COMMUNI CATI ONS Thermal Decomposition of Methyl Oleate Hydroperoxides and Identification of Volatile Components by Gas ChromatographyMass Spectrometry E. SELKE, E.N. FRANKEL, and W.E. NEFF, Northern Regional Research Center, Federal Research, Science and Education Administration, U.S. Department of Agriculture, Peoria, Illinois 61604 ABSTRACT The role of methyl olea te hydroperoxides as precursors of volatile compounds was investigated by thermal decomposition in the injector port of a gas chromatograph attached to a computerized mass spectrometer. The major volatile compounds identified correspond to those formed from triolein heated in air at 191 C. INTRODUCTION
In a previous investigation on the volatile compounds formed from triolein heated in air at 192 C, the major components identified were those expected from the decomposition of the four isomeric 8-, 9-, 10-, and ll-oleic hydroperoxides (I). Although hydroperoxides are the first major product of autoxidized oleate and related compounds, they are rapidly decomposed at temperatures exceeding 100 C (2), producing 90% polymeric and 10% volatile materials (3). Some nonperoxidic secondary products in heated fats such as dimers and polymers are known to be rich sources of volatile carbonyl compounds (4) and to decrease flavor and oxidative stability of soybean oil (5). Therefore. there is a question as to whether hydroperoxides are even formed at cooking
temperatures of 190 C and above. If hydroperoxides are formed as intermediates at high temperatures, there is no information about their finite existence and about the kinetics that control their decomposition. In this report direct evidence for the precursors of major volatiles was obtained by thermally decomposing pure methyl oleate hydroperoxides directly in the injector port of a gas chromatograph attached to a computerized mass spectrometer (GC-MS). With this technique (l) volatiles were separated, identified, and their relative proportion estimated by direct analysis of micro samples. EXPERIMENTAL METHODS
The hydroperoxides from methyl oleate autoxidized in O 2 at 40 C to a peroxide value 1&
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l'lG. I. Gas chromatogram of volatile compounds from thermally decomposed methyl oleate hydroperoxides.
511
512
COMMUNICATIONS TABLE I Comparison of Yolatiles from Decomposed Oleate Hydroperoxides and Heated Triolein
aSee Figure I for numbered peaks. bTentative identification based on GC elution and MS without reference compound.
of 1051 were purified by column partition chromatography (6). The hydroperoxides (checked for purity by thin layer chromatography) were analyzed for isomeric composition by GC-MS (7): 27% 8-,23% 9-, 23% 10-, and 27% II-OOH isomers. A neat sample of oleate hydroperoxides (3.8 ,ul) was inJectd into the same GC-MS system used previously (I >. The GC parameters were similar except the injector port temperature was 210 C, and temperature programming was mitiated at 25 C instead of -60 C. Identifications of volatile compounds were based on mass spectra matched manually and by computer with those of reference compounds and were confirmed by GC· retentIOn data. RESULTS AND DISCUSSION
The GC-MS computer-generated chroma togram depicts 19 peaks of interest (Fig. I). Eleven of these peaks are due to the same compounds previously identified from heated triolein (Table I). If the relative concentrations of only the nonester compounds are normalized, the relevant volatiles from oleate hydroperoxides correspond to those of heated triolein. These compounds, together with the methyl ester fragments, represent 93.6% of the relative total peak area of Figure I. The remaining LIPIDS, YOLo 13, NO.7
peaks are due to minor components which were too small to identify reliably. These results clearly indicate that oleate hydroperoxides are the major precursors of volatiles produced from triolein even at 192 C. However, these data are insufficient to prove that hydroperoxidation is the only route by which such products form. The well-recognized mechanism of carboncarbon scission on either side of the alkoxy radical intermediate produced from hydroperoxides (8) was checked by matching the concentration of cleavage products expected from each part of the oleate hydroperoxide isomers
7CH2CHO
The yields of cleavage products arising from each side of the hydroperoxide isomers were in remarkably good agreement. This mechanism accounts for aU the volatiles listed in Table I except for heptanal, 2-nonenal, and methyl nonanoate. Although the isomeric composition of the oleate hydroperoxides was
513
COMMUNICATIONS TABLE II Decomposition of Methyl Oleate Hydroperoxide Isomers Yield %
Cleavage products
B
A
CH 3(CH2 h-CH=CH-t-gH-:- (CH2 )6COOMe , 8
A: B:
2-Undecenal (I. 7%) + Me heptanoate (1.5%): Decanal (3.9'7
