Volatile Components from Triolein Heated in Air'
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Purchased by U. S. Dept. of Agric. for Official Use. 3987 Reprinted from the JOU1l.NAL OF TEtE AMERICAN OIL CHEMISTS' SOCIETY, Vol. 54, No.2, Pages: 62-67 (1977) Volatile Components from Triolein Heated in Air' E. SELKE, W.K. ROHWEDDER, and H.J. DUTTON, Northern Regional Research Center, ARS, USDA, Peoria, Illinois 61604 ABSTRACT AND SUMMARY tion of volatiles in only one or two classes of compounds. Relative odor and flavor significance of each oleate volatile In a continuing study to identify volatile odor con could be better assayed if their relative proportions were stituents and their precursors from heated soybean known, i.e., all the volatiles collected, separated, and identi- oil, the following model triglycerides were heated to fied during one analysis. 192 C in air for 10 min: (a) pure triolein, (b) a This paper is part of a continuing study to charactenze mixture of triolein (25%Hristearin, and (c) a ran volatile compounds (room odors) which develop when domly esterified triglyceride composed of oleic (25%) soybean oil (SBO) is heated to cooking. oil temperature and stearic acids. Each model system produced the (20). To pursue that objective, each major fatty aCid of same major compounds which were identified as SBO, as its pure and mixed triglyceride, is. heated to heptane, octane, heptanal, octanal, nonanal, cooking oil temperature and the resulting volat~es are cO,I 2-decenal, and 2-undecenal. These seven compounds lected and identified. This approach of volatile analYSIS apparently are unique to the oxidation of the oleate using individual pure compounds intensifies, t~e l:ss con fatty acid in each triglyceride sample. Minor volatile centrated SBO volatile compounds-thUS aidIng In their compounds from oxidized triolein included saturated identification-and may possibly establish each volatile's and unsaturated aldehydes and n-hydrocarbons and fatty acid precursor(s). A previous report described the saturated primary alcohols, methyl ketones, gamma identification and quantitation of volatiles from heated lactones, and monobasic acids. Incorporation of tristearin (21). This study covers the identification of stearic acid in the triglycerides noticeably increased volatiles from heated pure triolein, randomly esterified the amounts of saturated minor compounds and the oleo-stearin and a mixture of triolein-tristearin. The syn range of their carbon chain lengths. Decomposition thesized and mixed triglycerides were prepared to deter products characteristic of the oxidation of stearate mine if both triglyceride systems quantitatively and quali were apparent among decomposition products tatively produced the same volatiles. The oleic content in associated with the oxidation of oleate. the mixed and synthesized triglycerides was chosen to be comparable to that found in SBO, so that in parallel experi INTRODUCTION ments involving oleic and other major SBO fatty acids the Oxidation of oleic acid and its ester derivatives has been level of the oleate volatiles may be anticipated. under investigation for many years. Up until the mid 1940s, those investigations focused on the mechanism(s) of oxygen EXPERIMENTAL PROCEDURES addition to the monounsaturated molecules and on the Pure triolein was purchased from the Nu-Chek-Prep., primary oxidation products-hydroperoxides (1). There Inc. (Elysian, MN), and deodorized at 1 mm pressure. ~or after, the investigations shifted to hydroperoxide decompo 1.5 hr at 185-220 C prior to use. The randomly estenfled sition and the secondary oxidation products formed. triglyceride (oleo-stearin) was prepared from oleic acid Although the secondary decomposition products included (Hormel, Austin, MN), stearic acid (Matheson-ColeI?an ~nd many classes of compounds, the studies covered mainly Bell, Norwood, OH), glycerine, and an estenficatton aldehydes probably because they are major odor and flavor catalyst paratoluene sulfonic acid-according to Wheeler et constituents of oxidized fats, and because aldehydes are al. (22). The synthesized triglyceride was pUrified by liquid easily characterized via their 2,4-dinotrophenylhydrazine solid (Silica Gel) chromatography, then deodorized as (DNPH) derivatives. The major carbonyl compounds pre· above. The mixed triglyceride was prepared by adding dieted from the decomposition of the four oleate isomeric triolein (Hormel) to tristearin (Anderson Gayton Foods, hydroperoxides, where the OOH substituent is located at Jacksonville IL) which was previously purified by crystal the 8, 9, 10, and 11 carbon atom position, are octanal, lizing three 'times in acetone. This third model triglyceride nonanal, 2-decenal, and 2-undecenal (2a,2b). The presence was also deodorized as above. of those four aldehydes in oxidized lipids containing the Fatty acid compositions [by gas liquid chromatography oleate unsaturation has been well documented (2-11). (GLC)1 of each of the three model triglycerides were as Aliphatic aldehydes identified but not expected from the follows: pure triolein (Nu-Chek-Prep, Inc.)