Influence of mode stirrer and air renewal on controlled microwave drying of sliced zucchini

An on-line temperature-controlled microwave (MW) system, based on infrared thermography, was developed for drying zucchini slices. Moreover, the effects of different configurations, including turntable rotation, mode stirrer and air renewal, alone or in combination, on drying kinetics and quality of dried product were evaluated. The combination of stirrer and air renewal reduced the drying time of zucchini slices by about 48% with respect to the basic configuration and by 22% with respect to air renewal only. The lowest oscillation of temperatures about the fixed level (65 °C) was found when operating with the stirrer mode, because of the improved uniformity of the MW distribution. The samples treated by stirrer and air renewal configurations showed less variation in colour: the decrease of brightness was about 19% with respect to fresh samples and the value of global variation of colour, ΔE, was 18. It is important to note that air renewal in MW processing is an almost zero energy-cost option.