University of Manitoba, Department of Foods and Nutrition, Winnipeg, Manitoba, R3T 2N2 Canada, E-mail: przybyl@ms.umanitoba.ca
a Federal University of Pelotas, Pelotas, Rio Grande do Sul, Brazil
ABSTRACT
Canola oils with modified fatty acid composition were evaluated in this study. It has been observed that modification of fatty acid composition also cause changes in the content of tocopherols and sterols. Canola oils with modified fatty acid composition showed different storage stability as measured by peroxide value, off-flavour formation and conjugated dienes. Lower contribution of polyunsaturated fatty acids in some oils caused lower stability when compared to regular canola oil. Changes in tocopherols and sterols content during accelerated storage were dependent on the type of oil but not directly related to the contribution of unsaturated fatty acids.
KEYWORDS: Tocopherols, off-flavour, linolenic acid, sterols
INTRODUCTION
Recent trend in modification of vegetable oils is to modify fatty acid composition as main aim to improve nutritional and performance quality. Canola oil with modified fatty acid composition, lower content of linolenic acid and elevated contribution of oleic acid, separately or in combination, showed slight improvement in oxidative stability. Further evaluation of data showed that improvement was not “proportional” to the decrease in linolenic and linoleic acid content. Similar results were found within other modified oils. Oilseed breeders assumed that modification of fatty acid composition will not effect other components in oils. As shown, oxidative stability can not be directly related to changes in PUFA only.
MATERIALS AND METHODS
Oils. In this project canola oils with modified fatty acid composition such as: low linolenic acid (1.95% 18:3; LLCAN); low linolenic and high oleic acids (2.1% and 78%, respectively; HOLLCAN), high oleic content (77.8%; HOCAN); regular canola (CAN), soybean (SOY) and sunflower (SUN) oils; low linolenic acid flax oil (LLFLAX) were used.
Methods: Fatty acid composition was analysed by AOCS method Ce 1-62; Peroxide value by AOCS procedure Cd 8-53; Conjugated dienes by AOCS Ti 1a-64; Off-flavour components by method described by Przybylski (1991); Tocopherols and tocotrienols by normal phase HPLC, AOCS method Ce 8-89.
Accelerated Storage: Oils were stored in glass beakers without cover. Beakers were filled with oil untill ratio of surface area to volume was one. Oils were stored in an oven with enforced circulation at 650C without light presence and under fluorescent light, 2500 Lux, at 350C.
RESULTS AND DISCUSSION
Evaluated oils for their oxidative stability showed different resistance to oxidation as measured by peroxide value (Fig 1 and 2). Canola oils with lowered content of linolenic acid showed better oxidative stability however this improvement was not proportional to the reduction of this acid. In all storage conditions canola oil with reduced amount of linolenic acid and increased contribution of oleic acid (HOLLCAN) showed the best storage stability. Using off-flavour formation as measurement of stability, smaller differences between these oils were observed during storage with light presence. This indicates that other components may have accelerated decomposition of hydroperoxides because off-flavour components are the secondary products formed during oxidation of unsaturated fatty acids. These compounds with catalytic effect can be directly related to rancid and/or off-flavour formation in oils/fats and fat containing foods.
Similar behaviour of oxidative stability was observed when conjugated dienes were measured (data are not included in this proceeding version of paper) as assessment of oxidation. These products are formed as primary products during oxidation of unsaturated fatty acids.
These data shown that reduction of the content of polyunsaturated fatty acids in oils/fats can only partially improve stability and prevent formation of rancidity. Extended work done on evaluation of the effect of other components on oil oxidative stability, showed that fatty acid composition can answer only 50% of this characteristics of vegetable oil (Zambiazi 1997).
Table 1. Kinetic of tocopherols changes in modified canola and selected oils during accelerated storage (ppm/hr).
Oil α− Tocopherol γ−Tocopherol δ−Tocopherol Total
Light Dark Light Dark Light Dark Light Dark
CAN 1.47 1.18 0.68 0.60 0.0 0.0 2.21 1.77
LLCAN 1.31 0.79 0.45 0.58 0.0 0.0 1.76 1.37
HOCAN 1.04 0.88 0.31 0.13 0.0 0.0 1.35 1.01
HOLLCAN 0.85 0.72 0.22 0.10 0.0 0.0 1.07 0.82
SOY 1.19 0.71 1.48 1.04 1.0 0.50 3.66 2.24
SUN 1.53 1.27 0.0 0.0 0.0 0.0 1.53 1.27
LLFLAX 0.62 0.40 0.19 0.18 0.0 0.0 0.81 0.58
The total tocopherol content of listed in Table 1 oils was 565, 468, 601, 893, 986, 421, 173 mg/kg, respectively. Rate of disappearance of tocopherols during storage of oils was linolenic acid content dependent. Such oils as canola and soybean showed the fastest rate of depletion of tocopherols during storage at both conditions. Modified canola oils despite lower content of linolenic acid showed relatively fast degradation rate of tocopherols. This can be explain by the increase of the amount of linoleic acid at in oils where the amount of linolenic was reduced (LLCAN). Isomers of tocopherols did not disappeared equally in analysed oils as has been suggested by antioxidative activity when individual components were evaluated (Lea, 1960). It has been found that when mixture of tocopherols is present in an oil, the highest rate of depletion will have an isomer which is present at the highest concentration.
CONCLUSIONS
Results of this study clearly showed that modification of fatty acids in oils is also followed by changes in composition and content of other components. The highest changes were observed in tocopherols and sterols. This can explain why oils with lowered content of linolenic did not show expected improvement in oxidative stability. Effect of other component, mainly minor compounds, can overweight reduction in the content of linolenic acid in oil.
REFERENCES
1. American Oil Chemists’ Society, Official Methods and Recommended Practices, Fourth Edition, Champaign, IL, 1990.
2. Lea, C.H. 1960. J. Sci. Food Agric. 11:212-216.
3. Przybylski, R. 1991. Proceedings of the Eighth International Rapeseed Congress, Saskatoon, pp.861-866.
4. Zambiazi, R.C. 1997. The Role of Endogenous Lipid Components on Vegetable Oil Stability. Ph.D. Thesis, University of Manitoba, Winnipeg, Manitoba, Canada.
