## 3 Results
### Effect of PEF on the Concentration of Headspace Onion Volatiles  
Figure 6 depicts an example GC-MS total ion chromatogram (TIC) of the headspace of the sodium phosphate buffer following 6 h of steeping with a PEF-treated onion bulb. Five sulfur-containing volatile compounds of interest were identified: 1-propanethiol, 3 disulfides (methyl propyl disulfide, DPDS, and 1-propenl propyl disulfide), and dipropyl trisulfide. DPDS was the predominant volatile detected with the most abundant peak area in the headspace of the phosphate buffer following 6 h of steeping with onions.  
The average peak areas of all detected onion volatile compounds in the phosphate buffer increased in response to PEF treatment, particularly when the onion bulb was oriented with the root end positioned facing the high voltage electrode at the time of PEF (FE). The headspace concentration of DPDS did not increase significantly during the first 2 h of steeping for both untreated and PEF-treated onion bulbs. Following 4 and 6 h of steeping with onions pre-treated with PEF, the amount of DPDS increased significantly in the phosphate buffers (Figure 7). The greatest increase in DPDS (up to a 59-fold more than an untreated onion) was found in the headspace of the phosphate buffer steeped for 6 h with an onion treated with an electric field strength of 1.2 kV/cm (PEF 3) and positioned facing the high voltage electrode (FE) at the time of PEF application.  
Without a PEF pre-treatment on the onion, it was demonstrated that the peak area of DPDS in the headspace of the phosphate buffer over the course of 6 h of steeping was almost at the same level and never exceeded more than \(0.5\times 10^{7}\) (Figure 7). In fact, changes in the DPDS derived from untreated onions as a function of steeping duration were not statistically significant from one time point to another (\(p>0.05\)).  
The kinetic changes in the volatile compounds isolated in the headspace of the phosphate buffer during steeping with untreated onions were the slowest, with a kinetic rate constant (\(k\)) estimated at \((0.06\pm 0.01)\times 10^{7}\) peak area per hour (Table 2). In contrast, it was found that DPDS in the headspace of the phosphate buffer for PEF-treated onions increased linearly with the steeping duration (Figure 7). Overall, the results showed that the rate of DPDS increase (\(k\)) was accelerated, by at least 32- and 52-fold, in the headspace of the phosphate buffer steeped with onions PEF-treated at 0.6 (FE-PEF1) and 1.2 kV/cm (FE-PEF3), respectively, compared to untreated onion bulbs (Table 2).  
Figure 5: Representative photographs of onion scales from untreated (No PEF) and PEF-treated bulbs stained with tetrazolium salt and the percentage area fractions of viable cells (mean \(\pm\) standard error, \(n=3\); bars with different lower-case letters indicate a significant difference (\(p<0.05\)) between the onion orientations for each scale). Onion bulbs were positioned with either the root end facing the electrode (FE) or the root end not facing the electrodes (NFE) at the time of PEF application. Viable cells were stained red and dead cells were unstained. Number 1–5 indicates the fleshy scales and a terminal but in an onion bulb, as designated in Figure 1.