Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 3 Relationship between peroxide values and conjugated diene values of oxidized
vegetable oils.
Parr and Swoboda [26] have described an alternate spectroscopic method to
determine lipid oxidation of stored oils. In this assay, hydroperoxides of polyenoic
fatty acids as well as hydroxy and carbonyl compounds derived from them are converted
to more conjugated chromophores by two chemical reaction steps, namely
reduction and then dehydration (Fig. 4). These yield conjugable oxidation products
(COPs), which are measured and expressed as COP values. The first step of the
analytical procedure involves reduction of the carbonyl group by sodium borohydride,
which results in the disappearance of the characteristic ultraviolet absorption
of carbonyl compounds of oxidized polyenoic fatty acids (oxodienes). The decrease
in the absorption at 275 nm is known as oxodiene value. The next step of the COP
assay involves changes in the spectrum of the reduced compound to its dehydrated
counterpart which exhibits absorption maxima at 268 and 301 nm. The sum of these
absorbance changes at 268 and 301 nm yields the COP value whereas their relative
proportions define the COP ratio. For the calculation of oxodiene and COP results
the concentration of the final lipid solution also has to be taken into account.
B. Secondary Changes
The primary oxidation products (hydroperoxides) of fats and oils are transitionary
intermediates that decompose into various secondary products. Measurement of secondary
oxidation products as indices of lipid oxidation is more appropriate since
secondary products of oxidation are generally odor-active, whereas primary oxidation
products are colorless and flavorless. Secondary oxidation products include aldehydes,
ketones, hydrocarbons, and alcohols, among others. The following sections
describe common methods used for measuring secondary oxidation products of
lipids.
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