## 3 Results
### GC-MS Analysis  
GC-MS technology was used to analyze the changes of VOCs obtained in a non-destructive way after strawberries were damaged by impact. Analyzing the GC-MS data of nine groups of strawberry fruits with three extents of impact damage and three storage times after impact, 35 types of VOCs were identified, including 18 types of esters, 6 types of alkanes, 3 types of terpen, 3 alcohols, and 5 other substances. The relationship among the content of each VOC, the impact extent, and the storage time were further analyzed, and it was found that different impact damage extents and different storage times after impact caused some regular changes in the content of some VOCs. There were five VOCs whose relative content was related to the extent of impact damage suffered by the fruit (Figure 3), including acetic acid, hexyl ester, 1-hexanol, 2-hexen-1-ol, acetate, (E)-, dodecane, and decane. These VOCs were not detected when the impact height was 20 cm but were detected when the impact heights were 40 and 60 cm; the higher the impact height, the higher the substance content. However, some of them could be detected 4 and 8 h after impact but not detected at 24 h. This shows that these VOCs could be accumulated after the fruit is damaged by impact, but as the storage time \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**High**} & \multirow{2}{*}{**Feature** **Variables**} & \multirow{2}{*}{**Calibration**} & \multicolumn{4}{c}{**Calibration**} & \multicolumn{4}{c}{**Prediction**} & \multirow{2}{*}{**AB\_RMSE**} \\ \cline{3-3} \cline{5-10}  & & & **R\({}_{\text{c}}\)** & & **R\({}^{\text{2}}\)** & & **RMSEC** & & **R\({}_{\text{p}}\)** & **R\({}_{\text{p}}\)\({}^{\text{2}}\)** & **RMSEP** & **RPD** \\ \hline
20 cm & EN\({}_{100}\) & PLSR & 0.967 & 0.935 & 2.183 & 0.945 & 0.892 & 2.903 & 3.056 & 0.720 \\
20 cm & EN\({}_{\text{Max}}\) & LS-SVM & 1.000 & 1.000 & 0.000 & 0.996 & 0.991 & 0.823 & 10.743 & 0.823 \\
40 cm & EN\({}_{\text{Min}}\) & PLSR & 0.883 & 0.779 & 4.069 & 0.925 & 0.792 & 3.667 & 2.508 & 0.402 \\
40 cm & EN\({}_{\text{All}}\) & LS-SVM & 1.000 & 1.000 & 0.101 & 0.995 & 0.983 & 1.056 & 8.704 & 0.955 \\
60 cm & EN\({}_{\text{All}}\) & PLSR & 0.993 & 0.986 & 1.035 & 0.991 & 0.982 & 1.135 & 7.487 & 0.100 \\
60 cm & EN\({}_{\text{Sum}}\) & LS-SVM & 1.000 & 1.000 & 0.000 & 0.998 & 0.995 & 0.609 & 13.944 & 0.609 \\ all & EN\({}_{\text{Sum}}\) & PLSR & 0.799 & 0.638 & 4.692 & 0.755 & 0.568 & 5.456 & 1.523 & 0.764 \\ all & EN\({}_{\text{All}}\) & LS-SVM & 0.932 & 0.856 & 2.957 & 0.876 & 0.733 & 4.294 & 1.969 & 1.337 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Best models for the prediction of the time of occurrence of the impact on the fruit. increased after the impact, their content decreased, which may be related to the metabolic degradation of these VOCs in the fruit. Moreover, there were another six VOCs whose relative content increased with the storage time after impact (Figure 4), including ethyl acetate, D-limonene, butanoic acid, ethyl ester, hexanoic acid, ethyl ester, butanoic acid, 3-methyl-, ethyl ester, butanoic acid, 2-methyl-, and ethyl ester. Meanwhile, as shown in Figure 5, there were four VOCs, namely, acetic acid, butyl ester, octanoic acid, methyl ester, 1,4-cyclohexadiene, 1-methyl-4-(1-methylethyl)-, and naphthalene, 1,2,3,5,6,7,8,8a-octahydro-1,8a-dimethyl-7-(1-methylethenyl)-, which were not detected at 4 and 8 h after the impact but were detected as the storage time increased to 24 h.  
In addition, Figure 6 illustrates five VOCs whose relative contents decreased with increasing storage time after impact onset, including hexanoic acid, methyl ester; butylated hydroxytoluene; 1,6-octadien-3-ol, 3,7-dimethyl-; butanoic acid, methyl ester; and acetic acid, hexyl ester. There were two other VOCs among them whose content changed in relation to the extent of impact damage the fruit suffered.