Lipid Technology October 2007, Vol. 19, No. 10 229

Feature

Increasing gamma- and delta-tocopherols in oilsimproves oxidative stability

Kathleen Warner

Kathleen Warner is Lead Scientist for Edible Oil Research at the U.S. Department of Agriculture’s National Center for AgriculturalUtilization Research, 1815 North University Street, Peoria, IL 61604, USA; tel: +1-309-681-6584; fax: +1-309-681-6668; e-mail:kathleen.warner@ars.usda.gov

Summary

Over the past two decades, plant geneticists have revolutionized how fatty acid compositions of vegetable oils are optimized to improveoxidative stability and functionality. Now, the expertise of plant geneticists is reaching beyond altering fatty acids to changing other oilcomponents such as tocopherols. Basic lipid research on optimizing tocopherol profiles and ratios in vegetable oils is providing informationfor geneticists to breed the next generation of oxidatively stable vegetable oils. This review will discuss three studies; first, a basic study todetermine the oxidative stability provided by the addition of pure gamma and delta tocopherols to oils treated to remove naturaltocopherols; second, a practical study to evaluate the oxidative stability of mid-oleic sunflower oil from seeds modified by plant breeding tocontain high amounts of c- and d-tocopherols; and third, a frying test to determine the effects of gamma tocopherol addition.

IntroductionSunflower oil with its high levels of linoleic acid (75–80%) is notoxidatively stable. However, plant breeding of sunflowers hasmodified the fatty acid composition of sunflower oil to increaseoleic acid to high (>80%) and moderate (50–60%) amounts, whichhas significantly enhanced the oxidative stability of this oil. Thefatty acid composition is well known to appreciably affect theoxidative stability of vegetable oils, but does not account for itsstability entirely. Zambiazi (1) estimated that only about half ofthe stability of an oil can be explained by fatty acid composition.Therefore, some of the other 3% of oil components than triacyl-glycerols can influence stability. Tocopherols are well recog-nized as effective antioxidants both endogenously and as addi-tives, but the relative order of effectiveness of the four homolo-gues varies in different literature reports. In a review of eightstudies from the literature, Frankel (2) summarized the relativeorder of the antioxidant activities of tocopherols in vitro withthe most common order as gamma > delta > beta > alpha.

In previous oil stability tests, we found that soybean oils weremore stable when oxidized in the dark than sunflower oilsalthough the soybean oils contained 8–9% of the highly unstablelinolenic acid whereas the sunflower oils had no linolenic acid(3). A review of the tocopherol profiles of soybean and sunfloweroils showed widely different patterns between the two oils thatmay account for the relative instability of sunflower oil. Sun-flower oil is high in a-tocopherol, but low in c- and d-tocopherolsand soybean oil is low in a-tocopherol, but high in c- and d-toco-pherols. To determine if these differences in tocopherol profilescould help explain variations between the oxidative stabilitiesof sunflower and soybean oils, we conducted a basic study onthe effects of adding various ratios and amounts of pure a-, b-, c-and d-tocopherols to stripped soybean and sunflower oils.

Study 1 – Which tocopherol profile is better?Soybean or sunflower?In this basic study, commercially processed (refined, bleached,deodorized) soybean and sunflower oils were stripped (purified)

of all minor oil constituents leaving only the triacylglycerols (4).A mixture of pure tocopherols typical of soybean oil (120 ppm a,10 ppm b, 610 ppm c and 260 ppm d) was added to stripped sun-flower oil and soybean oil. Likewise, a mixture of pure tocopher-ols with the tocopherol profile typical of sunflower oil (610 ppma, 10 ppm b, 30 ppm c and 10 ppm d) was added to stripped soy-bean oil and sunflower oil. These four oils were oxidized at 608C,and peroxide values (PV) and total volatile compounds weremeasured as indicators of oxidation. Substituting the toco-pherol profile typical of soybean oil into sunflower oil and thatof sunflower oil into soybean oil significantly changed the stabi-lity of these oils on the basis of PV (Fig. 1). After 2 and 4 days ofstorage in the dark at 608C, purified soybean oil containing thesoybean tocopherol profile had significantly lower PV than the

i 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI 10.1002/lite.200700077

Figure 1. Peroxide values in stripped soybean or sunflower oilscontaining pure tocopherols typical of sunflower or soybean oiland aged at 608C.

