The microwave-assisted convective drying of kale (Brassica oleracea L. var. sabellica L.) using continuous and changeable power radiation

Convective drying is one of the most commonly used methods of food dehydration. This technique allows highly perishable products to be preserved; however, it is simultaneously time-consuming, energy-consuming and has a negative impact on the products' quality. In order to eliminate these drawbacks alternative drying methods have been sought. This article concerns the microwave-assisted convective drying of kale. Eight different drying processes were tested. The influence of both continuous and changeable application of microwaves on process kinetics (drying rate), energy consumption, and quality of product (color and ascorbic acid retention) was analyzed. Additionally, four thin-layer drying models were used to approximate the experimental data and an effective diffusion coefficient was calculated. It was found that the continuous application of microwaves positively influenced kinetics and energy consumption but may have an adverse effect on the products' quality. In contrast, intermittent-hybrid schedules were characterized by smaller kinetics advantages but a visibly better quality of product. Practical applicationsIn the face of current environmental problems (reduction of CO2 emissions) and increasing prices for electricity, the high energy consumption in convective processes is encouraging the search for alternative methods of drying food products that are prone to spoilage. Additionally, consumer awareness is growing, which is increasing the demand for high-quality, safe, food products. This work concerns the optimization of existing technology and the development of new technologies for food drying using microwaves. Microwave radiation is an effective source of energy that allows a significant reduction in drying time and energy consumption; but it can have a negative impact on product quality. The proposed solution is the intermittent use of microwaves; but the matter is not simple because the dielectric properties of food depend on many factors (moisture content, composition, temperature, etc.), which makes the effects of microwave drying difficult to predict. Thus, all research on this topic is valuable.