Lipid oxidation : stability of low linolenic acid canola cultivars and determination by HPLC analysis
Kay, elizabeth Mary
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The effect of plant breeding to alter the fatty acid composition of canola on the oxidative stability in experimental low linolenic acid genotypes, as compared to present day commercial canola cultivars, was investigated. Selected lipid composition factors and press characteristics which might be affected by this genetic manipulation were also investigated. HPLC techniques to determine the degree and state of oxidation in an oil were developed and applied to the canola oils in this study. Preliminary studies showed that oxidative stability was strongly influenced by content of non-triglyceride components, in particular phosphorus (phospholipid) content, and so variations were kept to a minimum by uniform processing. Surprisingly, there was no significant correlation between linolenic acid content and oxidative stability among the oils investigated. The high linoleic, low linolenic acid cultivar HLLL(UM), subsequently named Stellar, had ca. 3% linolenic acid and was the most stable, but HLLL(AC) had ca. 6% linolenic acid and was less stable than the commercial cultivars, Regent, Westar and Tobin which had ca. 8-10% linolenic acid. The oxidative stability was found to be most closely correlated to the iodine value and inherent stability values. The latter were calculated from the relative stabilities of the component fatty acids and their contents in the oils. The B. campestris cultivar, Tobin, exhibited greater stability than expected from its fatty acid composition, as compared to the B. napus cultivars. It is likely that genetic manipulation to alter the fatty acid composition also altered other characteristics. Cold press experiments showed that press characteristics at constant press parameters, as determined by throughput and residual oil content, were affected by seed moisture, species and cultivar. The experimental low linolenic acid cultivars had slightly poorer characteristics than other B. napus cultivars which could not be explained by seed moisture contents. Analysis of triglycerides by reverse-phase HPLC found that the distribution of triglyceride species was related to fatty acid coaposition, but in a complex fashion. It is likely that the genetic manipulation which altered the biosynthesis of the fatty acids also altered the biosynthesis of the triglycerides. Two complimentary HPLC techniques were developed. The amount of oxidised triglycerides separated and determined by analysis of straight oil on normal-phase HPLC was directly correlated to the peroxide value of the oil sample (R2 = 0.94). This technique was simple, rapid and reproducible, but provided little information on oxidation processes. The a.ount of hydroxy fatty acids separated by normal-phase HPLC after derivatisation was also directly correlated to the peroxide value of the sample (R2 = 0.99). This technique, while requiring longer preparation and analysis times, separated and determined individual hydroperoxide isomers from linoleic and linolenic acids. It thus provided information on their content and nature during oxidation.