Agric. Biol Chem., 47 (10), 2251 -2255, 1983 2251

Influence of the Positions of Unsaturated Acyl Groups in on Autoxidation

Dong Ki Park, Junji Terao and Setsuro Matsushita Research Institute for Food Science, Kyoto University, Uji, Kyoto 611, Japan Received March 3, 1983

The influence of the positions ofunsaturated acyl groups in the glycerides on autoxidation was analyzed in relation to synthesized and soybean . No discrepancies in rates of autoxidation of unsaturated acyl groups at different positions in the glycerides (between PLP and PPL, between PLnP and PPLn) was observed. Likewise, no discrepancies were observed before or after interesterification of synthesized mixtures or soybean oil triglyceride. In the case of trilinolein, peroxidation occurred at random at both the a- and ^-positions. (P, ; L, ; Ln, linolenic acid.)

Little has been reported about the factors in effect of the positions of unsaturated acyl structures which affect autoxidation. groups in glycerides on autoxidation with the Raghuveer and Hammond1* have reported use of synthesized triglycerides (1,3-dipal- that the stability of most decreases after mitoyl-2-olein and 1,2-dipalmitoyl-3-olein; randomization and proposed a theory based 1,3-dipalmitoyl-2-linolein and 1,2-dipalmi- on the hexagonal packing of the acyl chain in toyl-3-linolein; l ,3-dipalmitoyl-2-linolenin and glycerides in the molton state. They suggested l,2-dipalmitoyl-3-linolenin). The effect of in- that acyl groups at the Sn-1 and Sn-3 positions teresterification of triglyceride on autoxidation in glyceride are oxidized more rapidly than was also analyzed. thoseat Sn-2, and that the preferential place-

ment of unsaturated acyl groups at Sn-2 in MATERIALS AND METHODS many natural fats increases their stability. Catalano et al.2) analyzed the autoxidation of Materials. Oleic acid, linoleic acid, linolenic acid, 1,3- mono-unsaturated triglyceride isolated from dipalmitin, trilaurin, and trilinolein, all 99% pure, were natural fats and interesterifled fats with the use purchased from Sigma Chem. Co., Missouri. Trilinolein was purified by column chromatography, using Florisil of argentation thin layer chromatography. (100 ~200 mesh) to remove any peroxides. Soybean oil of They concluded that the composed commercial grade was provided by Nakarai Chem. Co. mainly of l,3-distearoyl-2-olein (SOS) have Ltd., Kyoto. Oxalyl chloride was purchased from greater resistance to oxidation than those con- Katayama Chem. Co., Osaka. Soybean oil triglyceride was taining a high percentage of l,2-distearoyl-3- prepared by the same method as that described in a olein (SSO). Hoffman et al?) have suggested previous paper.5) ,3-disaturated-2-unsaturatedthatl triglycer- Synthesis of triglyceride. l,3-Dipalmitoyl-2-linolein ides are more stable than l,2-disaturated-3- (PLP), PPL, POP, PPO, PLnP, and PPLn were synthe- unsaturated triglycerides when the respective sized by the same method as that described in a previous paper. 6) compositions of the fatty acids are identical. Lauet al.*] have reported that the glyceride Autoxidation. The sample (200 mg) was placed in a glass structure does not appear to affect the rate of vial and autoxidized by incubating at 37°C in the dark. oxidation of triglycerides by altering the The peroxide value was measured according to the method substance. used by Asakawa and Matsushita.7) Consumption of The purpose of this work is to clarify the dissolved oxygen was measured by the same method as 2252 D.K.Park, J. Terao and S. Matsushita that described in a previous paper.8) its peroxide value at intervals of elapsed time. No discrepancies in the rate of autoxidation Interesterification. Interesterification was carried out by the same method as that described in a previous paper.6) were observed for l,3-dipalmitoyl-2-linolenin (PLnP) and 1 ,2-dipalmitoyl- 3-linolenin Lipase hydrolysis of trilinolein monohydroperoxide. (PPLn) (curves A and B). Likewise, no discrep- Lipase hydrolysis of trilinolein monohydroperoxide was ancies were detected for l,3-dipalmitoyl-2- carried out by the same method as that described in a linolein (PLP) and l,2-dipalmitoyl-3-linolein previous paper.9) The products were then separated by thin layer chromatography on a Silica gel G layer (0.5 mm X100 thick), with a solvent system consisting of «-hexane, ethyl 12- ether, and acetic acid (60:40: 0.75, v/v). Spots were de- tected with a Shimadzu CS-910 dual-wavelength thin Ai ci/ layer chromatoscanner after the products had been spray- ed with a solution of 70% H2SO4 in saturated potassium dichromate,and heated at 110°C for 10 min. " I I

