Identification of α-tocopherol oxidation products in triolein at elevated temperatures

Article abstract of DOI:10.1021/jf001142f

The effect of high-temperature treatment on the stability of α-tocopherol (1) in triolein was assessed under a reduced-pressure atmosphere (4-40 mbar) simulating the deodorization step of the refining of vegetable oils. A marked degradation of i was observed, which increased with increasing temperature (180-260°C) and heating time (20-80 min). The degradation of I in triolein at 240°C was inhibited by the addition of the synthetic antioxidant TBHQ or when heating was performed under nitrogen atmosphere, indicating oxidative degradation. The oxidation products were isolated and identified as α-tocopherolquinone (2), 4a,5-epoxy-α-tocopherolquinone (3), and 7,8-epoxy-α-tocopherolquinone (4).

Full text of DOI:10.1021/jf001142f

1508
J. Agric. Food Chem. 2001, 49, 1508−1511
Id en tifica tion of r-Tocop h er ol Oxid a tion P r od u cts in Tr iolein a t
Eleva ted Tem p er a tu r es
Tom Verleyen,*,† Roland Verhe,† Andre Huyghebaert,‡ Koen Dewettinck,‡ and Wim De Greyt§
Department of Organic Chemistry, Coupure Links 653, 9000 Gent, Belgium; Department of Food
Technology and Nutrition, Coupure Links 653, 9000 Gent, Belgium; and De Smet Engineering,
Prins Boudewijnlaan 265, Edegem, Belgium
The effect of high-temperature treatment on the stability of R-tocopherol (1) in triolein was assessed
under a reduced-pressure atmosphere (4-40 mbar) simulating the deodorization step of the refining
of vegetable oils. A marked degradation of 1 was observed, which increased with increasing
temperature (180-260 °C) and heating time (20-80 min). The degradation of 1 in triolein at 240
°C was inhibited by the addition of the synthetic antioxidant TBHQ or when heating was performed
under nitrogen atmosphere, indicating oxidative degradation. The oxidation products were isolated
and identified as R-tocopherolquinone (2), 4a,5-epoxy-R-tocopherolquinone (3), and 7,8-epoxy-R-
tocopherolquinone (4).
Keyw or d s: R-Tocopherol; R-tocopherolquinone; epoxy-R-tocopherolquinone; thermoxidation; triolein
INTRODUCTION
Studies on the tocopherol balance during deodoriza-
tion showed that upon refining of vegetable oils, toco-
pherols are not only partially distilled but also oxidized
during the deodorization process (1). Tocopherols are the
most important natural antioxidants present in veg-
etable oils, protecting lipids from peroxidation. The
antioxidant properties of tocopherols have been ascribed
to the easy hydrogen transfer from their phenolic
hydrogen atom to a peroxyl radical (2). The hydrogen
transfer produces a tocopheroxyl radical, which com-
bines with another lipid peroxyl radical in a termination
reaction yielding nonradical tocopherol oxidation prod-
F igu r e 1. Structures of 1-4.
ucts (3, 4, 31).
Participation of tocopherols in antioxidation reactions
(eqs 1 and 2) leads to their degradation and consump-
tion, presumably by oxidation (5). A rapid tocopherol
The overall objective of this study was to investigate
the influence of high-temperature treatment on the
stability of tocopherols dissolved in a triacylglycerol
matrix and to identify the most important oxidation
products formed. Special emphasis was made to check
the thermal stability and to evaluate the susceptibility
of R-tocopherol to oxidation reactions at temperatures
>200 °C and low pressure during short heating experi-
ments simulating industrial deodorization conditions.
LOO• + TOH f LOOH + TO•
LOO• + TO• f TO-OOL
(1)
(2)
degradation at frying conditions and during microwave
heating has been observed, indicating the heat sensitiv-
ity of tocopherols (6-8). Relative stabilities of the
different tocopherol homolgues indicate that R-toco-
pherol is less stable than δ-tocopherol at high temper-
ature (9-11). Several tocopherol oxidation products
have been identified (12-18). However, little informa-
tion is available on the stability of tocopherols and on
the degradation products formed during oxidation of
tocopherols in a lipid matrix at high temperatures (5,
19, 20).
