Department of Foods & Nutrition, University of Manitoba, Winnipeg, Manitoba R3T 2N2, CANADA
Frying performance of vegetable oils is dependent on fatty acid composition and minor components. In this study evaluation of frying stability and changes in minor components have been assessed. The frying performance of regular canola oil was compared to high oleic low linolenic, low linolenic, and high oleic canola oils. Formation of free fatty acids, polar and polymer components were used as indices of frying stability. Modified canola oils did not show any improvement in frying stability compared to regular canola oil. Examination of minor components, particularly tocopherols showed that they were significantly altered during fatty acid modification. The changes in tocopherols could explain, in part, the reason for the lack of improvement in frying stability of the modified oils.
KEYWORD: Frying stability, fatty acids, tocopherols, vegetable oils.
A number of studies reported that modified vegetable oils show some improvement in frying stability over the corresponding regular oils (Dobarganes et al., 1993; Eskin et al.,1989; Warner and Mounts, 1993). However, no significant improvements in frying stability were shown by other researchers studying modified vegetable oils (Mounts et al., 1994a,b). These findings suggest fatty acid composition alone may not explain the stability of frying oils. Oilseed breeders have tended to focus their attention solely on altering fatty acid composition to improve the stability of vegetable oil with little or no attention given to the minor constituents in oils. Since frying stability of oil cannot always be accurately predicted on its fatty acid composition, it appears that minor components must play a significant role in oil stability. Any significant reduction in the levels of these minor constituents, particularly tocopherols, could dramatically alter the stability of the modified oils. This study compares the composition and frying stability of regular canola oil (CAN) with that of three modified canola oils; high oleic and low linolenic (HOLLCAN), low linolenic (LLCAN), and high oleic (HOCAN).
MATERIALS AND METHODS
Oil. Four commercially refined, bleached and deodorized canola oils were used in this study. A regular canola oil (CAN) and a low linolenic acid canola oil (LLCAN) ( CanAmera, Altona, Manitoba, Canada) and a high oleic canola oil (HOCAN) and high oleic and low linolenic canola oil (HOLLCAN) (Intermountain, Idaho Falls, Indiana, USA) All oils contained citric acid with no other preservatives were added.
Methods. Fatty acid composition was analysed by the AOCS official method Ce 1-62; (1990). Peroxide values (PV) were determined using the AOCS official method Cd 8-53 1990). Free fatty acids (FFA) were monitored by the AOCS official method Ca 5a-40 (1990) as well as using the Veri-FryRPro FFA-75 quick test method (Test Kit Technologies, Inc., New Jersey, USA). Total polar compounds (TPCs) were performed following the procedure of Petukhov (1996). High performance size exclusion chromatography (HPSEC) was performed on the polar fractions based on the method of Dobarganes and Perez-Camino (1988).
Frying Performance. Oils were heated at 175±2oC for 12 hours/day (for a total of six days) in a 2L capacity domestic deep fryer. Samples of fresh and heated oils were flushed with nitrogen and stored at -20oC until analysed. Samples were removed daily with the oil replenished to the 2L volume each morning.
RESULTS AND DISCUSSION
The freshly refined oils had FFA values less than 1% and PV of less than 1 meq/kg indicative of good quality oils. The fatty acid composition of the different canola oil is shown in Table 1.
Table 1. Percent fatty acid composition of regular and modified canola oils
Fatty acid CAN LLCAN HOCAN HOLLCAN
C18:1 57.4 62.7 74.4 74.8
C18:2 21.2 24.6 9.3 12.0
C18:3 10.2 3.0 6.7 3.1
Saturates 7.5 7.2 7.0 8.0
PUFAs 31.5 27.7 16.0 15.1
The total tocopherol content of CAN, LLCAN, HOCAN and HOLLCAN was 565, 468, 601 and 893 mg/kg, respectively. It was evident that the oils higher in polyunsaturated fatty acids (PUFA) were lower in total tocopherols.
FFAs increased in all oils during frying although there were no significant differences between any of the oils. With respect to total polar compounds (TPCs) however, LLCAN and HOCAN displayed faster rates of TPC formation compared to HOLLCAN and CAN (Figure 1). No significant differences were observed between CAN and HOLLCAN in spite of the much lower level of PUFA and higher levels of tocopherols in HOLLCAN compared to CAN (Table 1).
An examination of tocopherol changes during frying of canola oils is shown in Figure 2. Total tocopherols degraded at much faster rate in HOCAN and LLCAN compared either HOLLCAN and CAN. Tocopherol levels were reduced by 50% within 3 to 6 hours for HOCAN and LLCAN compared to more than 48 hours for HOLLCAN and over 72 hours for RCAN. Since tocopherols acts as antioxidants, oils in which tocopherols are rapidly degraded suggest a less stable oil. Among the canola oils, HOCAN and LLCAN exhibited faster rates of tocopherol degradation as well as significantly faster rates of TPC formation.
The results obtained from the HPSEC analysis suggested a steady increase in dimers and higher polymers during frying, however the data proved difficult to interpret.
Results of this study showed that fatty acid composition was not the sole determinant of frying stability. Variation in tocopherol levels and degradation rates appeared to explain some of the differences in frying stability between the canola oils. Of the oils examined CAN and HOLLCAN proved to be more stable to frying than either HOCAN or LLCAN. In future breeding programs plant breeders need to be cognizant of any changes in minor components accompanying modification of fatty acids.
1. American Oil Chemists' Society, Official Methods and Recommended Practices, Fourth Edition, Champaign, IL, 1990.
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6. Mounts, T.L., Warner, K., List, G.R., Neff, W.E. and Wilson, R.F. 1994b. J. Amer. Oil Chem. Soc. 71: 495-499.
7. Petukhov, I. 1996. Frying performance and storage stability of potato chips fried in genetically modified canola oils M.Sc Thesis, University of Manitoba, Winnipeg, Manitoba, Canada.