Food Lipids: Chemistry, Nutrition, and Biotechnology

distillate. In case of the distillation method, volatile substances are distilled off with
steam. Then the distillate is allowed to react with the TBA reagent in an aqueous
medium. The advantage of the distillation method is the absence of interfering substances.
In the extraction method, TBA-reactive substances (TBARSs) are extracted
from food material into an aqueous medium (i.e., aqueous trichloroacetic acid) prior
to color development with the TBA reagent. The main disadvantages of both of these
methods are long assay time and possibility of artifact formation. In the direct assay
method, lipid sample (oil) reacts with the TBA reagent and the absorbance of the
colored complex so prepared is recorded. The direct assay method is simple and
requires less time for sample preparation.
There are certain limitations when using the TBA test for evaluation of the
oxidative state of foods and biological systems because of their chemical complexity.
Dugan [28] reported that sucrose and some compounds in wood smoke react with
the TBA reagent to give a red color that interferes with the TBA test. Baumgartner
et al. [29] also found that a mixture of acetaldehyde and sucrose when subjected to
the TBA test produced a 532-nm absorbing pigment identical to that produced by
MA and TBA. Modifications of the original TBA test have been reported by Marcuse
and Johansson [30], Ke and Woyewoda [31], Robbles-Martinez et al. [32], Pokorny
et al. [33], Shahidi et al. [34,35], Thomas and Fumes [36], and Schmedes and Holmer
[37]. However, it has been suggested that TBARS values produce an excellent means
for evaluating the relative oxidative state of a system as affected by storage condition
or process variables [38]. Nonetheless, it is preferable to quantitate the extent of lipid
oxidation by a complementary analytical procedure in order to verify the results.
Several attempts have been made to establish a relationship between TBA values
and the development of undesirable flavors in fats and oils. It has been shown
that flavor threshold values correlate well with the TBA results of vegetable oils,
such as those of soybean, cottonseed, corn, safflower [17], and canola [5].
2. Oxirane Value
The oxirane oxygen or epoxide groups are formed during autoxidation of fats and
oils. The epoxide content is determined by titrating the oil sample with hydrobromic
acid (HBr) in acetic acid and in the presence of crystal violet, to a bluish green end
point. This method has been standardized by the American Oil Chemists’ Society in
their tentative method (Cd 9-57) [39], but it is not sensitive and lacks specificity.
The HBr may also attack �,�-unsaturated carbonyls and conjugated dienals, and the
reaction is not quantitative with some trans-epoxides. Fioriti et al. [40] found that
picric acid was the best of several acidic chromophores in its reaction with epoxides.
Despite a nonquantitative reaction, the product concentration followed Beer’s law.
This method has been found to be particularly well suited for the determination of
epoxides in heated fats and oils where the oxirane content is often less than 0.1%.
3. p-Anisidine Value
p-Anisidine value (p-AnV) is defined as 100 times the optical density measured at
350 nm in a 1.0-cm cell of a solution containing 1.0 g of oil in 100 mL of a mixture
of solvent and reagent, according to the IUPAC method [41]. This method determines
the amount of aldehyde (principally 2-alkenals and 2,4-alkadienals) in animal fats
and vegetable oils. Aldehydes in an oil react with the p-anisidine reagent under acidic
conditions. The reaction of p-anisidine with aldehydes affords yellowish products,
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