2009_Book_FoodChemistry

3.7 Changes in Acyl Lipids of Food 203
3.7.2.1.9 Secondary Products
The primary products of autoxidation, the monohydroperoxides,
are odorless and tasteless (such
as linoleic acid hydroperoxides; cf. 3.7.2.4.1).
Food quality is not affected until volatile
compounds are formed. The latter are usually
powerfully odorous compounds and, even in the
very small amounts in which they occur, affect
the odor and flavor of food.
From the numerous volatile secondary products
of lipid peroxidation the following compounds
will be discussed in detail
• odor-active carbonyl compounds
• malonic dialdehyde
• alkanes, alkenes
Odor-Active Monocarbonyl Compounds. Model
expriments showed that the volatile fractions
formed during the autoxidation of oleic, linoleic
and linolenic acid contain mainly aldehydes and
ketones (Table 3.31). Linoleic acid, a component
of all lipids sensitive to autoxidation, is
a precursor of hexanal that is predominant in
the volatile fraction. Therefore this substance,
since it can easily be determined by headspace
analysis, is used as an indicator for the characterization
of off-flavors resulting from lipid
peroxidation.
A comparison of the sensory properties (Table
3.32) shows that some carbonyl compounds,
belonging to side components of the volatile
fractions, may intensively contribute to an
off-flavor due to their low threshold values.
Food items containing linoleic acid, especially
(E)-2-nonenal, trans-4,5-epoxy-(E)-2-decenal
and 1-octen-3-one, are very aroma active.
The rapid deterioration of food containing
linolenic acid should not be ascribed solely to
the preferential oxidation of this acid but also
to the low odor threshold values of the carbonyl
compounds formed, such as (Z)-3-hexenal,
(E,Z)-2,6-nonadienal and (Z)-1,5-octadien-3-one
(Table 3.32). Aldehydes with exceptionally
strong aromas can be released in food by the
autoxidation of some fatty acids, even if they are
present in low amounts. An example is octadeca-
(Z,Z)-11, 15-dienoic acid (the precursor for
Table 3.31. Volatile compounds formed by autoxidation of unsaturated fatty acids (µg/g) a
Oleic acid Linoleic acid Linolenic acid
Heptanal 50 Pentane b + c Propanal b
Octanal 320 Pentanal 55 1-Penten-3-one 30
Nonanal 370 Hexanal 5,100 (E)-2-Butenal 10
Decanal 80 Heptanal 50 (E)-2-Pentenal 35
(E)-2-Decenal 70 (E)-2-Heptenal 450 (Z)-2-Pentenal 45
(E)-2-Undecenal 85 Octanal 45 (E)-2-Hexenal 10
1-Octen-3-one 2 (E)-3-Hexenal 15
1-Octen-3-hydroperoxide + c (Z)-3-Hexenal 90
(Z)-2-Octenal 990 (E)-2-Heptenal 5
(E)-2-Octenal 420 (E,Z)-2,4-Heptadienal 320
(Z)-3-Nonenal 30 (E,E)-2,4-Heptadienal 70
(E)-3-Nonenal 30 (Z,Z)-2,5-Octadienal 20
(Z)-2-Nonenal + c 3,5-Octadien-2-one 30
(E)-2-Nonenal 30 (Z)-1,5-Octadien-3-one + c
(Z)-2-Decenal 20 (Z)-1,5-Octadien-3- + c
hydroperoxide
(E,E)-2,4-Nonadienal 30 (E,Z)-2,6-Nonadienal 10
(E,Z)-2,4-Decadienal 250 2,4,7-Decatrienal 85
(E,E)-2,4-Decadienal 150
trans-4,5-Epoxy-(E)- + c
2-decenal
a Each fatty acid in amount of 1 g was autoxidized at 20 ◦ Cbyanuptakeof0.5 mole oxygen/mole fatty acid.
b Major compound of autoxidation.
c Detected, but not quantified.