## 3 Results and Discussion
### Effect of Storage Temperature and Fresh-Cut Style on Sulfur Compounds in Onion  
In order to analyze the correlation between the content of each sulfur compound and the sample, a hierarchical clustering analysis was conducted. As depicted in Figure 3, the sulfur compounds profile of the bulbs was significantly different from that of the rings and squares stored at any temperature, while the ring and square samples stored at the same temperature were clustered into one cluster. It was indicated that fresh-cut styles and storage temperature of onion had a great influence on their release of sulfur compounds, and different onion samples can be distinguished according to the content of sulfur compounds. For the same storage temperature, as shown in Table S1, at 4 \({}^{\circ}\)C, the total relative content of sulfur compounds was the lowest in the bulb samples and the highest in the square samples. However, at 20 \({}^{\circ}\)C and 25 \({}^{\circ}\)C, the content of sulfur compounds was the highest in the bulb samples but lowest in the ring samples. These results revealed that at  
Figure 3: Clustering heat map of volatile sulfur compounds in each sample. “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.  
a low temperature (4 \({}^{\circ}\)C), cutting onions into a ring or square shape will produce more sulfur. However, at higher temperatures (20 \({}^{\circ}\)C and 25 \({}^{\circ}\)C), fresh-cutting decreased the sulfur concentration. For the same cut style, as shown in Table S1, the total content of sulfur compounds in bulb samples at 20 \({}^{\circ}\)C and 25 \({}^{\circ}\)C was significantly higher than that after storage at 4 \({}^{\circ}\)C. The total content of sulfur compounds in onion rings was the highest at 20 \({}^{\circ}\)C, followed by 4 \({}^{\circ}\)C and finally 25 \({}^{\circ}\)C. The content of sulfur compounds in squared onion was the highest at 25 \({}^{\circ}\)C but lowest at 4 \({}^{\circ}\)C. These results indicated that when storing at the same cut style, the total content of sulfur compounds was higher in the sample stored at higher temperatures (20 \({}^{\circ}\)C or 25 \({}^{\circ}\)C). To more accurately find the difference between different samples, the content change of each compound was further analyzed. As shown in Figure 4, the patterns of their changes in concentration could be divided into six categories: (1) The concentrations of propan-1-thiol (Figure 4a) and dipropyl disulfide (Figure 4b) were higher in whole bulbs at 4 \({}^{\circ}\)C and 20 \({}^{\circ}\)C, but significantly lower in the bulb samples stored at 25 \({}^{\circ}\)C. However, the concentrations in rings and squares were lower in the samples stored at 4 \({}^{\circ}\)C and 20 \({}^{\circ}\)C but higher in the samples stored at 25 \({}^{\circ}\)C. (2) (_E_)-1-(prop-1-en-1-yl)-2-propyldisulfane, (_E_)-1-(prop-1-en-1-yl)-3-propyltrisulfane, 1-(1-(methylthio)propyl)-2-propyldisulfane, (_Z_)-1-(1-propenyldithio)propylpropylpropyl disulfide, and dipropyl trisulfide had the highest concentrations in the bulb samples stored at 20 \({}^{\circ}\)C and lowest concentrations in rings and squares samples stored at 4 \({}^{\circ}\)C, and the concentrations in rings and squares increased along with the increase in storage temperatures (Figure 4c-g). (3) Methyl 1-(1-propenylthio)propyl disulfide, 2-methoxy-5-methyl-thiophene, 6-ethyl-4,5,7,8-tetrathiaundecane, and 1-methyl-2-(1-(propylthio)propyl)disulfane had the highest concentrations in the whole bulb samples stored at 20 \({}^{\circ}\)C. However, for rings and squares, their concentrations decreased along with the increase in storage temperature (Figure 4h-k). (4) The concentrations of methyl propyl trisulfide increased along with the increase in storage temperatures (Figure 4l) in all samples.