2009_Book_FoodChemistry

1.2 Amino Acids 33
The isomers can be separated through the
sparingly soluble L-lysine sulfanilic acid
salt:
1.2.5.5 Phenylalanine
Benzaldehyde is condensed with hydantoin,
then hydrogenation using a chiral catalyst gives
a product which is about 90% L-phenylalanine:
(1.68)
(1.65)
Fermentation with a pure culture of Brevibacterium
lactofermentum or Micrococcus
glutamicus produces L-lysine directly:
1.2.5.6 Threonine
Interaction of a copper complex of glycine with
ethanal yields the threo and erythro isomers in the
ratio of 2:1. They are separated on the basis of
their differences in solubility:
(1.66)
1.2.5.4 Methionine
Interaction of methanethiol with acrolein produces
an aldehyde which is then converted to
the corresponding hydantoin through a Bucherer
reaction. The product is hydrolyzed by alkaline
catalysis. Separation of the resultant
racemate is usually not carried out since the
D-form of methionine is utilized by humans via
transamination:
(1.67)
(1.69)
D,L-threonine is separated into its isomers
through its N-acetylated form with the help of an
acylase enzyme.
Threonine is also accessible via microbiological
methods.
1.2.5.7 Tryptophan
Tryptophan is obtained industrially by a variation
of the Fischer indole synthesis. Addition
of hydrogen cyanide to acrolein gives 3-cyanopropanal
which is converted to hydantoin through
a Bucherer reaction. The nitrile group is then