Comparison of different drying techniques for Zanthoxylum bungeanum leaves: Changes in color, microstructure, antioxidant capacities, and volatile components

Abstract

Herein, the effects of different drying methods, namely, indoor natural drying (ND), vacuum freeze drying (FD), vacuum drying (VD), hot air drying (AD), and heat-pump drying (PD) on the qualities of Zanthoxylum bungeanum leaves were investigated. The VD-treated Z. bungeanum leaves retained their original color to a greater extent. The microstructure was significantly affected by the temperature, as evidenced by scanning electron microscopy. In addition, FD, and AD both led to high and relatively similar contents of the various components, including the total phenols, flavonoids, proteins, and sugars. The strongest antioxidant capacities resulted from AD treatment, with total phenolic and flavonoid contents as high as 65.07 mg GAE/g dw and 133.2 mg RE/g dw, respectively. Moreover, 107 volatile organic compounds (VOCs) were identified using headspace solid-phase microextraction–mass spectrometry coupled with gas chromatography–mass spectrometry and classified into eight types according to their structures. The different drying methods were found to produce variations in the number and relative contents of the VOCs, which were mainly olefins and alcohols. A comprehensive evaluation of the economic value and product quality concluded that AD is the most suitable drying method for Z. bungeanum leaves, owing to its high efficiency, low cost, and good product quality.