MATERIALS AND METHODS
Ch em ica ls. Pure R-tocopherol and silica gel 60 for chro-
matography were purchased from Merck (Darmstadt, Ger-
many), and technical grade triolein (OOO) was delivered by
Sigma Chemical Co. (St. Louis, MO). HPLC grade solvents
(hexane and isopropyl alcohol) were purchased from Merck and
were used without further purification. Water was distilled
twice and additionally purified with activated char.
Syn th esis of r-Tocop h er ol Oxid a tion P r od u cts. Several
oxidation products of R-tocopherol (1) (structure shown in
Figure 1) have been synthesized and isolated according to
literature reports. Alkaline ferricyanide is the most used
reagent for the preparation of R-tocopherol spirodimers and
R-tocopherol spirotrimers (12, 15, 21-23). Oxidation of R-to-
copherol with ferric chloride preferentially forms R-tocopherol
quinone (2) (24, 25). R-Tocopherol-R-tocopheroxyl dimer and
* Author to whom correspondence should be addressed
(telephone +32 9 264 59 50; fax +32 9 264 62 43; e-mail
tom.verleyen@rug.ac.be).
† Department of Organic Chemistry.
‡ Department of Food Technology and Nutrition.
§ De Smet Engineering.
10.1021/jf001142f CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/24/2001

Tocopherol Oxidation
J. Agric. Food Chem., Vol. 49, No. 3, 2001 1509
Ta ble 1. In flu en ce of Hea tin g Con d ition s on Tocop h er ol
Loss
R-tocopherol-R-tocopherol quinone spirodimer were isolated
from the oxidation of R-tocopherol with tert-butylhydroperoxide
(13, 14). Epoxy-R-tocopherols [4a,5-epoxy-R-tocopherol quinone
(3) and 7,8-epoxy-R-tocopherol quinone (4)] were prepared
according to a modified method of Csallany (26) and Ha (27).
Briefly described, R-tocopherol was epoxidized in an oxygen-
saturated acetonitrile/water mixture by addition of the radical
initiator azoisobutyronitrile (AIBN). Tocopherol oxidation
products (2-4) were separated from the crude oxidation
mixture by preparative HPLC using a Kontron chromatograph
(Serlabo, Brussels, Belgium) equipped with a UV detector (275
nm), a sample loop of 250 µL, and a normal phase silica column
(Lichrosorb, 250 × 20 mm internal diameter and particle size
5 µm, Optilab, Brussels). The flow rate of the mobile phase
(hexane/isopropyl alcohol, 95:5, v/v) was 15 mL/min, corre-
sponding to a column pressure of 25 bar. Collection of the
different fractions was performed manually.
temp
(°C)
time
(min)
pressure R-tocopherola R-tocopherol
expt
(mbar)
(ppm)
loss (%)
1
2
3
4
5
6
7
8
180
200
200
200
240
240
260
260
60
20
40
60
60
60
80
60
40
40
40
40
40
4-5
40
2070
2157
2118
2086
1996
2017
1895
1957
5.7
1.7
3.5
5.0
9.1
8.1
13.7
10.8
4-5
a
The initial level of R-tocopherol in all samples was 2195 ppm.
Ta ble 2. In flu en ce of Hea tin g Exp er im en ts on
Tocop h er ol Loss
1H NMR and 13C NMR spectra were recorded with a J EOL
J NM-EX270 spectrophotometer at 270 and 68 MHz, respec-
tively. The samples were dissolved in deuterated solvents with
tetramethylsilane as internal standard. All spectral data were
in accordance to literature reports.
Hea t Tr ea tm en t of Oil Sa m p les. The R-tocopherol stan-
dard was dissolved in triolein, and samples (15 g) were
transferred into a 50 mL flask and immersed in a preheated
oil bath. The flask was attached to a vacuum pump, and the
vacuum was regulated by use of a valve connected to a
manometer. After the heat treatment, the samples were
transferred into small bottles, purged with nitrogen, and stored
in the freezer until analysis. All results listed are the mean
values of two experiments, which showed a good reproduc-
ibility.