GC-MS analysis. Lipase hydrolyzed products were char- acterized by GC-MS according to the same method as that described in a previous paper.9) > -\ I /

RESULTS Autoxidative stability of synthesized triglyc- 0 3 6 9 12 F erides Day Figure 1 shows the autoxidative stability of Fig. 1. Autoxidative Stability of Synthesized Tri- synthesized triglycerides which contained un- glycerides. saturated acyl groups at different positions. All A, ALnP; B, PPLn; C, PPL; D, PLP; E, PPO; F, POP. Autoxidation was carried out by incubating at 37°C in the of these triglycerides were in a liquid state at dark. The extent of oxidation was determined by analysis room temperature. The degree of oxidation of of the peroxide value. P, palmitic acid; O, oleic acid; L, each triglyceride was determined by analysis of linoleic acid; and Ln, linolenic acid.

100-K

50-

4 Time(hr) Fig. 2. Consumption of Dissolved Oxygen during Autoxidation of Synthesized Triglycerides. A, PLnP; B, PPLn; C, PPL; D, PLP; E, POP; F, PPO. A solution consisting of 10/d oftriglyceride and 0.1 mg of methyl of linoleic monohydroperoxide was put into a 10ml test tube. After the addition of 4ml of 0.1 mborate buffer pH 7.0, the solution was blended by a Vortex mixer, and then sonicated with ultrasonic vibration for 2 min at 0°C. The solution (4ml) was put into a cell (inner volume of5 ml, closed system) with an oxygen electrode. After 1 min preincubation at 37°C, 100 ^1 ofFeSO4 solution and 100 fil ofascorbic acid were added to initiate the autoxidation reaction. The rate of consumption of dissolved oxygen was recorded with an oxygen electrode. Autoxidation of Triglycerides 2253

consumption of dissolved oxygen, using an (PPL) (curves C and D). Autoxidation of 1,3- oxygen electrode. No discrepancies in rates of oxidation were observed among triglycerides dipalmitoyl-2-olein (POP) and 1,2-dipal- before or after interesterification, despite the mitoyl-3-olein (PPO) occurred very slowly, random distribution of fatty acids in glycerides broughtThe autoxidativeabout by interesterification.stability of soybean oil and in these experimental conditions, no dis- triglycerides before and after interesterification was tested (Fig. 4). Autoxidative stability was crepancies in the rates of autoxidation were determined by analysis of the rate of con- sumption of dissolved oxygen, using an oxygen perceptible. Rates of consumption of dissolved electrode. No descrepancies in rates of oxi- dation were among soybean oil triglycerides oxygen during the autoxidation of synthesized before or after interesterification (curves A and B), despite the random distribution of fatty triglycerides were recorded (Fig. 2). Auto- acids in glycerides brought about by interes- terification (Table I). xidation was carried out at 37°C with the ad- dition of 1% methyl ester of linoleic monohy- droperoxide, because oxidation did not occur without an initiating agent. The extent of oxidation was measured by means of an oxy- gen electrode until the dissolved oxygen was completely consumed. No discrepancies in the rates of consumption of dissolved oxygen were detected for PLnP and PPLn (curves A and B). This was also the case for PLP and PPL compositions of soybean oil tri- (curves C and D), and for POP and PPO (curves E and F). 100-i utoxidative stability of triglyceride mixtures before and after interesteriftcation

