The effect of microwave vacuum drying process on citrus: drying kinetics, physicochemical composition and antioxidant activity of dried citrus (Citrus reticulata Blanco) peel

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Hot air drying (HAD) is the most commonly used method for citrus peel drying. However, the long time and low efficiency of this process leads to the degradation of product quality and a reduction in antioxidant activity. Therefore, in this study, to improve the drying efficiency and increase the product quality of dried citrus peel, the effects of microwave vacuum drying (MVD) were compared with HAD and vacuum freeze drying (VFD) with respect to their drying kinetics, bioactive compound profile and antioxidant capacity. The results showed that the drying rate decreased with increasing sample load but increased with increasing microwave power. The Page model was the best model for fitting the drying curve of citrus peel during MVD, and the highest rehydration rate (84.5%) was obtained at a microwave vacuum power of 600 W. The microstructure of citrus peel dried by MVD was assessed by scanning electron microscopy, and a loose structure with highly porous material was observed, whereas citrus peel dried by HAD was compact. The total phenolic content (10.48 ± 0.21 mg GAE/g DW), total flavonoid content (6.54 ± 0.41 mg RE/g DW) and oxygen radical absorbance capacity (279.29 ± 11.37 µmol TE/g DW) values obtained for citrus peel dried via MVD were significantly higher than those obtained using HAD (p < 0.05). Several phenolic acid substances and flavonoid profiles were identified and quantified by HPLC-DAD, including coumaric acid, nobiletin, and tangeretin. The nobiletin content observed after MVD was significantly higher than that measured after HAD and VFD. The overall results of this study revealed that MVD at lower microwave powers appears to be a good alternative to the conventional HAD of citrus peel.