R-tocopherol
R-tocopherol
(ppm)
loss (%)
initial R-tocopherol
2566
2326
2488b
2560
reference conditionsa
9.3
3.0
0.3
TBHQ (initial 4000 ppm)
N2 flush during whole expt
initial R-tocopherol
3421
3042
3086
3411
reference conditionsa
11.1
9.8
0.3
N2 flush until 240 °C is reached
N2 flush during whole expt
a
b
Heating conditions: 240 °C/5 mbar/80 min. TBHQ 1100 ppm
after heat treatment.
The impact of additional factors influencing the
tocopherol loss is shown in Table 2. Two sets of experi-
ments with different initial 1 contents (2566 and 3421
ppm) were studied. Heating of 1 under the reported
reference conditions resulted in remarkable losses of 1
of 9.3 and 11.1%, respectively, in accordance with the
results listed in Table 1. Addition of a high concentra-
tion (4000 ppm) of the synthetic antioxidant tert-
butylhydroquinone (TBHQ) to the sample during the
same heating procedure reduced the tocopherol loss to
only 3%. This protection was accompanied by a remark-
able decrease in TBHQ concentration to 1100 ppm.
In a third set of experiments, nitrogen was gently
blown through a capillary into the oil during the
complete heating experiment. Only a small amount of
nitrogen was blown into the oil in order to make sure
no distillation of 1 took place. Heating of the oil under
the gentle agitation of a nitrogen flow did not result in
any tocopherol loss (Table 2). The experiment under
nitrogen indicated that under specialized conditions,
where the atmosphere is constantly renewed, 1 is
thermally stable in a triacylglycerol matrix up to
temperatures of 240 °C. To check that the observed
tocopherol loss was not due to the oxygen initially
dissolved in the oil, it was nitrogen flushed, and after 3
min the nitrogen valve was closed. This heat treatment
resulted in a loss of 1 of 9.8%, which is similar to the
reference heating experiment.
HP LC An a lysis of Resid u a l Tocop h er ol. Analysis of
tocopherols by normal phase HPLC was carried out on an HP
series 1050 chromatograph (Hewlett-Packard, Avondale, PA).
A normal phase silica column (Alltima Silica 5U, 250 mm ×
4.6 mm i.d., particle size 5 µm, Alltech, Deerfield, IL) was
selected. The mobile phase, hexane/isopropyl alcohol (99.5:0.5),
was delivered at a flow rate of 1.5 mL/min corresponding to a
column pressure of 50 bar. A HP-1100 fluorescence detector
was used with the excitation and emission wavelengths set at
230 and 292 nm, respectively. Identification and quantification
of the different tocopherols were accomplished with pure
standards as reference compounds.
Isola tion of Tocop h er ol Oxid a tion P r od u cts fr om
Hea ted Tr iolein . Triolein (28 g) containing 3500 ppm of 1
was heated during 90 min at a temperature of 240 °C and a
reduced pressure of 6-7 mbar, which is similar to a treatment
of bleached oil in
a deodorization plant. After the heat
treatment, the triolein was methylated under nitrogen flow.
After 30 min, the solution was poured into water and extracted
twice with hexane. Methyl esters of oleic acid were distilled
(0.5 mbar, 135 °C), yielding a dark brown residue. The
concentrated residue was separated by preparative HPLC, as
above, to give six fractions (see Results). During a time course
of 15 min the column was eluted isocratically with hexane/
isopropyl alcohol 99.5:0.5, v/v; afterward, the polarity was
gradually increased in 5 min to a ratio of 99:1, v/v. The eluent
was monitored at different wavelengths between 290 and 240
nm.
Isola tion a n d Id en tifica tion of r-Tocop h er ol
Degr a d a tion P r od u cts. During the normal-phase
HPLC analysis of tocopherols with UV detection, the
appearance of unknown peaks in the chromatogram
could not be observed. It was supposed that the con-
centration of tocopherol oxidation products was too low
for their detection due to the abundance of the triolein.
