Comparing different processing methods in apple slice drying. Part 1. Performance of microwave, hot air and hybrid methods at constant temperatures

The drying curves, energy efficiency and colour variations of sliced apples dried at various temperatures and using different techniques were evaluated, i.e. microwave (35, 55 and 65 °C), hot air (35, 55, 65 and 75 °C) and combination of both (65 °C). The microwave (MW) tests also included comparison between air recirculation and continuous inlet of fresh air. Each drying method was evaluated at a fixed temperature level by means of a control system based on infrared thermography. The time required to complete the drying process at 65 °C varied from about 44 min for MW with fresh air ventilation (double with air recirculation), to 122 min for hybrid heating and 238 min for hot air. Drying kinetics was analysed by introducing a new semi-empirical model, capable of recovering the drying behaviour in terms of both mass loss and drying speed. The results showed that the hybrid drying mode, thanks to the reduced power of the microwaves, led to a lower drying rate than the microwave mode alone. Overall colour variations were minor in samples heated with MW to 65 °C (ΔE=19.8). No significant differences were found between the drying methods with regard to energy consumption. As expected, the fastest drying occurs as the temperature increases, which requires more heat generation within the test samples.