Drying rate control in microwave assisted processing of sliced apples

The most enhanced microwave systems for the preparation of quality-dehydrated fruits continuously adjust the power level in order to maintain the product temperature above a target value. As a result, typical drying curves that exhibit high drying rates in the middle stage are obtained. This can often lead to quality damage or undesirable changes on food colour and texture. In response to these issues, a microwave system is proposed that can realise drying processes keeping drying rates constant. This approach required a continuous temperature adjustment of the apple slices under test, whose temperature was detected by a computer-aided infrared thermography system. Since temperature corrections were required only during the middle stage of the process, the overall drying time was only slightly affected by the proposed control strategy. Nevertheless, compared to microwave drying with different constant temperatures (60, 70 and 80 °C), the resultant benefits of operating at constant drying rates included an improvement of texture and rehydration properties. No differences in colour of sliced apples were observed.