-98.9% oleic, oxidation of oleic acid and its ester derivatives are C 2-7 0.7% stearic, 0.1 % palmitic, 0.1 % linoleic, and 0.2% lino alkanals and C 6-9 2-alkenals (2-5,7-11). The "unexpected" lenic' esterified oleo-stearin-25.2% oleic, 72.5% stearic, alkanals may be attributed to oxidation-decomposition of and '2.3% palmitic; mixed triolein (HormeIHristearin higher to lower molecular weight aldehydes (2c, I 0, 12-15), 26.5% oleic, 66.2% stearic, and 7.3% palmitic. Double bond but no experimental evidence has been reported which locations (by reductive ozonolysis) (23) were as follows: might indicate the immediate precursors of the C 6-9 pure triolein->99.5% at the delta 9 ca~bon position, ~ trace 2-alkenals. at the delta 12 position; both the estenfied oleo-steann and In addition to aldehydes, other volatiles identified from the mixed triolein-tristearin-l00% at the delta 9 position. oxidized oleate include: mono- and dibasic acids (9,16,18), None of the three model triglycerides had any detectable methyl ketones (9,19), fatty acid esters (7,9, I 0), n-alcohols (4,9,10), gamma and delta lactones (9,17), and hydro isolated trans (by infrared-isolated trans absorption at 926 cm- 1), free fatty acids (by titration with NaOH), or carbons (4,9,10,19). Many of these compounds plus the mono- or diglycerides [by thin layer chromatography alkanals and 2-alkenals are suspected of contributing odors (TLC)-precoated-0.25 mm thick-Silica Gel TLC plates and flavors to oxidized lipids (2,8). Even though seven classes of compounds have been identified from oxidized developed in petroleum ether-diethyl ether-acetic acid oleate, most of the studies were limited to the identifica- mixture 70:25:5J. Specific details of the volatile collection and analysis B~eflY, 1 Presented at the AOCS meeting, Philadelphia, September, procedure have been reported (20,21). 2,cc of each 1974. model triglyceride was heated at 192 C In a stamless-steel 62 FEBRUARY, 1977 SELKE ET AL: HEATED TRIOLEIN VOLATILES 63 7 11 13 14 1& II 20 22 2421 32 3& 40 44 41 52 5& &0 &4 51 70 7274 7& 71 10 12 III II II II III III I hi II I 111I11 1111111 11111111111'1111111111 Illb II III II II I 1 II &I 10 12 15 17 IS 21 2& 30 34 3. 42 4S 50 54 51 &2 " 71 73 75 7771 11 I·Octanol .::12 lal \18 X10 , H17 A2 .::18 Nonanal 2·0ecenal Octanal I 2·Undecenal Ib] K12 K13AI3U4AI4KI5A15 AI2 \ H3 I , I H -45 0 25 50 75 100 ISO 225 Temperature. I I I I 25 50 75 100 Time. min. FIG. 1. Gas chromatogram of volatile compounds from thermally oxidized triolein (curve a) and randomly esterified oleo-stearin (curve b). Numbers 1-82 relate to GC peaks in both curves. H = hydrocarbons; A =aldehydes; K =methyl ketones; numbers adjacent to letters indicate number of carbon atoms in compound. ~1 =unsaturated in the 1 position, etc. The following GC peaks were the result of unresolved but identified compounds: Curve "a," peak 34-octanal and hexanoic acid; curve "b," peaks 22-nonane and I-nonene; 29-decane, and I-decene; 34-octanal and hexanoic acid; and 79-heptadecane and heptadecene. container for 10 min. During each of the ID-min heating favorable comparisons (names of compounds), along with periods, volatiles generated from the hot oils were con 13 similarity indexes, can be listed on a print-out sheet. The tinually swept out of the container with air and collected similarity index indicates how well the unknown spectrum directly on a gas chromatograph.!GC) column (glass-I 4 ft matches a particular known spectrum. The unknown's mass X 4 mm ID column, packed with 10% OV-17 on Chromo spectrum is then manually inspected and compared against sorb G) cooled to -60 C. Subsequently, the volatiles were the mass spectra of those compounds listed on the print-out separated by temperature programming (nonlinear from sheet for positive identification. At times none of the listed -60 C to 0 C then 2 C/min to 250 C) and as they eluted off compounds correspond to the unknown and at other times the GC column they were simultaneously monitored by a no compounds are even listed on the print-out sheet; under flame ionization detector, by smelling at the GC exit port those circumstances, manual interpretation of the unknown and by a mass spectrometer (MS) (nuclide 12-90 double spectrum is attempted. Fortunately, standard mass spectra focusing-magnetic scanning) set to scan from mle 10 to 450 were available in our library of standard spectra as well as in every 9 sec. Output of the MS was fed to a computer for the literature for each of the identified compounds, except later processing. Volatiles were identified from their mass in one instance. spectra and the identifications confirmed by comparing GC During the identification procedure, the technique of elution temperatures of the volatiles with those of authen mass chromatography (MC) is frequently used to determine tic compounds.