable compromise. Practical applications Low‐temperature MWAD is a promising alternative to conventional drying for food that contains temperature‐sensitive nutrients. This article provides a methodology for analyzing the kinetics of nutrient and color change during a MWAD process. A 3 2 factorial design was used to derive predictive correlations for drying time, nutrient half‐life, time constants characterizing color change, and electrical energy consumed during the drying process. The relative influences of microwave power and drying temperature could be readily discerned using these correlations. The reported study will help in developing and optimizing microwave drying processes for a variety of food products.