## 2 Results and Discussion
### Principal Component Analysis (PCA) and the Heatmap of Volatiles  
A PCA was performed using the volatile compounds data from all samples (Figure 4A and B). The first two principal components (PC1 and PC2) accounted for 61.03% of the total variation. PC1 and PC2 contributed to 45.47% and 15.56% of the total variation, respectively. The control strawberries in mostly late storage periods (H9D, B6D, and B9D) were separately grouped from other samples. As storage progressed, the strawberries were spread on the positive side of PC1. They were positively correlated with characteristic strawberry volatiles, such as 4-methoxy-2,5-dimethyl-3(2\(H\))-furanone, methyl butanoate, ethyl butanoate, methyl hexanoate, and ethyl hexanoate. Acetaldehyde and 2,3-butanedione,and hexanoic acid, that are responsible for the off-odor of strawberries, were spread on the positive side of PC1. These volatiles correlated with H9D, B6D, and B9D. Hexanal and 2-hexenal, that are responsible for the fruity odor of strawberries, were spread on the negative side of PC1. These volatiles correlated with the half-red strawberries during the early storage period (e.g., H0h, H6h, H1D, HT6h, and HT1D). Additionally, CO2-treated strawberries were spread on the negative side of PC2. Especially, CO2-treated bright-red strawberries were spread toward the negative side of PC2 compared to CO2-treated half-red strawberries. (_2E_)-2-hexenyl acetate and 3,7-dimethyl-1,6-ocatadien-3-ol concentrations positively correlated with CO2-treated strawberry groups. Thus, these may be used as potential markers of short-term CO2 treatment in strawberries. Previously, 3,7-dimethyl-1,6-ocatadien-3-ol was reported as a key component of the muscat table grape [43]. The concentration of 3,7-dimethyl-1,6-ocatadien-3-ol was higher in CO2-treated strawberry groups compared to the control groups, regardless of the ripening stage. Ten volatiles (4-methoxy-2,5-dimethyl-3(2H)-furanone, hexanoic acid, methyl octanoate, methyl butanoate, 2-heptanone, ethyl butanoate, butyric acid, hexyl acetate, ethyl octanoate, and hexyl butanoate) were selected from the loading plot based on their high variables importance in projection (VIP) values over 1, as they contributed to the discrimination of the strawberries samples.  
A dendrogram was obtained using cluster analysis (Figure 4C). Volatiles were grouped into two clusters. Cluster I included the characteristic strawberry volatiles (e.g., 4-methoxy  
Figure 4: **(A)** Score plot, **(B)** loading plot of the principal components analysis, and **(C)** hierarchical clustering and heatmap visualization of the changes in the volatile compounds in strawberries during storage. (H: half-red, B: bright-red, T: CO2 treated, h: hours, D: days). Green and red colors presented in the heatmap indicate values above and below the mean centered and scaled expression values, respectively. Black indicates values close to the mean. The total number of features was the same as the number of determined volatiles in the strawberries (n = 30), and the “non-specific filtering” option in the XLSTAT program was not used. Missing data were estimated using the nearest-neighbor algorithm. Hierarchical clustering was conducted based on Euclidian distances.  
2,5-dimethyl-3(2H)-furanone, ethyl butanoate, ethyl hexanoate, methyl butanoate, and methyl hexanoate) showing higher concentrations in bright-red strawberries, regardless of CO\({}_{2}\) treatment. Strawberries were grouped into two clusters. Cluster I included control strawberries at the late storage period (i.e., H9D, B6D, and B9D). Cluster II was divided into two subgroups. Subgroup I included CO\({}_{2}\)-treated half-red and bright-red strawberries, except B3D. Subgroup II included control half-red and bright-red strawberries, except CO\({}_{2}\)-treated half-red strawberries in the early storage period (i.e., HT6h-HT3D).