Drying kinetics characteristics and model fitting of exocarpium citreic grandis microwave vacuum drying

Exocarpium citreic grandis is the immature outer peel of grapefruit and contains active ingredients such as naringin and rhoifolin. The drying temperature can have a significant effect on the constituents. In order to explore the rules of moisture and quality changes in exocarpium citreic grandis during microwave vacuum drying, this study investigated the effects of different microwave intensities (0.75, 1.25, 1.50, 1.75, 2.00 W/g) on the drying characteristics and main component content of exocarpium citreic grandis. The results showed that, at a microwave intensity of 0.75 W/g, the drying process was controlled by a constant speed stage. At microwave intensities of 1.25 W/g and 1.50 W/g, the drying process was divided into three parts including acceleration stage, constant speed stage, and deceleration stage. When the microwave intensity increased to 1.75 W/g and 2.00 W/g, the drying process was divided into acceleration and deceleration stages with no constant speed drying stage. The resulted data suggested that the thin-layer drying kinetics model of exocarpium citreic grandis was fitted. The selection principle of the model was analyzed from four aspects including modification of the original model, transformation of dependent and independent variables, complex model, and initial and terminal conditions. Lewis, Page, Two-term exponential, and Weibull distribution models were selected for fitting analysis. The optimal thin-layer drying models were the Page model and the Weibull distribution model. The scale parameter α of the Weibull distribution function decreased with the increase in microwave intensity. The shape parameter β underwent slight changes with moisture migration mechanisms being the combined effect of surface moisture evaporation and internal moisture diffusion. The effective moisture diffusion coefficient Deff was increasing with the increase in microwave intensity. The activation energy Ea for microwave vacuum drying of exocarpium citreic grandis was 1.6799 kJ/mol·k. Microwave power had a significant effect on flavonoid compounds. When the microwave power was low, the main component content decreased with the increase in microwave intensity, but when the microwave power increased to 2.00 W/g, the drying temperature would produce small molecular phenolic compounds, leading to an increasing trend in total flavonoids, naringin, and rhoifolin contents. With the increase in microwave intensity, the surface of exocarpium citreic grandis presented more porous honeycomb structures, and the larger the pore size, the more evident the puffing effect. The study confirmed that microwave vacuum drying technology was suitable for the production and processing of exocarpium citreic grandis, which provided technical support for the high efficiency and high quality industrial production. © (2023), (Bio Tech System). All Rights Reserved.