## 3 Results and Discussion
### Highly Volatile Compounds Profiles of Onions under Different Fresh-Cut Styles by GC-IMS  
The difference comparison model was applied to compare the aroma variety of untreated onion (control group) and onions with different cutting styles (bulb, ring, and square) stored at 4 \({}^{\circ}\)C, 20 \({}^{\circ}\)C, and 25 \({}^{\circ}\)C (Figure 2). A topographic plot of fresh onions without any treatment was selected as a reference, and a topographic plot of other samples was deducted from the reference. If the volatile compounds were consistent, the background after the deduction was white, while red indicated that the concentration of the substance was higher than that in the reference, and blue indicated that the concentration of the substance was lower than that in the reference. As shown in Figure 2a, most of the signals in the topographic plot of onions appeared within a range of 100 and 1000 s of retention time, and significant differences in the aroma profile were obtained according to the variation in signals and colors. Onions after fresh-cut into rings and squares showed obvious improvements in the signals of aroma compounds. Moreover, the square samples showed more volatile components compared to those in untreated onions (Figure 2a). Principal component analysis (PCA) is built using signal strength to emphasize differences in volatile components of onions treated with different fresh-cut styles (Figure 2b). The distribution diagrams of the first two principal components confirmed by PCA are shown; the cumulative variance contribution rates were 39% and 22%. The control samples (untreated onions), all bulb samples, and the rings and squares samples stored at 4 \({}^{\circ}\)C were located at the negative semi-axis of the first principal component, while the ring and square samples stored at 20 \({}^{\circ}\)C and 25 \({}^{\circ}\)C were located at the positive semi-axis. The results  
Figure 2: **(a)** GC-IMS spectra of volatile organic compounds in onion samples. **(b)** Principal component analysis (PCA) diagram of onion samples. **(c)** Gallery plot (fingerprint) of volatile organic compounds in onion samples. Labels: “S-0” represents the untreated onion (control group); “S-1”, “S-2”, and “S-3” represent the onion bulbs, onion rings, and onion squares stored at 4 \({}^{\circ}\)C; “S-4”, “S-5”, and “S-6” represent the onion bulbs, onion rings, and onion squares stored at 20 \({}^{\circ}\)C; “S-7”, “S-8”, and “S-9” represent the onion bulbs onion rings, and onion squares stored at 25 \({}^{\circ}\)C.  
revealed that the volatile compounds of all onion bulbs and ring- and square-cut samples stored at 4 \({}^{\circ}\)C are relatively similar to those of the control, while the volatile compounds of onion rings and squares stored at 20 \({}^{\circ}\)C and 25 \({}^{\circ}\)C are similar but quite different from those of the control. Next, gallery plot analysis, as a fingerprinting technique, was used to show the differences in the volatile compounds of onions under different treatments more specifically and intuitively [27]. As shown in Figure 2c, each row shows all the signal intensities of one sample, and each column reveals the selected compound in different samples. Furthermore, colors represent the signal intensity (compound concentration) of the volatile molecules. The low intensities are expressed in white color, the high intensities in red, and the higher signal intensities in a deeper color. Accordingly, the differences in the volatile compounds in different onion samples were observed. Each sample had its own characteristic volatile components. As shown in Figure 2c, most of the volatile compounds had higher concentrations when stored at a higher temperature.