Food Lipids: Chemistry, Nutrition, and Biotechnology

and ketones. This ion is formed from the reaction of 2,4-dinitrophenylhydrazine (2,4-
DNDH) with an aldehyde or ketone, followed by the reaction of the resulting hydrozones
with alkali (Fig. 7), which is then analyzed spectroscopically at a given
wavelength. Many variations of this spectroscopic method have been reported
[45,46]. Each method offers an alternative solvent, wavelength, or workup to analyze
the quinoidal ion.
The analysis of individual carbonyl compounds is another method which has
recently gained popularity. Hexanal, one of the major secondary products formed
during the oxidation of linoleic or other �-6 fatty acids in lipid-containing foods
[47,48], has been used to follow lipid oxidation. Shahidi and Pegg [47] reported that
a linear relationship existed between hexanal content, sensory scores, and TBA numbers
of cooked ground pork, whereas St. Angelo et al. [49] established a similar
correlation for cooked beef. O’Keefe et al. [50] have used hexanal as an indicator
to assess oxidative stability of meat from broiler chickens fed fish meal. Supplementation
of high amounts of fish meal to the diet increased the hexanal content of
the thigh meat during storage. However, recent studies have shown that during oxidation
of marine oils which are rich in polyunsaturated fatty acids of the �-3 type,
large amounts of propanal are formed and that a good correlation exists between the
content of propanal and the amount of TBARS in such samples [51,52]. Therefore,
it is essential to use appropriate indicators when assessing stability of food lipids.
We recommend that hexanal be used when oils under investigation are rich in �-6
fatty acids while propanal would serve as a reliable indicator when oils high in �-3
fatty acids are being considered.
6. Hydrocarbons and Fluorescent Products
Studies of oxidized methyl linoleate and soybean oil [53] have revealed that saturated
hydrocarbons could be detected when aldehydes are either absent or undetectable.
Snyder et al. [54] have reported that ethane, propane, and pentane are predominant
short chain hydrocarbons formed through thermal decomposition of soybean oil.
Correlations of flavor acceptability scores and pentane content determined by GC
Figure 7 Reaction steps in the production of hydrozones from carbonyls and 2,4dinitrophenylhydrazine.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.