2009_Book_FoodChemistry

222 3 Lipids
Table 3.45. Volatile compounds formed from heattreated
tristearin a
Class of Portion C- Major
compound number compounds
Alcohols 2.7 4–14 n-Octanol
n-Nonanol
n-Decanol
γ-Lactones 4.1 4–14 γ-Butyrolactone
γ-Pentalactone
γ-Heptalactone
Alkanes 8.8 4–17 n-Heptadecane
n-Nonane
n-Decane
Acids 9.7 2–12 Caproic acid
Valeric acid
Butyric acid
Aldehydes 36.1 3–17 n-Hexanal
n-Heptanal
n-Octanal
Methyl 38.4 3–17 2-Nonanone
ketones
2-Heptanone
2-Decanone
a Tristearin is heated in air at 192 ◦ C.
Methyl ketones are obtained by thermally
induced β-oxidation followed by a decarboxylation
reaction (Fig. 3.37). Aldehydes
are obtained from the fragmentation of hydroperoxides
by a β-scission mechanism
(Fig. 3.38) occurring nonselectively at elevated
temperatures (compare the difference
with 3.7.2.1.9).
Unsaturated aldehydes with a double bond
conjugated to the carbonyl group are easily
degraded during the deep frying process
(Formula 3.95). Addition of water results in
the formation of a 3-hydroxyaldehyde that
is split by retro aldol condensation catalyzed
by heat. Examples of this mechanism are the
degradation of (E,Z)-2,6-nonadienal to (Z)-
4-heptenal and acetaldehyde, as well as the
cleavage of 2,4-decadienal into 2-octenal and
acetaldehyde.
(3.95)
Fig. 3.37. Autoxidation of saturated fatty acids. Postulated
reaction steps involved in formation of methyl
ketones
Some volatiles are important odorous compounds.
In particular, (E,Z)- and (E,E)-2,4-
decadienal are responsible for the pleasant
deep-fried flavor (cf. 5.2.7). Since these aldehydes
are formed by thermal degradation
of linoleic acid, fats or oils containing this
acid provide a better aroma during deep
frying than hydrogenated fats. However, if
a fat is heated for a prolonged period of
time, the volatile compounds produce an
off-flavor.