## 2 Results and Discussion
### Effect Of Microwave Pretreatment on Oxidative Stability of Oil  
The oxidative stability of vegetable oil is defined as the resistance to oxidation during processing and storage [31]. The induction period (IP) is an important parameter in identifying the oxidative stability of oil. As shown in Figure 2, MW pretreatment on peanut significantly (\(p\) < 0.05) increases the IP of extracted oil. Oil extracted from untreated peanut has the lowest IP (6.34 \(\pm\) 0.10 h). With 1, 2, 3, 4, 5 min MW pretreatment on peanut, the IP of extracted oil increased to 11.24 \(\pm\) 0.08 h, 13.96 \(\pm\) 0.68 h, 15.38 \(\pm\) 0.59 h, 16.50 \(\pm\) 0.23 h and 17.05 \(\pm\) 0.31 h, respectively. These results concur with previous research [9; 10; 17; 18; 20]. The oxidative stability of vegetable oils is influenced by many factors, mainly fatty acid composition, antioxidants and minor compounds [10]. As mentioned before, the darkening of extracted oil from MW-treated peanut may be due to the formation of phospholipid non-enzymatic browning products, which are known to have strong antioxidant activity [27]. Tocopherols and phytosterols have also been reported to contribute to the increased oxidative and shelf life of vegetable oils [32; 33]. Five minutes of MW treatment on peanut could inactivate 65% lipoxygenase activity and 78% lipase activity [34]. The inactivation of oxidative enzymes may possibly contribute to the higher oxidative stability of oil. With MW pretreatment on peanuts, increased tocopherols and phytosterols content, possible phospholipid non-enzymatic browning products and inactivation of  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**MH Time** & **Phytosterols** & \multicolumn{3}{c}{**Tocophenols**} \\ \cline{2-6}
**(min)** & **Total** & \(\alpha\)**-Tocopherol** & \(\gamma\)**-Tocopherol** & **\(\delta\)**-Tocopherol** & **Total** \\ \hline
0 & 273.55 \(\pm\) 2.51 \({}^{\mathrm{f}}\) & 13.87 \(\pm\) 0.27 \({}^{\mathrm{e}}\) & 3.97 \(\pm\) 0.06 \({}^{\mathrm{e}}\) & 0.58 \(\pm\) 0.02 \({}^{\mathrm{c}}\) & 18.42 \(\pm\) 0.29 \({}^{\mathrm{e}}\) \\
1 & 298.49 \(\pm\) 3.26 \({}^{\mathrm{e}}\) & 13.93 \(\pm\) 0.17 \({}^{\mathrm{e}}\) & 4.00 \(\pm\) 0.07 \({}^{\mathrm{e}}\) & 0.60 \(\pm\) 0.04 \({}^{\mathrm{c}}\) & 18.53 \(\pm\) 0.22 \({}^{\mathrm{e}}\) \\
2 & 310.33 \(\pm\) 5.21 \({}^{\mathrm{d}}\) & 15.82 \(\pm\) 0.06 \({}^{\mathrm{d}}\) & 4.37 \(\pm\) 0.03 \({}^{\mathrm{d}}\) & 0.60 \(\pm\) 0.04 \({}^{\mathrm{c}}\) & 20.79 \(\pm\) 0.08 \({}^{\mathrm{d}}\) \\
3 & 325.13 \(\pm\) 6.76 \({}^{\mathrm{c}}\) & 16.79 \(\pm\) 0.22 \({}^{\mathrm{c}}\) & 4.58 \(\pm\) 0.08 \({}^{\mathrm{c}}\) & 0.66 \(\pm\) 0.01 \({}^{\mathrm{b}}\) & 22.03 \(\pm\) 0.25 \({}^{\mathrm{c}}\) \\
4 & 339.16 \(\pm\) 3.73 \({}^{\mathrm{b}}\) & 20.38 \(\pm\) 0.03 \({}^{\mathrm{b}}\) & 5.05 \(\pm\) 0.11 \({}^{\mathrm{b}}\) & 0.69 \(\pm\) 0.03 \({}^{\mathrm{b}}\) & 26.12 \(\pm\) 0.12 \({}^{\mathrm{b}}\) \\
5 & 363.35 \(\pm\) 4.22 \({}^{\mathrm{a}}\) & 21.36 \(\pm\) 0.24 \({}^{\mathrm{a}}\) & 5.66 \(\pm\) 0.04 \({}^{\mathrm{a}}\) & 0.86 \(\pm\) 0.02 \({}^{\mathrm{a}}\) & 27.88 \(\pm\) 0.27 \({}^{\mathrm{a}}\) \\ \hline \hline \end{tabular}  
* Values represent means \(\pm\) standard deviation of triplicate tests. Values in the columns with different superscripts \({}^{\mathrm{a}-\mathrm{f}}\) are significantly different (\(p\) < 0.05).  
\end{table}
Table 2: Effect of microwave pretreatment on free phytosterols and tocopherols content (mg/100 g) in oil.  
oxidative enzymes are responsible for the improvement of the oxidative stability of extracted oil. Longer shelf life will provide stronger market competitiveness for oil products.