230 October 2007, Vol. 19, No. 10 Lipid Technology

other three samples. On the other hand, stripped sunflower oilwith the sunflower tocopherol profile developed significantlyhigher PV than the other three samples. The substitution of thesunflower tocopherol profile into soybean oil resulted in signifi-cantly increased PV at both 2 and 4 days compared to soybeanoil with the soybean tocopherol profile. Adding soybean toco-pherols into sunflower oil resulted in significantly decreased PVcompared with sunflower oil with sunflower tocopherol profile.Frankel reported the same effect on the formation of conjugateddienes with c-tocopherol (2). A positive effect of substituting soy-bean tocopherols into sunflower oil was noted for total volatilecompounds (data not shown).

Study 2 – Breeding sunflowers to contain highamounts of gamma- and delta-tocopherols

In the study just discussed, we found that after oxidation in thedark, oils containing different levels of added pure tocopherols(low a, low b, high c, and high d) had significantly better stabilitythan oils containing added high a, low b, low c and low d puretocopherols. These results agreed with those from most litera-ture studies on tocopherols (2) showing that c-and d-tocopherolswere better antioxidants than a-tocopherol on the basis of PV.From these results, we recommended to sunflower plant geneti-cists that they develop sunflower seeds with increased amountsof gamma and delta tocopherols to potentially enhance the oxi-dative stability of sunflower oil. Dr. Jerry Miller, a plant geneti-cist with the U.S. Department of Agriculture in Fargo, ND, USAcrossed mid-oleic sunflower seeds with sunflower seeds natu-rally high in c- and d-tocopherols. Mid-oleic sunflower seedswere chosen for altering their tocopherol profile rather thanhigh linoleic sunflower seeds because mid-oleic sunflower isnow the new commodity sunflower in the USA. The tocopherolcompositions of the resulting mid-oleic sunflower oils and amid-oleic sunflower oil control with the regular sunflower toco-pherol profile are listed in Table 1. The control had the typicaltocopherol profile found in sunflower oils with high a-, and lowc- and d-tocopherols. The a-tocopherol levels in the modifiedmid-oleic oils ranged from 33 to 393 ppm, the c tocopherol ran-ged from 333 to 680 ppm and the d tocopherol ranged from 64to 221 ppm. As the amount of a tocopherol decreased, the levelof b tocopherol decreased correspondingly; whereas the levels ofc- and d-tocopherol increased. Most of these modified mid-oleicsunflower oils had tocopherol compositions similar to the toco-pherol profile (low a, high c and d) found in soybean oils. All theoils had oleic acid contents in the range of 58–67%. To analyzethe effects of modified tocopherol profiles, we extracted the oilfrom the seeds and evaluated the crude oils for oxidative stabil-ity in oven storage tests at 608C. After aging at 608C, oils were

measured for peroxide values and hexanal content as indicatorsof oxidation. We found that when the c-tocopherol content ofmid-oleic sunflower oil was increased from its regular level of20 ppm to 300–700 ppm in the modified oils, the oxidation ofthe oils was decreased significantly compared to mid-oleic sun-flower oil with its regular low c- and d-tocopherol levels (Fig. 2).A positive correlation coefficient of 0.99 was calculated forincreasing a tocopherol and increasing PV. On the other hand,negative correlation coefficients of -.99 and -.98 were calculatedfor increasing c and d tocopherols and decreasing PV. The modi-fied oils had a-tocopherol contents of up to 300 ppm withoutnegatively affecting the stability of the oil. This finding may beimportant because a tocopherol is a precursor in foods for vita-min E and is known to help protect oil from photosensitized oxi-dation. Oils such as sunflower with high a tocopherol levels aremore stable to light oxidation than oils such as soybean withlow a tocopherol levels (3). A mid-oleic sunflower oil (sample C)in this study with a tocopherol profile of 472 ppm c-, 95 ppm d-and 289 ppm a-tocopherol had one of the best oxidative stabili-ties indicating that mid-oleic sunflower oil could be more oxida-tively stable because of the high c- and d-tocopherols and still bea good source of Vitamin E because the a-tocopherol content wasnot decreased to low levels. The hexanal contents of the modi-fied oils showed a similar pattern as with the peroxide values;however, the differences between the oils were not as distinct(data not shown). Studies have been conducted on tocopherolsto determine how and where they function, for example, as achain-breaking antioxidant (initiation, propagation, or termina-tion phase) or as a preventive antioxidant (peroxide decomposi-tion). Apparently, c- and d-tocopherols were more effective ininhibiting the formation of primary oxidation products (perox-ides) than on inhibiting secondary oxidation products (hexanal).Hexanal was significantly higher in regular mid-oleic sunfloweroil with 18 ppm c-tocopherol than in the oils with c-tocopherolin the 333-680 ppm range.

