## 3 Results and Discussion
### Effect of PEF Processing Intensities on the Volatile Concentrations
Figure 4: Mean (\(n=8\)) ion counts of methyl propyl disulfide (MPDS), dippropyl disulfide (DPDS), Propyl propenyl disulfide (PPrDS), and methyl propenyl disulfide (MPrDS) in control and PEF-treated onion samples. Peak areas have been standardised to the internal standard. Bars with different letters indicate statistically significant difference (Fisher’s LSD, \(p<0.05\)). Error bars represent standard error of the mean. After 24 h incubation following PEF treatment, the concentration of thiosulfinates significantly increased in all the onion varieties. The highest relative concentration of thiosulfinates upon PEF treatment was obtained at different treatment intensities in the onion varieties. The highest concentration of thiosulfinate in the spring onions was obtained after 24 h incubation with a high-PEF treatment. However, the highest concentration of thiosulfinate in the Yellow Sweet Spanish bulbs was obtained upon 24 h incubation with a medium-PEF treatment. Application of high-PEF treatment intensity resulted in a relatively low concentration of thiosulfinate after 24 h of the PEF treatment in the bulbs. The relative drop in the concentration of thiosulfinate in the Yellow Sweet Spanish bulbs could be due to the extensive disintegration of the plasma and vacuole membrane resulting in the leaching of the alkenyl cysteine sulfoxides from the cell cytoplasm, thus reducing the available precursor substrate for the reaction [29; 30]. 2-Methyl-2-pentenal (2M2P) was the only aldehyde detected in the onion samples in this study. 2M2P is a self-condensation product formed from propanal (Figure 2) and is reported to contribute a fruity flavour to onion samples [31]. PEF treatment increased the concentration of 2M2P immediately after the treatment, and the concentration of 2M2P was afterwards found to decrease across all onion cultivars after 24 h incubation. The reduction in the concentration of 2M2P was found to be significantly higher upon the application of medium- and high-PEF treatment intensities. This reduction in the  
Figure 5: Mean (\(n=8\)) ion counts of dippropyl trisulfide (DPTS), propyl propenyl trisulfide (PPrTS), and methyl propyl trisulfide (MPTS) in control and PEF-treated onion samples. Peak areas have been standardised to the internal standard. Bars with different letters indicate statistically significant difference (Fisher’s LSD, \(p<0.05\)). Error bars represent standard error of the mean. concentration of 2M2P after 24 h incubation after PEF treatment could be due to evaporation occurring during incubation. The application of PEF treatment significantly (\(p\leq 0.05\)) increased the concentration of disulfide compounds, categorised in Group 2 (Figure 4). Block [19] reported that the disulfides are formed upon the rearrangement of the alkyl thiosulfinates, which further depend on the degree of cell damage and time duration taken for the enzymatic reaction to occur. The impact of PEF treatment intensities on the concentration of disulfide compounds was found to be in the order of dipropyl disulfide (DPDS) followed by propenyl propenyl disulfide (PPrDS), methyl propenyl disulfide (MPDS) and finally methyl propenyl disulfide (MPrDS). The impact of PEF treatment intensities was found to be similar for the concentration of DPDS and MPDS, at both time points measured (Figure 4). The concentration of DPDS and MPDS increased (2-3 fold) immediately after PEF treatment, irrespective of the electric field strength. This implies that the reaction cascade initiated upon PEF treatment has not reached completion at T0. However, the concentration of DPDS and MPDS increased enormously (7-30 fold) after 24 h of incubation and was dependent on the applied electric field strength. The application of PEF treatment was found to have a different impact on the concentration of the PPrDS and MPrDS (Figure 4). The concentration of PPrDS and MPrDS increased with elevating electric field strength, measured at both time points across all onion varieties.