Methylation of the triolein and subsequent distillation
of the methyl oleate were used to obtain a residue highly
concentrated in tocopherol and tocopherol oxidation
products. This residue was separated by use of prepara-
tive HPLC to give six fractions. The first three fractions
RESULTS
The influence of different heating conditions on the
R-tocopherol degradation is listed in Table 1. After 60
min of heating, the loss of 1 at 180 and 200 °C was ∼5%,
but it increased to 8.1 and 13.7% at 240 and 260 °C,
respectively (experiments 1, 4, 5, and 7). The operating
pressure (40 or 4 mbar) had little influence on the
tocopherol loss (experiments 5-8) with slightly higher
tocopherol retention at lower pressure. Tocopherol loss
gradually increased with increase in heating time
(experiments 2-4).

1510 J. Agric. Food Chem., Vol. 49, No. 3, 2001
Verleyen et al.
had NMR spectra containing the typical ester function
and two unsaturated carbons: H NMR (CDCl3) δ 2.3
The results listed in Tables 1 and 2 indicate a marked
loss of 1 upon heating of the model system even if the
concentration of oxygen was rather low. Heating of the
model system at 240 °C during 80 min resulted in a loss
of 1 of 9%. The loss of 1 was strongly related to the
heating temperature and the heating time. The head-
space pressure (4-40 mbar) and, consequently, the
oxygen concentration above the oil hardly influenced the
tocopherol loss. Additional experiments, using the syn-
thetic antioxidant TBHQ and flushing with nitrogen,
were performed to check if the tocopherol loss is
inevitable due to a thermal breakdown or has to be
explained by an oxidation of the tocopherols. Addition
of TBHQ to the model system significantly reduced the
tocopherol loss to only 3%. Probably TBHQ, which also
is an effective antioxidant, reduced the tocopherol loss
by competing with tocopherols to capture radicals
formed in the model system during the heating experi-
ment. Experiments under nitrogen indicated that under
specialized conditions, where the atmosphere is con-
stantly renewed, 1 is thermally stable in a triacylglyc-
erol matrix up to temperatures of 240 °C. Tocopherols
could only be protected against degradation when
nitrogen, as inert gas, was constantly blown through
the triolein.
It is very difficult to compare these data with litera-
ture reports as previous studies at high temperatures
have dealt with tocopherol losses during conditions
similar to frying temperatures of 180 °C. In all studies
an almost linear decrease of tocopherols in relation to
the heating time was observed. Murkovic et al. (5) and
Barrera-Arellano et al. (11) reported a tocopherol de-
crease of almost 10% per hour at 180 °C, leading to
complete tocopherol depletion after 9-10 h.
The level of polymeric triacylglycerols has not been
quantified but is expected to be insignificant at these
low levels of tocopherol degradation. The total content
of polymeric triacylglycerides in deodorized oils rich in
polyunsaturated fatty acids (e.g., soybean oil) ranged
between 4 and 7% and was found to be independent of
the deodorization temperature (28, 29). Only in the
absence of tocoperols can a higher level of thermal and
oxidative triacylglycerol polymers be expected (30).
Work is in progress in our laboratories to study the
effect of fatty acid unsaturation on the rate of tocopherol
degradation.
In this research 2-4 were identified as the major
oxidation products formed during the heating experi-
ments in the model system. This indicates a degradation
of R-tocopherol proceeding after formation of adducts
with peroxyl radicals (eq 2) at the 5-, 7-, and 8a-
positions, respectively. Breakdown of these peroxyl
adducts will generate epoxy species with carbon-
centered radicals that capture oxygen and form 8a-
hydroperoxy derivatives. Further degradation of 8a-
hydroperoxy-4a,5-epoxy-R-tocopherol, 8a-hydroperoxy-
7,8-epoxy-R-tocopherol, and 8a-hydroperoxy-R-tocopherol
will generate 3, 4, and 2, respectively. Murkovic et al.
