Lipid oxidation : stability of low linolenic acid canola cultivars and determination by HPLC analysis
Date
1988
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Degree Level
Doctoral
Abstract
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.
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Degree
Doctor of Philosophy (Ph.D.)
Department
Food Science
Program
Food Science