# Antioxidant Properties of Soybean Oil Supplemented with Ginger and Turmeric Powders
## 3 Results and Discussion  
### Phenolic Characterization of Ginger- and Turmeric-Supplemented Soybean Oils  
The total phenolic content was not detected in the refined soybean oil used as control, in agreement with other authors [49], even if a total phenolic content respectively of 0.13 mM GAE/L [50], 200 mg GAE/kg [10; 11], 139 mg GAE/g [12] was determined in other commercial soybean oils. This could be associated not only to the different conditions of phenolic extraction, but also to the commercial refining grade, since it has been demonstrated that the total phenolic content of crude soybean oil decreased during refining steps [51]. In this study, the total phenolic content of soybean oil increased only after adding ginger and turmeric powders containing mostly ginger-derived 6-gingerol and 6-shogaol, and turmeric-derived curcumin [31; 35; 41]. The high oil solubility of these hydrophobic compounds has been confirmed by several studies [7; 52]. Based on the comparison between 6-gingerol content of ginger-supplemented oils at the first cycle (Table 1) with that of the corresponding powders analyzed in our previous study [41], the solubility in oil for all three ginger powders was 100%. Comparing curcumin content of turmeric-supplemented oils at the first cycle (Table 1) with that of the corresponding powders analyzed in our previous study [41], the oil solubility for commercial product (43%) was significantly lower than that for freeze-dried powders derived from rhizomes and peels (89% and 100%, respectively). In this regard, the food processing could affect the bioaccessibility and bioavailability of phytonutrients [53]. The contents of total polyphenols as well as bioactive compounds statistically differed (\(p<0.001\)) among oil samples at each enrichment cycle (Table 1). The phenolic amounts of all samples in the three oil enrichment cycles were much higher than those found in soybean oils flavored with thyme and rosmary extracts (0.93 and 0.38 mg GAE/g, respectively; [10]), and with microwave-assisted olive leaf extract (0.07 mg GAE/g; [8]) for effectively preventing the lipid oxidation. Soybean oils with turmeric powders such as TR and TP had the greatest total phenolic content at Cycle 1 and Cycle 2, while TC and GP achieved the best phenolic yields at Cycle 3. The powder preparation and structure could affect the bioactive compounds extractability during oil enrichment [53]. In this regard, turmeric-supplemented soybean oils especially TR and TP achieved the maximum yields of total polyphenols and curcumin in the first two cycles probably due to the high lipid solubility of turmeric-derived curcumin [54]. In fact, the percentage reductions of phenolic and curcumin contents in TR (respectively 50% and 77% at Cycle 2; 79% and 97% at Cycle 3) and TP (respectively 58% and 76% at Cycle 2; 81% and 96% at Cycle 3) enhanced at increasing oil enrichment cycles and were higher than those of TC (respectively 19% and 18% at Cycle 2; 33% and 40% at Cycle 3). Regarding ginger-supplemented soybean oils, the amounts of total polyphenols and 6-gingerol were greater in GP whose percentage reductions (respectively 21% and 30% at Cycle 2; 38% and 79% at Cycle 3) were lower than those of GR (respectively 46% and 83% at Cycle 2; 66% and 100% at Cycle 3) and GC (respectively 32% and 70% at Cycle 2; 52% and 96% at Cycle 3). Instead, the 6-shogaol content was stable over cycles. The 6-shogaol is a compound derived from fresh ginger during thermal drying or storage [31]. The highest 6-shogaol amount in GC could be attributed to the different drying procedure [45].