# Antioxidant Properties of Soybean Oil Supplemented with Ginger and Turmeric Powders
## 3 Results and Discussion
### Oxidative Stability of Ginger- and Turmeric-Supplemented Soybean Oils  
In a previous study [56], it has been analyzed the peroxide value (PV) and induction period (IP) for respectively monitoring the primary and secondary phases of lipid oxidation of the same commercial refined soybean oil with/without BHT and ginger and turmeric powders during storage at 62 \({}^{\circ}\)C for 28 days. The PV parameter increased slowly in the first 14 days and quickly at the end, while the IP index linearly decreased with increasing days of storage under accelerated oxidation conditions. Moreover, the presence of synthetic and natural antioxidants contributed to significantly decreasing the PV values and increasing the IP values of the soybean oil during thermal storage. In this study, the IP index was considered as reference parameter for not only comparing the oxidative stability of soybean oil samples but also indirectly evaluating the antioxidant effectiveness of ginger and turmeric powders. The averaged IP values were significantly different (\(p<0.001\)) among oil samples at each enrichment cycle (Table 3). The ginger- and turmeric-supplemented soybean oils had higher IP values than the control C and the reference antioxidant BHT only at Cycle 1 probably due to a more than 50% percentage reduction of their phenolic contents and antioxidant activities at Cycle 2 and Cycle 3 (Tables 1 and 2, respectively). Generally, the degradation of phenolic antioxidants in oils and fats during thermal storage contributed to the formation of the lipid oxidation-related products [57]. Moreover, some authors showed that the curcuminoids contents in turmeric extracts subjected to different extraction temperatures (60-90 \({}^{\circ}\)C) and times (15-180 min) achieved the maximum at 90 \({}^{\circ}\)C for 60 min but decreased under extended heating [58]. Hence, phenolic compounds such as 6-gingerol  
Figure 1: Correlation of the antioxidant activity (2,2-diphenyl-1-picrylhydrazyl—DPPH, ferric reducing ability of plasma—FRAP) with the concentration of 6-gingerol and curcumin in soybean oils supplemented with ginger (**a**) and turmeric (**b**) powders, respectively. and curcumin at low concentrations could be more susceptible to thermal degradation during our Rancimat test carried out at 110 \({}^{\circ}\)C, thus, contributing to a further increased oxidation rate of ginger- and turmeric-soybean oils, respectively (Table 3). As regards cycle 1, the IP increments of soybean oils supplemented with antioxidant powders were as follows: TR (+5.4 h) \(>\) TP (+4.09 h) \(>\) GP (+3.5 h) \(>\) GR (+2.9 h) \(>\) GC (+2.7 h) \(>\) TC (+1.4 h) \(>\) BHT (+0.33 h). These values were much greater than those of soybean oils containing low (0.1 mg/g) and high (1.50 mg/g) dosage of red chicory extract (+0.8 h and +1.4 h, respectively) at the same experimental conditions of Rancimat instrument [5]. Moreover, they were also higher than those of soybean oils supplemented with 300 mg/kg of olive leaf extract (+0.5 h) [9], 1823 mg/kg of olive leaf extract encapsulated by Arabic gum (+0.4 h) [4] or 1000 mg/kg of protein hydrolyzes isolate from cow's intestine (+2.2 h) [8]. Among oil samples, TR and TP had the highest oxidative stability at Cycle 1, while GP achieved the best performance at Cycle 2 and Cycle 3 as a consequence of their greater phenolic contents (Table 1) and antioxidant activity (Table 2). In this regard, several authors confirmed the high correlation between the oxidative stability of soybean oils added with plant extracts and their phenolic composition [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15]. Regarding turmeric-supplemented soybean oils, TR and TP showed greater oxidative stability than that of TC only for the first two cycles where achieved the highest contents of total polyphenols and curcumin (Table 1). In this regard, the decrease in the oxidation rate of soybean oil after adding curcumin extracted from turmeric rhizome has been previously observed [7].