Temperature control for microwave vacuum drying of raspberries

In order to optimize the microwave vacuum drying of raspberries, the temperature variation of different constant power microwave vacuum conditions was investigated. The conventional microwave vacuum drying (MVD) and the three different steady temperature control scheme was selected. A microwave intermittent vacuum drying scheme (MIVD), microwave vacuum drying power curves control scheme (MVD-T), and microwave vacuum drying power curves with intermittent control scheme (MIVD-T) were developed and conducted. As shown, temperatures under control schemes were all kept at more moderate ranges compared to constant power, where MIVD-T showed less fluctuation by adopting power curve changing with time as input variable. It was found that temperature control schemes could improve the appearance, as MVD-T and MIVD-T both reduced carbonization on the exterior and interior of raspberries. For antioxidant properties, there were differences between different schemes, in which MIVD-T could maintain more total monomeric anthocyanin (TMA) compounds and keep other antioxidant compounds similar to other drying methods. Control schemes such as MIVD, MVD-T, and MIVD-T were effective in obtaining more steady temperature materials than conventional microwave vacuum drying and were beneficial for drying quality.