Effects of Different Drying Methods on the Quality Characteristics of Dried Zanthoxylum armatum Leaves

In order to ensure improved drying quality of Zanthoxylum armatum (Z. armatum) leaves with low processing cost and high nutritional value, the effects of different drying methods on the quality of Z. armatum leaves were studied, so as to provide a reference basis for its development and utilization.

With Z. armatum leaves as test materials, 5 different drying methods were set, including natural drying (ND), hot air drying (AD), heat pump drying (PD), vacuum drying (VD), and vacuum freezing drying (FD). The effects of 5 different drying methods on their drying characteristics, functional characteristics, microstructure, content of polyphenol, flavonoid, protein, fat, and total sugar, as well as antioxidant capacities and volatile compounds, were investigated.

The moisture ratios of Z. armatum leaves under 5 different drying methods all exhibited an exponential downward trend, and the time required to reach the end of drying was 32, 7, 6, 16 and 28 h for ND, AD, PD, VD and FD, respectively. During the drying period, the faster the water diffusion, the greater the drying rate and the shorter the elapsed time, and the drying rate was PD>AD>VD>FD>ND. The effects of different drying methods on the color of Z. armatum leaves was VD>ND>PD>AD>FD, where AD and FD could better retain their chlorophyll content at 12.74 and 12.85 mg∙g^-1, respectively. The microstructure showed that temperature had a significant effect on the internal structure of Z. armatum leaves, which was more seriously damaged by high temperature, and FD exhibited a porous structure with better water-holding capacity of 5.85 g∙g^-1. ND had the highest retention of sugars at 6.53 g/100 g, resulting in a higher water-solubility of 35.93%. PD retained its protein and fat better, with content of 2.43 and 4.86 g/100 g, respectively. The retention of polyphenols and flavonoids in dried Z. armatum leaves was high, with polyphenol content ranging from 72.16 to 109.50 mg∙g^-1 and flavonoid content ranging from 45.60 to 82.23 mg∙g^-1, where AD and FD were high and relatively close in polyphenol and flavonoid content. The scavenging capacities of DPPH∙, ABTS⁺∙ and the power of FRAP reduction were FD>AD>PD>ND>VD, with positive correlation between the content of polyphenols, flavonoids and antioxidant capacities. Simultaneously, the volatile compounds were determined by headspace solid-phase microextraction-mass spectrometry coupled with gas chromatography-mass spectrometry, and a total of 49 species were detected, which could be classified into seven categories according to structures, including olefins, ketones, aldehydes, esters, alcohols, benzenes, and phenols. It was found that the types, numbers, and relative content of volatile compounds in Z. armatum leaves varied under 5 different drying methods, among which olefins were dominant, with a total of 26 species and relative content ranging from 58.02% to 75.18%.

Based on the comprehensive quality indexes of materials and actual operation cost, it was found that AD was more suitable for drying Z. armatum leaves, with higher efficiency, lower cost, as well as relatively better quality of color and antioxidant capacities.