Food Chemistry

5.3 Individual Aroma Compounds 379

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)

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 cleav-

age of unsaturated fatty acids, a mixture of alco-

hols and aldehydes results, in which the aldehy-

des 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, pine-

like 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 decarboxy-

lation. 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′+CoASH

(5.25)

Esters are significant aroma constituents of many

fruits. They are synthetized only by intact cells:

(5.26)