(5) studied the kinetics of the epoxytocopherol quinone
formation at several temperatures in corn oil triacyl-
glycerides and found its content to reach levels >1000
ppm after 5 h of heating at 180 °C. Even longer heating
times resulted in a reduction of the epoxytocopherol
quinone yielding other dimeric oxidation products. Only
upon working under an inert atmosphere, where all
oxygen is excluded, was no degradation of 1 at temper-
atures up to 240 °C observed. Any trace of oxygen
1
(t, 2H, OCCH2-, J ) 7.6 Hz), 3.7 (s, 3H, -COOCH3),
5.3 ppm (m, CH2CHdCH-); 13C NMR (CDCl3) δ 173
(CdO), 130.1, 129.9 (-CHdCH-), 50.6 (-OCH3). These
components were identified as residual methyl oleate
and oxidation products of methyl oleate according to
their spectral data. No other functional groups indicat-
ing tocopherol or tocopherol oxidation products could be
detected in the NMR spectra of these fractions.
Fraction 4 eluted as a sharp peak at 21 min and was
identified as residual R-tocopherol by comparison with
reference standard 1: UV λmax 292 nm; IR (KBr) 2930,
2874, 1600, 1461, 1420, 1382, 1151, 1110 cm-1; 1H NMR
(CDCl3) δ 1.79 (t, 2H, Ar-CH2CH2C, J ) 7.26 Hz), 1.96
(s, 3H, Ar-CH3), 2.01 (s, 3H, Ar-CH3), 2.08 (s, 3H, Ar-
CH3), 2.58 (t, 3H, CH2CH2-Ar, J ) 7.26 Hz), 4.1 (s, 1H,
-OH); 13C NMR δ 149.36 (C6), 140.55 (C8b), 126.7,
124.9, 123.0, 117.3 (C4a, C5, C7, C8), 75.0 (C2).
The fifth fraction eluted at 25.4 min, had a deep red
color, and was identified as 2 from its spectral data: UV
λmax 276 nm; IR (NaCl) 3495 (OH), 2925, 2845, 1642
(-CdO), 1460, 1374, 1309 cm-1; 1H NMR (CDCl3) δ 2.0
(s, 6H, 2 × Ar-CH3), 2.04 (s, 3H, Ar-CH3) 2.54 (t, 2H,
-CH2-Ar, J ) 4.95 Hz); 13C NMR (CDCl3) δ 187.7
(C8b), 187.3 (C6), 144.5 (C4a), 140.4, 140.2, 140.1 (C7,
C5, C8), 72.6 (C2)
Fraction 6 eluted at 33.2 min and had no spectral data
that corresponded to any of the previous reported
tocopherol oxidation products. The 13C NMR spectral
data revealed two conjugated keto functions (194.3 and
193.9 ppm), one unsaturation in conjugation with the
keto (141.2 and 140.9 ppm), methoxy and hydroxy
function. Combining these data, the component was
expected originally to contain an epoxy group, which
was opened during the methylation reaction. Formation
of the quinone could occur during the methylation or
due to an easy hydrolysis in the oil. To confirm this
component originated from epoxy-R-tocopherol quinone
and the sample cleanup hydrolyzed the epoxy function,
authentication of this fraction was checked by use of
standard epoxytocopherol quinone. A mixture of 3 and
4 was synthesized and treated with the same sample
cleanup as the triolein. Spectral data of the hydrolyzed
epoxy-R-tocopherol quinone fraction completely matched
the spectral data of the isolated fraction 6: UV λmax 273
nm; IR (NaCl) 1682, 1461, 1377 cm-1; 1H NMR (CDCl3)
δ 3.66 (s, 1H, OH), 2.96 (s, 3H, -OCH3), 1.96 (s, 6H, 2
× -CH3), 1.63 (s, 3H, -CH3), 1.26 (m); 13C NMR (CDCl3)
δ 194.3, 193.9, 141.2, 140.9, 65.8, 63.5. The identity of
this degradation product of epoxytocopherone has not
been established.
