## 3 Results and Discussion  
### Volatile Changes of WOF According to Types of Vegetable Oil and Frying Temperatures  
SPME coupled with GC-MS was applied to compare the differences in volatile compounds produced according to different temperatures and types of vegetable oils. Table S1 lists the volatile compounds identified from WOF samples and their relative peak areas and RIs. There were 162 volatile compounds identified across all WOF samples: 5 acids, 14 alcohols, 28 aldehydes, 1 benzene derivative, 3 esters, 19 furrans/furanones, 20 hydrocarbons, 20 ketones, 5 lactones, 6 nitrogen-containing compounds, 7 pyrazines, 3 phenols, and 31 sulfur-containing compounds. In general, the total content of volatile compounds in all WOF samples increased with the temperature, with the increase being the smallest for soybean oil. Figure 1 shows the quantitative composition of identified volatile compounds in the WOF samples according to chemical functional groups. Chemical groups, such as aldehydes, furans/furanones, and sulfur-containing compounds, were major volatile groups in all WOF samples, which is consistent with a previous study [3]. They demonstrated that these compounds were the most important contributors to the flavor of WOF. The composition of each chemical group showed different changes depending on the temperatures; for examples, the proportions of aldehydes, sulfur-containing compounds, and hydrocarbons decreased, while those of furans/furanones and ketones increased with increasing temperature. These changes in chemical groups could contribute to the flavor characteristics of WOF. Aldehydes and sulfur-containing compounds are generally composed of low odor thresholds and unique odor descriptions. In particular, furans/furanones, which have cooked and caramel flavor notes, were found to contribute significantly to the characteristic flavor formation of deep-fried oil with Welsh onion [3]. Figure 2 shows the changes in major chemical groups, such as aldehydes, furans/furanones, and sulfur-containing compounds, according to the types and temperatures of the frying oil. Aldehydes can be the dominant odor compounds in oils used to fry Allium  
Figure 1: Quantitative composition of identified volatile compounds in WOF (Welsh onion fried) samples according to chemical functional groups. Each stacked bar is 100% wide. Each figure represents the changes in volatile compounds of WOF in different vegetable oils according to the heating temperatures. species, mainly due to their low odor thresholds [1] and unique odor descriptions, such as waxy, green, and grassy. Both aliphatic and branched-chain aldehydes are generally formed via specific reactions, such as lipid oxidation or Strecker degradation. Lipid oxidation begins with the generation of hydroperoxide and progresses via mechanisms involving free radicals. There are high contents of oleic acids and linoleic acids in corn and canola oils, which are easily oxidized to hydroperoxides [10]. Hydroperoxides are then decomposed into aldehydes, acids, hydrocarbons, and other compounds [2]. This study found that the contents of aldehydes were the highest at 190 \({}^{\circ}\)C in WOF based on corn, canola, and palm oils, but not for soybean oil. High temperatures can enhance lipid oxidation by increasing the breakdown of hydroperoxides, which produces free radicals [11]. This increases the oxidation rate of fatty acids, which enhances aldehyde formation. The total amount of aldehydes was lowest in WOF prepared using palm oil. The proportion of saturated fatty acids relative to unsaturated fatty acids is higher in palm oil than in other vegetable oils, including soybean, corn, and canola oils. Compared with saturated fatty acids, unsaturated fatty acids, such as linoleic acid and linoleic acid, are more susceptible to oxidation due to the presence of multiple double bonds [12]. Aldehydes, such as hexanal, 2,4-decadienal, and (E)-oct-2-enal, are lipid-degraded compounds of linoleic acid [13], and their contents were largest in corn oil rich in linoleic acid.