Figure 3 shows the autoxidative stability of 50-

trilaurinA and trilinolein mixtures before and V

after interesterification. The extent of oxida- B> tion was estimated by analysis of the rate of

1 2 Time(hr) Fig. 4. Autoxidative Stability of Soybean Oil Tri- glycerides before and after Interesterification. A, interesterified soybean oil triglyceride; B, soybean oil triglyceride. The extent of autoxidation was determied by analysis of the rate of consumption of dissolved oxygen. The conditions were the same as those described in Fig. 2.

Table I. Fatty Acid Composition of 20 4 0 60 80 Soybean Oil TG before and AFTER INTERESTERIFICATION Time(min) Fig. 3. å 'Autoxidative Stability of Trilaurin and Tri- 16:0 18:0 18:1 18:2 18:3(mol%) linolein Mixtures before and after Interesterification. T 9.7 3.7 23.3 54.0 8.1 A, interesterified triglycerides; B, a catalyst (sodium Before p QJ {2 2{4 ?()2 6J methoxide) added to triglyceride mixtures; C, the mix- ture of both triglycerides; D, trilinolein. The degree of T 9.8 3.8 23.5 54.1 7.9 autoxidation of each sample was determined by analy- p 9.6 4.0 22.6 54.4 7.9 sis of the rate of consumption of dissolved oxygen. The conditions of oxidation were the same as those described T, total fatty acid distribution; p, fatty acid at 2- in Fig. 2. position. 2254 D.K. Park, J. Terao and S. Matsushita glycerides before and after interesterification is shown in Table I. No changes in total fatty acid composition were observed. Unsaturated fatty acid in soybean oil triglycerides appeared to be located almost exclusively at the 2- position of glycerides. It was noted in partic- ular that virtually all of the linoleic acid was located at the 2-position (70%). During the interesterification, some of this fatty acid shifted from the 2-position to the 1- and 3- « **å ? FA positions in the course of random distribution. Saturated fatty acids increased from 1.9 to 13.6% and linoleic acid decreased from 70.2 to 54.5% at the 2-position after interesterifi- cation. These results indicate that the position of the unsaturated fatty acids in glycerides has noeffect on autoxidation. nalysis of lipase hydrolyzed products Trilinolein monohydroperoxide was hydro- lyzed with pancreatic lipase10) and hydrolyzed