DISCUSSION
Studying the stability of 1 at high temperature in a
real vegetable oil matrix is very difficult due to the
abundance of several interfering minor components
(e.g. other tocopherols, sterols, and minor phenolic
compounds) that might synergize or antagonize the
oxidation of this tocopherol and interfere with the
purification as well. Therefore, the research was carried
out in a model system, in which 1 was dissolved in
triolein and the oil was subjected to heat treatments
under vacuum while still avoiding distillation of the
tocopherols. The origin of the tocopherol loss was
investigated by changing the working conditions; tem-
perature, residual pressure, time, additional antioxi-
dants, and nitrogen purging.

Tocopherol Oxidation
J. Agric. Food Chem., Vol. 49, No. 3, 2001 1511
(16) Yamauchi, R.; Kato, K.; Ueno, Y. Free-radical scaveng-
ing reactions of R-tocopherol during the autoxidation of
methyl linoleate in bulk phase. J . Agric. Food Chem.
1995, 43, 1455-1461.
present will immediately oxidize 1. These results also
support previous observations that tocopherol loss dur-
ing deodorization is partly due to oxidative degradation.
(17) Yamauchi, R.; Matsui, T.; Kato, K.; Ueno, Y. Reaction
product of R-tocopherol with methyl linoleate-peroxyl
radicals. Lipids 1990, 25, 152-158.
(18) Yamauchi, R.; Miyake, N.; Kato, K.; Ueno, Y. Reaction
of R-tocopherol with alkyl and alkylperoxyl radicals of
methyl linoleate. Lipids 1993, 28, 201-206.
(19) Chu, Y.; Lin, W. Effect of soybean pretreatment on the
colour quality of soybean oil. J . Am. Oil Chem. Soc.
1990, 67, 368-372.
(20) Lai, M.; Lin, W.; Chu, Y.; Chen, S.; Kong, K.; Chen, C.
The mechanism of color reversion in soybean salad oil.
J . Am. Oil Chem. Soc. 1989, 66, 565-571.
LITERATURE CITED
(1) Verleyen, T.; Verhe, R.; Huyghebaert, A.; Dewettink, K.;
De Greyt, W. Tocopherol balance during deoderization.
Abstracts: 91st AOCS Annual Meeting & Expo, San
Diego, CA, April 25-28. Int. News Fats, Oils Relat.
Mater. 2000, 11, S97.
(2) Kamal-Eldin, A.; Appelqvist, L. The chemistry and
antioxidant properties of tocopherols and tocotrienols.
Lipids 1996, 31, 671-701.
(3) Liebler, D.; Baker, P.; Kaysen, K. Oxidation of vitamin
E: evidence for competing autoxidation and peroxyl
radical trapping reactions of the tocopheroxyl radical.
J . Am. Oil Chem. Soc. 1990, 112, 6995-7000.
(4) Mukai, K.; Sawada, K.; Kohno, Y.; Terao, J . Kinetic
study of the prooxidant effect of tocopherol. Hydrogen
abstraction from lipid hydroperoxides by tocopheroxyls
in solution. Lipids 1993, 28, 747-752.
(5) Murkovic, M.; Wiltschko, D.; Pfannhauser, W. Forma-
tion of R-tocopherolquinone and R-tocopherolquinone
epoxides in plant oil. Fett/ Lipid 1997, 99, 165-169.
(6) Yuki, E.; Ishikawa, Y. Tocopherol contents of nine
vegetable frying oils, and their changes under simulated
deep-fat frying conditions. J . Am. Oil Chem. Soc. 1976,
53, 673-676.
(7) Pongracz, G. Hitzestabilita¨t der tocopherole. Fat. Sci.
Technol. 1988, 90, 247-251.
(8) Marquez-Ruiz, G.; Polvillo, M.; J orge, N.; Ruiz-Mendez,
M.; Dobarganes, M. Influence of used frying oil quality
and natural tocopherol content on oxidative stability of
fried potatoes. J . Am. Oil Chem. Soc. 1999, 76, 421-
425.
(9) Yoshida, H.; Tatsumi, M.; Kajimoto, G. Relationship
between oxidative stability of vitamin E and production
of fatty acids in oils during microwave heating. J . Am.
Oil Chem. Soc. 1991, 68, 566-570.