Study 3 – gamma-tocopherol effects on fryingThe current worldwide trend of avoiding hydrogenated oils con-taining nutritionally undesirable trans isomers has increasedthe need for other stable frying oils. Although c-tocopherol is

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Table 1. Tocopherol compositions of crude mid-oleic sunflower oils.

Alpha Beta Gamma Delta

(ppm)

A 393 82 333 64B 356 78 364 68C 289 56 472 95D 69 24 680 221E 33 12 656 188

Control 909 34 18 3

See text for explanation of samples A-E and control.

Figure 2. Peroxide values of crude mid-oleic sunflower oils con-taining various levels of gamma-tocopherols and aged for up to4 days at 608C.

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effective in inhibiting oxidation in a liquid salad oil, we wantedto determine if the effect was similar on the stability of fryingoils and fried food. In our study, potato chips were fried in amodel oil system of triolein with different levels of added pure c-tocopherol (5), then the chips were stored at 608C and evaluatedfor odor attributes by sensory analysis and for volatile com-pounds by purge and trap gas chromatography-mass spectrome-try. Oils sampled after 1, 3, and 6 hours of frying time showedthat c-tocopherol significantly inhibited polar compound pro-duction in the triolein used for frying. In addition, c-tocopherolinhibited oxidation of the fried food even when only very lowlevels of retained c-tocopherol were present in the frying oil orpotato chips. Formation of nonanal, a major volatile compoundformed from the oxidation of oleic acid, was inhibited by c-toco-pherol in aged potato chips. Odor analysis of the aged potatochips showed that samples with 0 ppm c-tocopherol had a ran-cid odor after aging for 4 days. Potato chips fried in triolein with400 ppm c-tocopherol had no rancid odors; however, as the levelof c-tocopherol decreased in the triolein and in the potato chipsduring aging, a plastic odor characteristic of oxidized oleic acidwas detected.

Conclusions

In our first study, we found that the oxidative stability of sun-flower oil could be enhanced if we substituted its natural toco-pherol profile with the profile typical of soybeans. We extendedthis basic study to a practical application in which plant geneti-

cists modified the tocopherol profile of mid-oleic sunflowers tohave increased levels of c- and d-tocopherols. The resulting oilshad significantly improved oxidative stability compared to thecontrol oil with low levels of c- and d-tocopherols and highamounts of a-tocopherol. These results could have significantimpact on the utilization of mid-oleic sunflower oil in the fryingindustry with enhanced oxidative stability and in productsrequiring longer shelf life. In addition, sunflower oil wouldremain a good source for vitamin E because of moderateamounts of a-tocopherol. Finally, c-tocopherol was shown toeffectively inhibit oxidation in both frying oil and aged friedfood.

Future research on altering tocopherol profiles

Our future research on genetically modified tocopherol profilesof sunflower seeds will include growing larger quantities of mid-oleic sunflower seeds bred to increase levels of c- and d-tocopher-ols. These seeds will be commercially processed into refined,bleached, deodorized oil and then the oil will be evaluated foroxidative stability and for frying stability.

Research on modifying tocopherols in sunflowers could alsobe extended to alter the tocopherol profiles in other oilseeds toenhance their oxidative stabilities. Table 2 shows the tocopherolcompositions of some of the major vegetable oils. Soybean oil isunique for its high amounts of the very effective antioxidant, d-tocopherol and it also contains high amounts of c-tocopherol.Canola (low erucic acid rapeseed), corn and cottonseed also havesubstantial amounts c-tocopherol. On the other hand, safflower,rice bran, palm and peanut are all low in c-tocopherol. If thetocopherol contents of these oilseeds can be genetically modi-fied, their oxidative stabilities could very well be enhanced byincreasing c- and d-tocopherols.

References

1. Zambiazi, R.C. et al. (1998) Lipid Technol., 10, 58–62.2. Frankel, E.N. (2005) Antioxidants, in Lipid Oxidation, 2nd

edition, The Oily Press, Bridgewater (UK), 209–258.3. Warner, K. et al. (1989) J. Am. Oil Chem. Soc., 66, 558 –564.4. Warner, K. (2005) J. Agric. Food Chem., 53, 9906–9910.5. Warner, K., et al. (2003) J. Agric. Food Chem., 51, 623–627.

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Table 2. Tocopherol compositions of major vegetable oils.

Alpha Beta Gamma Delta

(ppm)

Canola (low erucic acidrapeseed)

175 0 415 10

Corn 100 10 400 20Cottonseed 350 0 300 0Palm 190 0 0 0Peanut 110 0 130 5Rice Bran 330 18 60 0Safflower 340 0 35 5Soybean 120 10 610 190Sunflower 610 10 30 10