Headspace volatile analysis
While most analytical methods give information on nonvolatile oxidation products such as hydroperoxides, conjugated dienes and nonvolatile aldehydes in oil, the headspace volatile analysis determines volatile oxidation products formed in the headspace of oil sample. In fact, volatile oxidation products are very important for perception of oil quality because they are responsible for off‐odors and flavors (Kerrihard et al., 2015). Therefore, in this study, headspace volatiles were analyzed to investigate the effect of 0.1% OOE on preventing the formation of volatile oxidation products in SBO and FO.
Table 4 shows headspace volatile analysis results for stripped SBO and stripped FO with the four antioxidants. SBO samples were analyzed after storing for 21 days at 25 ºC and for 7 days at 40 ºC, and FO samples were analyzed after 10 days at 25 ºC and 5 days at 40 ºC. The instrument was set to monitor 22 headspace volatiles including pentanal, hexanal, heptanal, octanal, nonanal, 2-propenal, t -2-pentenal,t -2-hexenal, t -2-heptenal, t -2-octenal,t -2-decenal, t -2-undecenal, t,t -2,4-heptadienal,t,t -2,4-nonadienal, t,t -2,4-decadienal, 1-pentanol, 1-hexanol, 1-octen-3-ol, n-butyl methacrylate, 2-ethylfuran, 2-pentylfuran, and decane. There were no detectable lipid oxidation-related volatiles in the initial stripped SBO. After storage at 25 ℃ for 21 days, pentanal, hexanal, t -2-pentenal,t -2-heptenal, t -2-octenal, 1-octen-3-ol, andt,t -2,4-heptadienal were observed in control stripped SBO, while pentanal, hexanal, t -2-pentenal, and t -2-octenal were detected in the stripped soybean oil with antioxidant treatments. It is worthwhile to note that all four treatments were very effective in preventing the formation of t -2-heptenal, which was the most abundant volatile (12.30 ± 0.64 µg/g) in the control stripped SBO and is known to be the key odorant in edible oils along with pentanal and hexanal (Xu et al., 2017). Formation of 1-octen-3-ol andt ,t -2,4-heptadienal was also effectively prevented by all four antioxidants. Other volatiles including pentanal, hexanal,t -2-pentenal, and t -2-octenal were significantly reduced by the four antioxidants. The overall effectiveness of 0.1% OOE in reducing volatile oxidation products of SBO at 25 ℃ was very similar to 0.1% Toco, 0.02% BHT and 0.1% RE. After 7 days at 40 ℃, in addition to the 7 volatiles observed at 25 ℃, t -2-hexenal was also detected in the headspace of the control, and hexanal was the most abundant volatile (10.77 ± 0.34 µg/g) followed byt -2-heptenal (6.97 ± 0.97 µg/g). All four antioxidants prevented the formation of these volatiles, as only hexanal was detected in the samples at significantly lower levels compared to the control, and their effectiveness was very similar.
The control stripped FO at day 0 did not have any detectable volatiles, however after storage at 25 ℃ for 10 days, five volatiles, pentanal, hexanal, t -2-pentenal, t -2-heptenal, andt ,t -2,4-heptadienal were detected.t ,t -2,4-Heptadienal was detected at the highest concentration, which was consistent with the earlier studies with FO heated at 80 ℃ (Yang et al., 2017) and with eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) heated at 110 °C (Peinado et al., 2016).t ,t -2,4-Heptadienal is known to be one of the major volatiles associated with oxidized FO odor (Sullivan & Budge, 2012; Venkateshwarlu et al., 2004). 0.1% OOE, 0.02% BHT and 0.1% Toco completely prevented the formation oft ,t -2,4-heptadienal during storage of FO for 10 days at 25 ℃, while 0.1% RE was not very effective in preventing this volatile. Hexanal was not detected in FO samples with 0.1% OOE and 0.1% Toco while it was present in FO samples with 0.02% BHT and 0.1% RE at 0.07 ± 0.00 and 0.18 ± 0.02 µg/g, respectively, after 10 days at 25 ℃. FO with 0.1% RE had higher amounts of t -2-pentenal (1.29 ± 0.32 µg/g), hexanal (0.18 ± 0.02 µg/g) and t ,t -2,4-heptadienal (2.09 ± 0.79 µg/g) compared to FO with other antioxidants. The lower antioxidant activity of 0.1% RE than other antioxidants in FO was also observed with PV, CDV and p -AV. One more volatile,t -2-octenal, was observed in the control FO after 5 days at 40 ℃. OOE effectively prevented the formation of all the volatiles except for pentanal, which was detected at 0.02 ± 0.01 µg/g. 0.1% RE was found to be the least effective in preventing the formation of pentanal, hexanal,t -2-pentenal, and t,t -2,4-heptadienal during storage of FO at 40 ℃ among the four antioxidants. Overall results showed that 0.1% OOE was very effective in preventing the formation of volatile oxidation products during storage of FO at 25 and 40 ℃, which was similar to 0.02% BHT and 0.1% Toco, and better than 0.1% RE.