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.