J Am Oil Chem Soc
efficient at inhibiting oxidation in emulsion systems than
their parent compounds.
to the oxidation study. The tocopherol contents of stripped
soybean oil were determined as described previously [13].
However, little research has been performed on the
oxidation of bulk oils with hydroxycinnamic acids and
their decarboxylated products as antioxidants. Sinapic acid
(3,5-dimethoxy-4-hydroxycinnamic acid) and its decar-
boxylated product 4-vinylsyringol are good examples of
such antioxidant compounds. In the present study, the
peroxide value (PV) and thiobarbituric acid reactive sub-
stances (TBARS) assay were performed to measure the
concentrations of hydroperoxides and malondialdehydes in
soybean oils containing different antioxidants (a-tocoph-
erol, sinapic acid, or 4-vinylsyringol). The effect of syn-
ergistically inhibiting oxidation between any two
Synthesis of 4-Vinylsyringol
4-Vinylsyringol (4-VS) was synthesized by a modified
method described in previous study [11]. Sinapic acid
(100 mg) was dissolved in 2 mL of N,N-dimethylform-
amide (DMF) containing 20 mg of sodium acetate as a
catalyst for the decarboxylation reaction. The reaction was
carried out at 130 °C in an oil bath for 15 min. After
cooling to room temperature, 4-VS was extracted with n-
hexane (2 mL each time) several times. The solvent was
removed by flushing with nitrogen, and 4-VS obtained was
stored at -70 °C for further experiments.
1
antioxidants was also determined. Furthermore, H NMR
was used to compare the antioxidant activities of
a-tocopherol, sinapic acid, and 4-vinylsyringol in stripped
soybean oil by monitoring the degradation of polyunsatu-
rated fatty acids (PUFA), the evolution of conjugated
forms, and the formation of aldehydes during oxidation.
Oxidation Process
Different antioxidants, namely a-tocopherol, sinapic acid,
or 4-VS, were added to 10 g of SSO at a concentration of
500 ppm. In order to determine synergistic antioxidant
effect between two antioxidants, soybean oil containing
250 ppm of a-tocopherol and 250 ppm of sinapic acid
(SBO-toco ? SA), 250 ppm of a-tocopherol and 250 ppm
of 4-VS (SBO-toco ? VS), or 250 ppm of sinapic acid and
250 ppm of 4-VS (SBO-SA ? VS) were submitted to
oxidation. The oil (6 g) was placed into a Petri dish
(35 9 10 mm) and exposed to air for 19 days in an oven at
60 °C in the dark. Oxidized samples were flushed with
nitrogen and frozen at -70 °C until required for analysis.
Experimental Procedures
Materials
Soybean oil was purchased from a local market (Daejeon,
South Korea). Silicic acid, deuterated chloroform (99.9
atom % D, containing 0.1 % v/v TMS) and dimethyl
sulfoxide-d6 (99.9 at.% D, contains 0.03 % (v/v) TMS)
were obtained from Sigma-Aldrich (St. Louis, MO, USA).
Activated charcoal for column chromatography was
obtained from Daejung Chemical Reagent (Daejung
Chemicals & Metals Co., Ltd). N,N-Dimethylformamide
(DMF) was purchased from TCI (Tokyo Chemical Industry
Co., Ltd).
Peroxide Value (PV) and Thiobarbituric Acid Reactive
Substances Value (TBARS Value)
Peroxide values of oxidized oils were measured using the
IDF method described in the previous study [14]. TBARS
values were determined as described previously [15].
Preparation of Stripped Soybean Oil
1H-NMR Experiment
Stripped soybean oil (SSO) was obtained by passing soy-
bean oil through a vacuum liquid chromatographic column
(3.5 cm diameter, 20 cm length) [12]. Samples were pre-
pared by diluting 120 g of soybean oil with 120 mL of n-
hexane. The lower layer of the column was packed with
15 g of silicic acid to remove minor diacylglycerol (DAG),
monoacylglycerol (MAG), and free fatty acid (FFA) in
soybean oil, and an upper layer of 30 g of active charcoal
was used to remove tocopherols. After eluting the oil with
300 mL of n-hexane, the solvent was removed using a
vacuum rotary unit (RE 111, Bu¨chi, Flawil, Switzerland) at
40 °C. Traces of n-hexane were removed by flushing with
nitrogen. Stripped soybean oil was stored at -70 °C prior
1H-NMR experiments were carried out using a Bruker
Avance III 600 spectrometer operated at 600.23 MHz.
Typically, 200 mg of oil was mixed with 400 lL of CDCl3
containing 0.1 % tetramethylsilane as an internal standard,
and this mixture was placed in a 5-mm diameter NMR
tube. The acquisition parameters used were: spectral width
12,335.5 Hz, number of scans 16, and acquisition time
2.656 s. The experiment was carried out at 28 °C. All
spectra were processed using ACD labs NMR Processor,
version 10.0. Chemical shifts (d) were referred to the TMS
at d = 0 ppm. Conjugated forms (–CH=CH–CH=CH–),
hydroperoxides (–OOH), and aldehydes (–CHO) can be
presented in mol/mol oil directly by normalizing the peak
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