## 2 Results and Discussion
### Effect of Microwave Pretreatment on Physicochemical Properties of Oil  
Color plays a very important role in the sensory properties of oil. As shown in Table 1, the MW treatment significantly (\(p<0.05\)) affected the Lovibond color of extracted peanut oils. With the increasing MW treatment time, the color of extracted oil gradually changed from light yellow (R 0.00/Y 1.20, Control) to light brown (R 0.40/Y 3.47, 5 min MW treatment). Darkening of oils produced from roasted seeds was also reported by other researchers [18; 24; 25]. The changes in color may be due to the formation of phospholipid non-enzymatic browning products. MW pretreatment not only resulted in the darkening of oil but also led to an increased browning index [19], which was consistent with the result of oil extracted from roasted pine nut [26]. High correlation between the browning reaction markers (absorbance, fluorescence, and pyrrolyzed phospholipid content) could indicate that the brown compounds in the roasted seed oil are due to the occurrence of a Maillard type browning reaction of phospholipids [27].  
Acid Value (AV) is a measure of the free fatty acids concentration in oil, which could be used to evaluate the freshness of oilseed, crude oil and product oil. As can be seen from Table 1, the AV of extracted oil from untreated peanut was 0.31 \(\pm\) 0.02 mg KOH/g oil. As the peanuts were being MW treated, the AV of extracted oil was significantly (\(p<0.05\)) increased. With 5 min MW treatment, the extracted oil reached the maximum AV (0.47 \(\pm\) 0.00 mg KOH/g oil). Although it was 0.16 mg KOH/g higher than the control, it was still far below the limit in the Codex standard which allows for the presence of AV up to 4 mg KOH/g in cold-pressed and virgin oils. With the MW pretreatment,  
Figure 1: Effect of microwave pretreatment on the oil extraction yield. Different characters (a–e) on top of the line indicate significant (\(p<0.05\)) differences among samples with different treatment times.  
the decreased triacylglycerlycerols (TAG) and increased free fatty acids were observed [24]. It is indicated that the increasing AV of extracted oil from MW-treated peanut may be due to the hydrolysis of TAG, as reported by other researchers [9; 25; 28].  
Perozoide value (PV) measures the quantity of peroxides in the oil, which serves as an indicator index of the primary oxidation product formation. Very low PV (0.54 \(\pm\) 0.08 meq O\({}_{2}\)/kg oil) was determined in the oil extracted from the control sample (Table 1). The PV of the oil significantly (\(p\) < 0.05) increased with the increasing MW treatment time. The peanuts were MW treated for 5 min and the extracted oil reached the maximum PV (5.30 \(\pm\) 0.08 meq O\({}_{2}\)/kg oil), which was also within the Codex standard of cold-pressed and virgin oils (PV < 15 meq O\({}_{2}\)/kg oil). Similar results were observed in the MW pretreatment of rapeseed, Chilean hazelnuts and sunflower seed [9; 18; 25]. The effect of MW pretreatment on PV changes of extracted oil during the storage has also been studied. Even the initial PV of extracted oil from MW-treated rapeseed (0.85 mmol/kg) was higher than the initial PV of oil from untreated rapeseed (0.70 mmol/kg), but when stored for 32d, the PV of MW-treated oil (12.83 mmol/kg) was lower than the PV of untreated oil (20.22 mmol/kg) [29]. It can be inferred that MW pretreatment samples produced lower secondary oxidation products than the untreated samples. This result was consistent with previous research [27; 28].