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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. [ABSTRACT FROM AUTHOR]

Four drying methods, hot air drying (HAD), microwave constant temperature drying (MWD), microwave vacuum temperature‐controlled drying (MIVD‐T50), and freeze drying (FD), were investigated for their effects on raspberries in terms of drying time, color, microstructure, rehydration, water activity, phenolic compounds, flavonoids, anthocyanins, and DPPH scavenging ability. It was found that microwaves combined with vacuum took the shortest time. The color of MIVD‐T50 is similar to that of FD and much better than that of HAD and MWD. A smooth surface and fluffy and porous structure inside were observed in the FD and MIVD by scanning electron microscopy (SEM) images. FD has the highest energy consumption, followed by HAD, MIVD‐T50 and MWD. The retention rates of total phenols and flavonoids in HAD, MWD and FD were lower than those in MIVD‐T50, and FD had the advantage of retaining anthocyanin content. In general, MIVD‐T50 has significance for the commercial production of raspberry. Practical applications: The use of natural antioxidants in food processing not only enhances the shelf life of products but also improves their nutritional value. Raspberry is a rich source of antioxidants and is widely used in food processing. However, the drying process is crucial in preserving its antioxidant quality, and different drying methods affect the antioxidant content differently. According to the results of the experiments, the drying method has a considerable impact on the energy usage and quality of dried raspberries. Compared with the HAD and MWD treatments, both FD and MIVD‐T50 showed better nutrient content preservation and antioxidant activity, which may be caused by the lower temperature drying process of FD or the stronger efficiency of MIVD‐T50. However, the drying time of FD is longer, and the energy consumption is higher. Therefore, MIVD‐T50 is a promising raspberry drying technology with low energy consumption, a short drying time, and high quality. It has huge development potential and broad commercial scale production market prospects. [ABSTRACT FROM AUTHOR]