## 3 Results and Discussion
### Volatile Changes of WOF According to Types of Vegetable Oil and Frying Temperatures
Octanal, nonanal, (E)-dec-2-enal, and (E)-undec-2-enal were found in considerable amounts in canola oil with a high oleic acid content. [PERSON] et al. [14] reported that octanal, nonanal, 2-decenal, and 2-undecenal were the characteristic carbonyls for oleic- acid-rich oils via the thermal oxidation of oleic acid triglycerides. In the present study, the contents of certain aldehydes, such as hexanal, (2E,4E)-deca-2,4-dian, 3-methylbutanal, and 2-phenylacetaldehyde, increased significantly in all WOF samples with increasing temperature. Hexanal was generated via thermal oxidative degradation of unsaturated fatty acids. 3-Methylbutanal was formed via the thermally induced Strecker degradation of leucine [15], whereas 2-phenylacetaldehyde was derived from phenylalanine via a similar pathway [16]. The results of this study indicated that the contents ofurans/furanones increased with temperature. Furans/furanones are produced via various pathways, such as Maillard reaction, carbohydrate pyrolysis, and lipid oxidation, that are significantly influenced by temperature. The contents offuran-2-carbaldehyde (furfurral),furan-2-ylmethanol (furfuryl alcohol), 1-furan-2-yl)ethenone (2-acetylfuran), 5-methylfuran-2-carbaldehyde (5-methylfurral), and 5-(hydroxyl-methyl)furan-2-carbaldehyde (HMF), 2-pentylfuran, and 1-(furan-2-yl)-2-hydroxy-ethanoneone increased when WOF was exposed to a higher temperature. In particular, furural and HMF, which are mainly produced by the degradation of monosaccharides, were major furan derivatives. In addition, they can originate from carbonyl compounds produced by the oxidation of lipids [17]. Figure 2: Changes of major chemical groups (aldehydes, furrans/furanones, and sulfur-containing compounds) in WOF (Welsh onion fried) samples depending on different temperatures: (**A**) WOF prepared at 140 \({}^{\circ}\)C, (**B**) at 165 \({}^{\circ}\)C, (**C**) at 190 \({}^{\circ}\)C. Asterisk indicates a statistically significant difference (\(p\)-value \(<\) 0.05) in the changes of chemical groups compared to the samples prepared at a lower temperature (140 or 165 \({}^{\circ}\)C). Among vegetable oils, the contents of furans/furanones were higher in WOF prepared using corn and canola oils, which have high proportions of unsaturated fatty acids. Furans can be generated from various precursors, including unsaturated fatty acids [18]. [PERSON] et al. [19] demonstrated the content of furans related to heating time in the linoleic model system. Unsaturated fatty acids are more prone to oxidation than are saturated fatty acids, and hydroperoxides and their oxides decompose faster to produce more carbonyl compounds. The abundance of furan derivatives in WOF prepared using corn and canola oils might be explained by the large amounts of unsaturated fatty acids in these oils. The characteristic aromas of Allium species are commonly attributed to sulfur-containing volatiles [20]. We found that the main sulfur-containing compounds of the WOF samples included (methyldisulfanyl)methane (dimethyl disulfide), 1-(methyl-disulfanyl)propane (methyl propyl disulfide), 1-(propyldisulfanyl)propane (dipropyl disulfide), (methyltrisulfanyl)methane (dimethyl trisulfide), and (methyl-tetrasulfanyl)methane (dimethyl tetrasulfide). These compounds can be generated by the thermal degradation of sulfur-containing amino acids [3]. The volatile chemicals in the species of Allium genus are produced through the hydrolysis of nonvolatile sulfur-containing compounds, termed S-alk(en)yl-L-cysteine sulfoxides (CSs) [19]. The main CSs in Welsh onion are S-propenyl-L-cysteine sulfoxide, S-methyl-L-cysteine sulfoxide, and S-propyl-L-cysteine sulfoxide [21]. CSs are stored in the cytosol of storage mesophyll cells, and during thermal or enzymatic tissue damage, they produce their corresponding sulfenic acids, which are further converted into various sulfur-containing compounds, including dimethyl sulfides, methyl propyl sulfides, and dipropyl sulfides [22].