Author:
METWALLI ALI A. M.,LAMMERS WIM L.,VAN BOEKEL MARTINUS A. J. S.
Abstract
Homocitrulline arises from the reaction between cyanate and the
ε-amino
group of lysine residues; in milk, cyanate derives from heat-induced urea
breakdown.
Since homocitrulline levels were unknown in heated milk, its formation
was studied
in the temperature range 100–150°C. Firstly, an analysis method
based on ion-exchange
chromatography using an amino acid analyser was developed. Homocitrulline,
liberated from milk protein by enzymic hydrolysis, was eluted well
separated from other amino acids and could be readily distinguished using
this
technique. Secondly, homocitrulline levels were determined for various
time–temperature
combinations in samples of milk, milk to which 10 mm-urea had
been
added, and milk that was made urea-free. No homocitrulline was formed in
urea-free
milk, while homocitrulline formation was stimulated by urea addition. Upon
prolonged heating, we found extensive subsequent breakdown of homocitrulline.
The
kinetics of homocitrulline formation was quite complicated, but an approximation
was possible as the initial formation reaction could be modelled using
zero order
reaction kinetics. The apparent activation energy for homocitrulline formation
was
estimated at ∼90 kJ/mol. In general, the levels of homocitrulline
to be expected
in heated milk appeared to be quite low: ∼0·3 mm
in in-bottle sterilized milk, and
<0·01 mm in UHT sterilized milk.
Publisher
Cambridge University Press (CUP)
Subject
Animal Science and Zoology,General Medicine,Food Science
Cited by
11 articles.
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