## 3 Results and Discussion
### Effect of PEF Processing Intensities on the Volatile Concentrations  
The effect of PEF treatment had a significant (\(p\leq 0.05\)) effect on the concentration of volatile compounds (Table S1). The concentration of volatile compounds detected in the control samples was found to be similar across the two-time points analysed. However, the concentrations of volatile compounds detected in the PEF-treated samples were found to be dependent on the applied electric field strength at each time point (treatment effect, Table S1). The detected volatile compounds have been grouped into three groups, based on their position in the enzymatic reaction cascade (Figure 2). Figure 1: PCA biplot of first and second principal components of PEF treatment intensities and volatile compounds produced by (**A**) Red Bunching, (**B**) Ishikura, and (**C**) Yellow Sweet Spanish bulb onions, analyzed at 0 h and after 24 h. Each data point represents independent onion sample (\(n=8\)). Group 1 contains the highly reactive propanethial S-oxide, 2-methyl-2-pentenal, and an intermediate compound, propenyl propyl thiosulfinate (Figure 3). Group 2 contains the disulfide compounds, namely dipropyl disulfide, propenyl propyl disulfide, methyl propenyl disulfide (Figure 4). Group 3 contains the trisulfide compounds, namely dipropyl trisulfide, propenyl propyl trisulfide, and methyl propyl trisulfide (Figure 5). The propanethial s-oxide (PSO) in onions is interesting due to its tear-inducing nature (lachrymatory factor). In the onion cultivars tested, PEF treatment intensities had a considerable effect on the concentrations of PSO measured at T0. The concentration of PSO increased with greater applied electric field strengths (>0.3 kV/cm) for all three cultivars (Figure 3). [PERSON] et al. [7] reported that the critical electric field strength (_Ec_) required to rupture the onion cell and tonoplast membrane was between 0.2 and 0.33 kV/cm, supporting the current result. Increasing electric field strength resulted in enhanced cell membrane permeabilisation facilitating the occurrence of enzymatic reactions, such as the reactions between allinase and alkenyl cysteine sulfoxides (precursor), resulting in high concentrations of PSO being observed. After 24 h following PEF treatment, the impact of PEF treatment on PSO concentrations was found to be similar or lower than the impact at T0. The application of PEF treatment intensities has resulted in the enzymatic reaction, generating 1-propenesulfic acid as the first intermediate compound, which is further catalysed by lachrymatory factor synthase (LFS) to form propanethial s-oxide (Figure 4) [26; 27]. The reduction in the concentration of PSO during 24 h incubation after PEF treatment, could be due to hydrolysis or loss due to its high vapour pressure (41.2 \(\pm\) 0.2 mmHg at 25 \({}^{\circ}\)C) [28]. Figure 2: Enzymatic production cascade of onion flavour compounds. Self and cross condensation products of propenyl, propyl, and methyl cysteine sulfoxides. Some reaction products are not shown for clarity. Adapted from [24; 25]. Figure 3: Mean (\(n\) = 8) ion counts of propanethial S-oxide (PSO), propenyl propyl thiosulfinate (PrPthiosulfinate), and 2-methyl-2-pentenal (2M2P) 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. The alkyl thiosulfinates are rapidly formed via self- or cross-condensation of sulfenic acids upon enzymatic initiation of the reaction cascade [25]. The concentration of propenyl propenyl thiosulfinate, detected at T0, was found to increase with an increase in electric field strength in the spring onions (Red Bunching and Ishikura) (Figure 5). However, no effect of PEF treatment was observed in the Yellow Sweet Spanish bulbs at T0. The differences observed could be due to the physiological properties of the cultivar and the maturity of the bulb tissue.