## 4 Conclusions  
This study investigated the effects of the temperature (140, 165, and 190 \({}^{\circ}\)C) and type of vegetable frying oil (soybean, corn, canola, and palm oils) on the formation of volatile compounds and hazardous compounds, such as PAHs and acrylamide. In general, the degree of temperature affected the generation of acrylamide as well as that of volatile compounds, whereas the formations of volatile compounds and both hazardous compounds, PAHs and acrylamide, were highly related to the type of vegetable oils. Frying heating process, using various oils, including soybean, corn, canola, and palm oil, have been applied in industry as well as home cooking. Thus, it is important to understand the impacts of temperatures and type of oils on the generations of volatile compounds and hazardous substances. Volatile compounds of WOF samples were formed via a variety of chemical reactions, such as thermal oxidative degradation of lipids and the Maillard reaction. Certain chemical groups, such as aldehydes, furans/furanones, and sulfur-containing compounds were identified as the major compounds in WOF samples. The contents of volatile compounds, in particular, aldehydes derived from fatty acids, and thiol and thiophenes, were highly dependent on the composition of unsaturated fatty acids in the frying oils. When the contents of PAHs and acrylamide produced in Welsh onion fried using vegetable oils at different temperatures were determined, that of acrylamide showed a significant difference depending on both heating temperatures and type of vegetable oils, while those of PAHs were affected only by the type of vegetable oils. In general, PAHs were detected at lower than the EU maximum tolerable limit, and acrylamide was detected below 36.46 \(\mathrm{\SIUnitSymbolMicro g}\)/kg. These results can be used as basic data for the industrial process of frying Welsh onion and utilized to develop legal regulations on the production of acrylamide when frying Welsh onion products. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Compound**} & \multirow{2}{*}{**Heating Temp.**} & \multicolumn{4}{c}{**Concentrations 1 (\(\mathrm{\SIUnitSymbolMicro g}\)/kg)**} \\  & & **Soy Bean Oil** & **Corn Oil** & **Canola Oil** & **Palm Oil** \\ \hline \multirow{3}{*}{Acrylamide} & 140 \({}^{\circ}\)C & N.D. 3 a 2 & N.D. a & N.D. a & N.D. a \\  & 165 \({}^{\circ}\)C & 7.31 \(\pm\) 0.80 b & N.D. a & N.D. a & N.D. a \\  & 190 \({}^{\circ}\)C & 14.72 \(\pm\) 2.68 c & 21.74 \(\pm\) 8.98 b & 25.38 \(\pm\) 3.76 b & 36.46 \(\pm\) 12.83 b \\ \hline \hline \end{tabular}  
* All results are expressed as mean \(\pm\) standard deviation for three replicates. 2 Mean values with different letters show significant differences (\(p<0.05\)) within a row using [PERSON]’s multiple comparison test. 3 Not detected.  
\end{table} 
Table 2: The formation of acrylamide in WOF (Welsh onion fried) samples at 190 \({}^{\circ}\)C.  
**Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/foods11091335/s1](https://www.mdpi.com/article/10.3390/foods11091335/s1)]([https://www.mdpi.com/article/10.3390/foods11091335/s1](https://www.mdpi.com/article/10.3390/foods11091335/s1)), Table S1: Volatile compounds identified in welsh onion (_Allium fistulosum_ L.) fried  
Conceptualization, Y.-S.K., S.-M.L. and H.-M.K. ; methodology and formal analysis, H.-M.K., S.-J.M. and M.-Y.J. ; data curation, H.-M.K., S.-M.L. and M.-K.P. ; writing--original draft preparation, H.-M.K. and [PERSON]-K.P. ; writing--review and editing, Y.-S.K. and M.-K.P. ; visualization, H.-M.K. and M.-K.P. ; supervision, Y.-S.K. All authors have read and agreed to the published version of the manuscript.