## 3 Results and Discussion
### The Volatile Fingerprints of Shitiake Mushrooms under Different HAD Processing by HS-GC-IMS  
The GC-IMS technology was used to develop the fingerprints (Figure 2) and visually analyze the differences in the volatile components of shitiake mushrooms processed at different drying rates and different drying degree. The three-dimensional (3D) topographic map of the volatile components was presented in Figure 1(a), with the drift time as the X-axis, the retention time as the Y-axis, and the peak intensity as the Z-axis. It could be seen that the varieties of volatile compounds in the three samples were similar, but the signal intensities were different. The content of volatile substances in sample B (shown in the black rectangle) was significantly higher than those in sample A and C.  
Figure 1: The moisture content (**a**) and drying rate curves (**b**) of shitiake mushrooms during hot air drying. Then to clearly show the different signal intensity, the GC-IMS topographic map was obtained (Figure 2b) through the top view of 3D topographic plot and by normalizing the ion migration time and the position of the reactive ion peak (RIP), using the relative drift time as the abscissa and the volatile matter retention time as the ordinate. It presented a global volatile fingerprint with 2D plots. Each point on the right of RIP represented a volatile organic compound. It could be seen that the retention time of most volatile compounds in all samples was 100-600 s, and the relative drift time was 1.0-1.5. Varieties of the volatile compounds in the three samples were also similar, as shown in 3D topographic map. The same substance might produce multiple signal points, including dimers  
Figure 2: The fingerprints of volatile flavor compounds of shitake mushrooms under different hot air drying conditions determined by GC-IMS: (**a**) 3D topographic plot, (**b**) topographic plot, and (**c**) gallery plot (The number in the Figure refers to the number in Table 1). and trimmers. This also was observed in the GC-IMS topographic maps of Tricholoma Matsutake Singer [25; 27]. The color of the signal point represented the concentration of the substance, white indicated lower density, red indicated higher density, and darker colors indicated greater density [21]. It was showed that signal intensities of sample B were higher than those in sample A and C, especially those in the black rectangle. This indicated that the contents of volatile compounds in sample B were distinct from sample A and C, although they were similar with respect to the varieties of volatile compounds. In order to effectively recognize discrepancy information between the three samples, all signals in topographic map were identified. A total of 55 volatile components were identified in the three samples, including 32 known components (Table 1) and 23 unknown components based on the existing database. Among the 32 known components, aldehydes, esters, alcohols, ketones, acids, and sulfur compounds were the main volatile flavor components. All of the 55 signals were used to develop a gallery plot to compare the differential volatile compound information of the three samples (Figure 2c). Each row in the gallery plot was represented as all the signal peaks selected in a sample, and each column was represented as the signal peaks of the same volatile substance in different samples. From the Figure 2c, it could be seen that few compounds show similarity among the three samples. Among the 55 components, 29 components (in the red rectangle), including acetoin, hexanal, (E)-2-hexenal, 1-octen-3-ol, 2-methylpropanal, 2-methyl-1-propanol, heptanal, ben