Food Chemistry

5.3 Individual Aroma Compounds 377

Fig. 5.27.Formation of aldehydes during isoleucine biosynthesis (according toPiendl, 1969).→main pathway
→side pathway of the metabolism

A study involving the yeast Saccharomyces
cerevisiaeclarified 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 thatα-ketobutyric acid, de-
rived from threonine, can be converted into iso-
leucine. Butanal and 2-methylbutanal are formed
by side-reaction pathways.
2-Acetolactic acid, obtained from the conden-
sation of two pyruvate molecules, is the inter-
mediary product in the biosynthetic pathways
of valine and leucine (Fig. 5.28). However,
2-acetolactic acid can bedecarboxylated 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 atα-keto-4-methyl valeric acid to

form 3-methylbutanal (Fig. 5.28).

The enzyme that decarboxylates theα-keto-

carboxylic acids to aldehydes has been detected

in oranges. Substrate specificity for this decar-

boxylase is shown in Table 5.29.

Table 5.29.Substrate specificity of a 2-oxocarboxylic

acid decarboxylase from orange juice

Substrate Vrel(%)

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