## Materials and Methods
### Pulsed Electric Field Treatments  
Before PEF treatment, the outer \"papery\" scales (also known as tunic layers, Figure 1) and the outermost fleshy scale (scale 0, Figure 1) of the onions were carefully removed by hand and the bulbs were washed with water. For every PEF treatment, an onion bulb (48.57 \(\pm\) 8.31 g) was positioned in the middle of the PEF treatment chamber (Figure 2). The PEF chamber (German Institute of Food Technologies, Quakenbruck, Germany) consisted of two parallel stainless-steel electrodes (100 mm length \(\times\) 50 mm height \(\times\) 5 mm thick each) with an 80 mm gap between them (electrode gap). The onion was fully immersed in a sodium phosphate buffer solution (0.01 M NaH\({}_{2}\)PO\({}_{4}\), pH 7.0, with an electrical conductivity of 1.4 mS/cm), which always resulted in an onion-to-buffer ratio of 50:250 in the treatment chamber. The onion was then immediately treated using the PEF apparatus (ELCRACK HVP 5 system; DIL, German Institute of Food Technologies, Quakenbruck, Germany) under a batch treatment configuration. In the present study, onions positioned at two different orientations with respect to the PEF electrodes were compared (Figure 2), in which an onion bulb was positioned with either the root end (or disc stem) facing the high voltage electrode (hereafter referred to as \"FE\") or the root end not facing the electrodes (hereafter referred to as \"NFE\"). Untreated onion bulbs were used as controls (hereafter  
Figure 1: Schematic structure of an onion bulb. Figure 2: Orientation of an onion bulb positioned inside the PEF chamber during treatment. The onion bulb was positioned with either the root end facing the electrode (FE) or the root end not facing the electrodes (NFE). “X” indicates the equatorial area of the onion bulb where specimens were later taken for cryogenic-scanning electron micrographs (cryo-SEM) evaluation. referred to as \"No PEF\" onions), and similar to PEF-treated onions, they were submerged in the sodium phosphate buffer at the same onion-to-buffer ratio for no more than 3 min until further use. For each onion orientation, onions were treated independently with three different PEF conditions (considering the intensity of the electric field strength applied and pulse number required to achieve the required specific energy input) and compared to untreated (\"No PEF\") samples as a control (Table 1). The PEF 1 treatment involved treating onions with an electric field strength of 0.6 kV/cm in conjunction with a specific energy input of 6 kJ/kg. For the PEF 2 treatment, onions were treated at the same level of field strength (0.6 kV/cm), but the specific energy input was increased 10-fold to 60 kJ/kg. Onions were also treated at a higher field strength of 1.2 kV/cm and a specific energy input of 6 kJ/kg (PEF 3). These PEF conditions fall within the range of electric field strengths and specific energy inputs that have been shown capable of rupturing the plasma membrane of onion cells [15] and substantially impacted on the cell integrity of the onion [14; 16]. Input voltages and the number of square wave bipolar pulses were adjusted to achieve the aforementioned treatment intensities, while a constant pulse width of 20 us and a frequency of 50 Hz were used for all the PEF processing conditions applied. Experiments were carried out at 20 \({}^{\circ}\)C and temperature changes due to PEF treatments were found to be negligible. For every PEF processing condition tested in this study, at least three independent PEF experiments using individual whole onion bulbs were conducted (\(n=3\)). Untreated and PEF-treated onions, along with the sodium phosphate buffer, were transferred to an enclosed container (LabServ, Thermo Fisher Scientific, Scoresby, VIC, Australia) where onions were removed for photo capturing (as detailed below) after 0, 1, 2, 4, and 6 h of steeping in the sodium phosphate buffer. Moreover, a small volume of buffer (5 mL) was removed simultaneously at each steeping time point for headspace volatile analysis (see Section 2.5).