## 3 Results and Discussion
### Analysis of Real Samples  
Flavor-enhanced oils are usually produced by the traditional pressing process, mainly including roasting, pressing, and filtering. Heterocyclic volatiles are commonly produced by the Maillard reactions among proteins, amino acids, and sugars during oilseed roasting [40]. Pyrazines are one group of the main Maillard reaction products and also give the main source of roasting-like aroma in flavor oils [6]. Flavor-enhanced sesame and peanut oils are the main representatives of such oils, while rapeseed oil also has a good market in China for its unique spicy and roasted flavor, occupying more than 30% of the entire Chinese rapeseed oil market [41]. For the quantification of pyrazines in the three market  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Compound** & **Spiked Level** & **Recovery 1** & **Spiked Level** & **Recovery 1** & **Spiked Level** & **Recovery 1** \\  & **(ng/g)** & **(\%)** & **(ng/g)** & **(\%)** & **(ng/g)** & **(\%)** \\ \hline
2-methyl-pyrazine & 342 & 104.4 \(\pm\) 15.0 & 1710 & 106.1 \(\pm\) 4.13 & 3420 & 98.5 \(\pm\) 6.9 \\
2,5-dimethyl-pyrazine & 642 & 99.8 \(\pm\) 7.7 & 3210 & 111.3 \(\pm\) 13.0 & 6420 & 93.9 \(\pm\) 12.5 \\
2,6-dimethyl-pyrazine & 382 & 109.8 \(\pm\) 6.0 & 1910 & 104.3 \(\pm\) 4.2 & 3820 & 100.9 \(\pm\) 13.7 \\ Ethly-pyrazine & 364 & 105.2 \(\pm\) 2.9 & 1820 & 95.3 \(\pm\) 9.5 & 3640 & 97.1 \(\pm\) 14.4 \\
2-ethyl-6-methyl-pyrazine & 330 & 104.8 \(\pm\) 8.4 & 1652 & 105.5 \(\pm\) 6.5 & 3305 & 94.2 \(\pm\) 10.7 \\
2-ethyl-5-methyl-pyrazine & 85 & 97.8 \(\pm\) 8.3 & 425 & 93.4 \(\pm\) 7.9 & 851 & 113.6 \(\pm\) 5.5 \\ Trimethyl-pyrazine & 336 & 112.9 \(\pm\) 3.1 & 1680 & 109.7 \(\pm\) 5.6 & 3360 & 97.9 \(\pm\) 10.6 \\
2,6-diethyl-pyrazine & 256 & 96.1 \(\pm\) 6.5 & 1280 & 107.8 \(\pm\) 8.1 & 2560 & 115.4 \(\pm\) 7.2 \\
3-ethyl-2,5-dimethyl-pyrazine & 252 & 102.2 \(\pm\) 6.7 & 1260 & 112.3 \(\pm\) 15.7 & 2520 & 106.7 \(\pm\) 7.9 \\
2-ethyl-3,5-dimethyl-pyrazine & 314 & 112.0 \(\pm\) 14.0 & 1570 & 106.7 \(\pm\) 6.3 & 3140 & 91.9 \(\pm\) 9.4 \\
3,5-diethyl-2-methyl-pyrazine & 284 & 105.3 \(\pm\) 10.3 & 1420 & 109.1 \(\pm\) 6.23 & 2840 & 110.5 \(\pm\) 6.5 \\ Isopropyl-pyrazine & 308 & 98.7 \(\pm\) 7.1 & 1540 & 95.34 \(\pm\) 9.1 & 3080 & 93.0 \(\pm\) 10.2 \\
5H-5-Methyl-6,7-dihydrocopopenta- & 98 & 96.5 \(\pm\) 5.1 & 490 & 109.2 \(\pm\) 6.7 & 980 & 111.7 \(\pm\) 11.0 \\ pyrazine & & & & & & \\ \hline \hline \end{tabular} \({}^{1}\) Recovery calculated from three concentration and three replicates.  
\end{table}
Table 2: Recoveries of 13 pyrazines in refined rapeseed oil at different concentration levels.  
product samples, MHS-SPME-arrow-GC-MS were carried out as set up above. Prior to analysis of the samples, the standard calibration solution of pyrazines and 3-methyl-pyridine with known concentrations were analyzed to calibrate the detector responses. Taking 2-ethyl-5-methyl-pyrazine in flavor-enhanced peanut oil as an example, Figure 3 showed the change of peak areas for four successive extractions. An exponential decrease in the target compound with the number of extractions was observed. The average concentrations and standard deviations of pyrazines together with R\({}^{2}\) and slope of the linear plot ln A\({}_{\rm i}\) versus i  
the highest concentration were 2-methyl-pyrazine and 2,5-dimethyl-pyrazine in three oils. As described by previous studies [1; 11], pyrazines may be the best indicator to measure roasted flavor intensity. It is now clear also that sesame oil has the strongest roasted flavor as the concentration of pyrazines is the highest.