2009_Book_FoodChemistry

5.3 Individual Aroma Compounds 379
Alcohol formation in plants and microorganisms
is strongly favoured by the reaction equilibrium
and, primarily, by the predominance of NADH
over NAD + . Nevertheless, the enzyme specificity
is highly variable. In most cases aldehydes >C 5
are only slowly reduced; thus, with aldehydes
rapidly formed by, for example, oxidative cleavage
of unsaturated fatty acids, a mixture of alcohols
and aldehydes results, in which the aldehydes
predominate.
5.3.2.2 Hydrocarbons, Esters
Fruits and vegetables (e. g., pineapple, apple,
pear, peach, passion fruit, kiwi, celery, parsley)
contain unsaturated C 11 hydrocarbons which play
a role as aroma substances. Of special interest
are (E,Z)-1,3,5-undecatriene and (E,Z,Z)-1,3,5,8-
undecatetraene, which with very low threshold
concentrations have a balsamic, spicy, pinelike
odor. It is assumed that the hydrocarbons
are formed from unsaturated fatty acids by
β-oxidation, lipoxygenase catalysis, oxidation of
the radical to the carbonium ion and decarboxylation.
The hypothetical reaction pathway from
linoleic acid to (E,Z)-1,3,5-undecatrieneis shown
in Formula 5.25.
R—CO—SCoA+ R ′ —OH
→ R—CO—O—R ′ (5.25)
+ CoASH
Esters are significant aroma constituents of many
fruits. They are synthetized only by intact cells:
Fig. 5.29. Biosynthesis of (E,Z)-2,4-decadienoic acid
ethyl ester in pears (according to Jennings and
Tressl, 1974)
Alcohol dehydrogenases (cf. 2.3.1.1) can reduce
the aldehydes derived from fatty acid and amino
acid metabolism into the corresponding alcohols:
R—CH 2 —OH + NAD ⊕
⇋ R—CHO+NADH + H ⊕ (5.24)
(5.26)