FORMATION OF HOMOCITRULLINE DURING HEATING OF MILK

Homocitrulline arises fi om the reaction between cyanate and the epsil on-amino group of lysine residues; in milk, cyanate derives from heat- induced urea breakdown. Since homocitrulline levels were unknown in he ated milli, its formation was studied in the temperature range 100-150 degrees C. Firstly, an analysis method based on ion-exchange chromato graphy using an amino acid analyser was developed. Homocitrulline , li berated from milli protein by enzymic hydrolysis, was eluted well sepa rated from other amino acids and could be readily distinguished using this technique. Secondly, homocitrulline levels were determined for va rious time-temperature combinations in samples of milk, milk to which 10 mM-urea had been added, and milk that was made urea-free. No homoci trulline was formed in urea-free milk, while homocitrulline formation was stimulated by urea addition. Upon prolonged heating, we found exte nsive subsequent breakdown of homocitrulline. The kinetics of homocitr ulline formation was quite complicated, but an approximation was possi ble as the initial formation reaction could be modelled using zero ord er reaction kinetics. The apparent activation energy for homocitrullin e formation was estimated at similar to 90 kJ/mol. In general, the lev els of homocitrulline to be expected in heated milk appeared to be qui te low: similar to 0.3 mM in in-bottle sterilized milk, and < 0.01 mM in UHT sterilized milk.