productsA were observed on a thin layer chro- matography plate (Fig. 5). It has previously been confirmed that hydrolyzed trilinolein monohydroperoxide, releases dilinoleoylglyc- Fig. 5. Lipase Hydrolyzed Products of Trilinolein eride, monolinoleoylglyceride, and linoleic Monohydroperoxide on Thin Layer Chromatography. acid, along with the respective monohydro- Experimental system: I, linoleic acid and linoleic acid peroxide (linoleic acid monohydroperoxide, monohydroperoxide; II, monohydroperoxide; III, hy- monohydroperoxide, and di- drolysis products of trilinolein monohydroperoxide; IV, glyceride monohydroperoxide).10) In this ex- hydrolysis products of trilinolein; V, monoglyceride and monoglyceride monohydroperoxide. Developed with n- periment, spots a and b were well resolved hexane-ethyl ether-acetic acid (60 : 40 : 0.75, v/v/v). (Fraction No. III). Spot a was found to be negative to a potassium iodide solution. The hydrogenated product of spot a was identified was identified as 2-monohydroperoxylinole- as a trimethylsilyl derivatives by means of gas oylglyceride.10) It has been shown that pan- chromatography mass spectrometry. In the creatic lipase is specific for the primary link- mass spectrum, characteristic fragment ions of age in triglyceride.12) Thus yields of linoleic monostearoylglyceride appeared at m/z 487 acid and linoleic acid monohydroperoxide, [M-15], 399 [M-103, loss of CH2OSi- or monoglyceride and monoglyceride mono- (CH3)3], 341, 267, 218 [(CH3)3SiOCH2CH= hydroperoxide, depend on the location of CH2OSi(CH3)3], 129, and 103, respectively. the hydroperoxylinoleoyl group in the glyc- Therefore, spot a was confirmed to be 2- erol moiety. The relation ratios of these prod- monolinoleoylglyceride. Spot b was found to uctswere calculated with a chromatoscanner. be positive when sprayed with potassium io- The relative ratio of linoleic acid and linoleic didesolution. Through observation of the acid monohydroperoxide was found to be 6 : characteristic fragment ions m/z 173 (9-hy- 4, and that of monoglyceride and monoglyc- droxy isomer), and 259 (13-hydroxy isomer) eride monohydroperoxide, 5:3. These re- after hydrogenation and methanolysis, spot b sults indicate that autoxidation occurs at ran- Autoxidation of Triglycerides 2255 dom at both the a- and ^-positions of the No discrepancies in rates of autoxidation were observed before or after interesterifl- cation of triglycerides (Figs. 3 and 4). DISCUSSION REFERENCES It has been shown that the degree of per- oxidation is identical at the a- and /^-positions 1) K. G. Raghuveer and E. G. Hammond, /. Am. Oil of the glycerol moiety of trilinolein during hem. Soc, 44, 239 (1967). 2) M. Catalino, M. D. Felice and V. Sciancalipore, Inc. autoxidation.13) Through analysis of the rel- liment, 14, 89 (1975). ative ratios of lipase hydrolyzed products 3) G. Hoffman, J. B. A. Strnik, R. G. Polman and C. (monoglyceride and monoglyceride hydro- .Van Ossten, Zesz. Probl. Postepow Nauk Roln., C

A 146, 93 (1973). peroxide, and linoleic acid and linoleic acid W 4) F. Y. Lau, E. G. HammondandP. F. Ross, /. Am. monohydroperoxide) obtained from trilinolein Oil Chem. Soc, 59, 407 (1982). monohydroperoxide, it was confirmed that 5) D. K. Park, J. Terao and S. Matsushita, Agric. Biol. autoxidation occurs at random at both the a- Chem., 47, 121 (1983). and j6-positions of the glycerol moiety of tri- 6) D. K. Park, J. Terao and S. Matsushita, Yukagaku, linolein (Fig. 5). 32, 418 (1983). The effects of the position of unsaturated 7) T. Asakawa and S. Matsushita, Lipids, 16, 965 (1981). acyl groups in glycerides on autoxidation car- 8) D. K. Park, J. Terao and S. Matsushita, Agric. Biol. ried out with synthesized triglycerides (POP Chem., 47, 2243 (1983). and PPO, PLP and PPL, and PLnP and PPLn) 9) D. K. Park, J. Terao and S. Matsushita, Agric. Biol are shown in Figs. 1 and 2. No discrepancies in Chem., 45, 2071 (1981). rates of autoxidation were observed among 10) J. Terao and S. Matsushita, Agric. Biol. Chem., 45, 601 (1981). triglycerides in which the unsaturated acyl ll) C. B. Johnson and R. T. Holman, Lipids, 1, 371 groups were located at different positions in (1966). the glycerides. In addition, these results in- 12) C. Litchfield, "Analysis of Triglyceride," Academic dicate that selective attacks of oxygen mol- Press Inc., New York, 1972, p. 173. ecules on the unsaturated acyl groups in glyc- 13) D. K. Park, J. Terao and S. Matsushita, Agric. Biol. Chem., 45, 2443 (1981). erides do not occur.

moiety.