380 5 Aroma Compounds
Fig. 5.30.Biosynthesis ofγ-andδ-lactones from oleic and linoleic acid (according toTressl et al., 1996) (1) R-
γ-decalactone, (2) S-δ-dodecalactone, (3) R-δ-decalactone, (4)γ-decalactone, (5) R-(Z)-6-γ-dodecenelactone, (6)
R-γ-nonalactone
Acyl-CoA originates from the β-oxidation of
fatty acids and also occasionally from amino acid
metabolism. Figure 5.30 shows an example of
how ethyl (E,Z)-2,4-decadienoate, an important
aroma constituent of pears, is synthesized from
linoleic acid.
Table 5.30 gives information on the odor thresh-
olds of some esters. Methyl branched esters, from
the metabolism of leucine and isoleucine, were
found to have very low values. The odor thresh-
olds of the acetates are higher than those of the
corresponding ethylesters.
When fruits are homogenized, such as in the pro-
cessing of juice, the esters are rapidly hydrolyzed
by the hydrolase enzymes present, and the fruit
aroma flattens.
5.3.2.3 Lactones...............................................
Numerous lactones are found in food. Some of
the representatives which belong to the typical
aroma substances of butter, coconut oil, and vari-
ous fruits are presented in Table 5.31.
Since the aroma of lactones is partly very pleas-
ant, these substances are also of interest for com-
mercial aromatization of food. In the homolo-
gous series ofγ-andδ-lactones, the odor thresh-
old decreases with increasing molecular weight
(Table 5.32).
The biosynthesis of lactones was studied using
the yeastSporobolomyces odorus and it was
shown that the results are valid for animal and
plant foods. Labelling with deuterium indicates