(10) Yoshida, H.; Kajimoto, G.; Emura, S. Antioxidant effects
of tocopherols at different concentrations in oils during
microwave heating. J . Am. Oil Chem. Soc. 1993, 70,
989-995.
(11) Barrera-Arellano, D.; Ruiz-Mendez, V.; Ruiz, G.; Dobar-
ganes, C. Loss of tocopherols and formation of degrada-
tion compounds in triacylglycerol model systems heated
at high temperature. J . Sci. Food Agric. 1999, 79, 1923-
1928.
(21) Nelan, D.; Robeson, C. The oxidation product from
R-tocopherol and potassium ferricyanide and its reaction
with ascorbic and hydrocloric acids. J . Am. Chem. Soc.
1962, 84, 2963-2965.
(22) Skinner, W.; Alaupovic, P. Oxidation products of vitamin
E and its model, 6-hydroxy-2,2,5,7,8-pentamethylchro-
man. V. Studies of the products of alkaline ferricyanide
oxidation. J . Org. Chem. 1963, 28, 2854-2858.
(23) Bolon, D. Generation of o-quinone methides in solution
trimerization. J . Org. Chem. 1970, 35, 715-719.
(24) Frampton, V.; Skinner, W.; Bailey, P. The production
of tocored upon the oxidation of R-tocopherol with ferric
chloride. J . Am. Chem. Soc. 1954, 76, 282-284.
(25) Frampton, V.; Skinner, W.; Cambour, P.; Bailey, P.
R-Tocopurple, an oxidation product of R-tocopherol. J .
Am. Chem. Soc. 1960, 82, 4632-4634.
(26) Csallany, A.; Ha, Y. R-Tocopherol oxidation mediated
by superoxide anion (O2-). I. Reactions in aprotic and
protic conditions. Lipids 1992, 27, 195-200.
(27) Ha, Y.; Csallany, A. R-Tocopherol oxidation mediated
by superoxide anion (O2-). I. Identification of the stable
R-tocopherol oxidation products. Lipids 1992, 27, 201-
205.
(28) De Greyt, W. Effect of physical refining on selected
minor components in vegetable oils. Doctoral thesis,
University of Ghent, 1998.
(29) Dobarganes, M.; Perez-Camino, M.; Marquez-Ruiz, G.
High performance size exclusion chromatography of
polar compounds in heated and non heated fats. Fat Sci.
Technol. 1988, 90, 308-310.
(12) Skinner, W.; Parkhurst, R. Oxidation products of vita-
min E and its model, 6-hydroxy-2,2,5,7,8-pentamethyl-
chroman. VII. Trimer formed by alkaline ferricyanide
oxidation. J . Org. Chem. 1964, 29, 3601-3603.
(13) Suarna, C.; Craig, D.; Cross, K.; Southwell-Keely, P.
Oxidation of vitamin E (R-tocopherol) and its model
compound 2,2,5,7,8-pentamethyl-6-hydroxychroman. A
new dimer. J . Org. Chem. 1988, 53, 1281-1284.
(14) Suarna, C.; Southwell-Keely, P. Effect of alcohol on the
oxidation of the vitamin E model compound, 2,2,5,7,8-
pentamethyl-6-chromanol. Lipids 1989, 24, 56-60.
(15) Yamauchi, R.; Kato, K.; Ueno, Y. Formation of trimers
of R-tocopherol and its model compound, 2,2,5,7,8-
pentamethylchroman-6-ol, in autoxidizing methyl li-
noleate. Lipids 1988, 23, 779-783.
(30) Lampi, A.-M.; Kamal-Eldin, A. Effects of R- and γ-toco-
pherols on thermal polymerization of purified high oleic
sunflower triacylglycerols. J . Am. Oil Chem. Soc. 1998,
75, 1699-1703.
(31) Lucarini, M.; Pedrielli, P.; Pedulli, G. Bond dissociation
energies of O-H Bonds on substituted phenols from
equilibration studies. J . Org. Chem. 1996, 61, 9259-
9263.
Received for review September 14, 2000. Revised manuscript
received December 6, 2000. Accepted December 29, 2000.
J F001142F