## 3 Results and Discussion
### Evaluation of PEF Treatment Intensities and Incubation Time on the Volatile Compounds  
Principal components analysis (PCA) was utilised to visualise the differences in the headspace concentrations of the detected volatile compounds due to different PEF treatment intensities and incubation time. The PCA biplots for each of the three onion cultivars explained between 83% and 92% of the variance on the first two principal components (PCs) (Figure 1). The trends explained by the PCs for the three cultivars studied were found to be similar. The first trend explained by PC1 is the effect of incubation time on the PEF-treated samples. The T0 samples are negatively correlated (left) on the PC1 axis, whereas the T24 samples are positively correlated (right). The magnitude of this effect was found to be greater at medium- and high-PEF intensities. The second trend explained by PC2 is the effect of PEF treatment intensities at T0. The two control samples (T0 and T24) are both negatively loaded on PC2, and an increase in electric field strength moved the scores of PEF treatment intensities towards the positive loading of PC2 for T0 samples. The impact of PEF treatment on volatile compounds was found to be different between the onion cultivars studied. The differences could have been caused due to their difference in the relative concentration of volatile precursors, which is based on the genotype and the growth environment (e.g., sulfur supply) [23]. In the Red Bunching onions, the variation along PC1 (67.6%) was found to be correlated with the incubation time upon PEF treatment (Figure 1A), whereas the PEF treatment intensities were discriminated from each other along PC2 (21.3%). The concentration of PSO, nonane, decane, 2M2P, and DMT were associated with the medium- and high-PEF treatment intensities for the T0 samples. In contrast, the higher concentrations of sulfur-containing compounds (thiosulfinates, disulfides, and trisulfides) were found to be associated \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**CAS Number**} & \multicolumn{2}{c}{**Volatile Compounds**} & \multirow{2}{*}{**M.W**} & **R.T a** & **Calculated** & \multirow{2}{*}{**Lit R1 c**} & \multirow{2}{*}{**E.I. d**} \\  & \multicolumn{2}{c}{**(Abbreviations)**} & & **(min)** & & **RI b** & \\ \hline
111-84-2 & Nonane & 128 & 6.9 & 895 & 900 & 128 \\
67-56-1 & Methanol & 32 & 7.2 & 910 & 920 & 32 \\
124-18-5 & Decane & 142 & 9.6 & 1007 & 1000 & 142 \\
1120-21-4 & Undecane & 156 & 12.9 & 1112 & 1100 & 156 \\
623-36-9 & 2-Methyl-2-pentenal (2M2P) & 98 & 15.5 & 1180 & 1160 & 98 \\
2179-60-4 & Methyl propyl disulfide (MPDS) & 122 & 17.6 & 1257 & 1224 & 122/80 \\
32157-29-2 & Propanehial S-oxide (PSO) & 90 & 17.7 & 1257 & - & 90 \\
638-02-8 & Dimethyl diiophene (DMT) & 112 & 18.4 & 1286 & 1253 & 111 \\
5905-47-5 & Methyl propenyl disulfide (MPPDS) & 120 & 19.4 & 1316 & 1327 & 120 \\
629-19-6 & Diproyl disulfide (DPPDS) & 150 & 21.9 & 1400 & 1370 & 150/108 \\
5905-46-4 & Propyl propenyl disulfide (PPrPDS) & 148 & 23.4 & 1460 & 1428 & 148 \\
17619-36-2 & Methyl propyl trisulfide (MPTS) & 154 & 25.3 & 1533 & 1516 & 154/112 \\
2217-02-9 & Fenchyl alcohol (S) e  & 81 & 26 & 1593 & 1574 & 81 \\
6028-61-1 & Dipropy trisulfide (DPTS) & 182 & 27.7 & 1700 & 1662 & 182/75 \\  & Propenyl proyl thiosulfinate & 180 & 28.6 & 1770 & - & 180 \\  & (PrPthiosulfinate) & & & & & \\
33922-73-5 & Propyl propenyl trisulfide (PPrTS) & 180 & 29.2 & 1817 & - & 180/74 \\ \hline \end{tabular}
\end{table} 
Table 1: Identification of volatile compounds in control and pulsed electric field (PEF)-treated onion cultivars. with the medium- and high-PEF treatment intensities at T24. The variations observed in both PCs of the Ishikura onions were found to follow a similar trend for the Red Bunching onions. The variation along PC1 (65.6%) was correlated with incubation time upon PEF treatment (Figure 1B) and the PEF treatment intensities were discriminated from each other along PC2 (26.5%).