2009_Book_FoodChemistry

5.3 Individual Aroma Compounds 377
Fig. 5.27. Formation of aldehydes during isoleucine biosynthesis (according to Piendl, 1969). → main pathway
→ side pathway of the metabolism
A study involving the yeast Saccharomyces
cerevisiae clarified the origin of methylpro-panal
and 2- and 3-methylbutanal. They are formed to
a negligible extent by decomposition but mostly
as by-products during the biosynthesis of valine,
leucine and isoleucine.
Figure 5.27 shows that α-ketobutyric acid, derived
from threonine, can be converted into isoleucine.
Butanal and 2-methylbutanal are formed
by side-reaction pathways.
2-Acetolactic acid, obtained from the condensation
of two pyruvate molecules, is the intermediary
product in the biosynthetic pathways
of valine and leucine (Fig. 5.28). However,
2-acetolactic acid can be decarboxylated in a side
reaction into acetoin, the precursor of diacetyl.
At α-keto-3-methylbutyric acid, the metabolic
pathway branches to form methylpropanal and
branches again at α-keto-4-methyl valeric acid to
form 3-methylbutanal (Fig. 5.28).
The enzyme that decarboxylates the α-ketocarboxylic
acids to aldehydes has been detected
in oranges. Substrate specificity for this decarboxylase
is shown in Table 5.29.
Table 5.29. Substrate specificity of a 2-oxocarboxylic
acid decarboxylase from orange juice
Substrate V rel (%)
Pyruvate 100
2-Oxobutyric acid 34
2-Oxovaleric acid 18
2-Oxo-3-methylbutyric acid 18
2-Oxo-3-methylvaleric acid 18
2-Oxo-4-methylvaleric acid 15