2009_Book_FoodChemistry

380 5 Aroma Compounds
Fig. 5.30. Biosynthesis of γ- andδ-lactones from oleic and linoleic acid (according to Tressl 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 thresholds
of some esters. Methyl branched esters, from
the metabolism of leucine and isoleucine, were
found to have very low values. The odor thresholds
of the acetates are higher than those of the
corresponding ethylesters.
When fruits are homogenized, such as in the processing
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 various
fruits are presented in Table 5.31.
Since the aroma of lactones is partly very pleasant,
these substances are also of interest for commercial
aromatization of food. In the homologous
series of γ- andδ-lactones, the odor threshold
decreases with increasing molecular weight
(Table 5.32).
The biosynthesis of lactones was studied using
the yeast Sporobolomyces odorus and it was
shown that the results are valid for animal and
plant foods. Labelling with deuterium indicates