Seasonal variations in the total nitrogen, non-protein nitrogen and urea nitrogen contents of Friesian and Jersey milk

Abstract

SummaryA large number of samples (433) from the milk of Friesian herds was taken during the period November 1968 – September 1969, and a smaller number (159) from Jersey herds during the dairy ‘season’ September 1969 – April 1970, and analysed for total nitrogen (TN) and for non-protein nitrogen (NPN). The Friesian milk sample varied in TN from 0·456 to 0·624% (average 0·531 ± 0·021%) and in NPN from 0·024 to 0·054% (average 0·035 ± 0·004%). The Jersey samples varied in TN from 0·467 to 0·718% (average 0·584 ± 0·036%) and in NPN from 0·022 to 0·051% (average 0·033 ± 0·006%).The average NPN value for all the milks was 0·034 ± 0·005%.There was a seasonal trend in NPN and TN for the Jersey and in TN but not in NPN for the Friesian milks. The proportion of NPN as a percentage of TN varied widely – for the Friesian samples from 4·9 to 10·2% (average 6·46%) and for the Jersey samples from 3·7 to 8·7% (average 5·74%).Urea N also was estimated in 125 of the Friesian and in all the Jersey milk samples. It varied from 0·012 to 0·029% (average 0·020 ± 0·004%) in the Friesian and from 0·009 to 0·031% (average 0·020 ± 0·004%) in the Jersey samples. For all the milks the urea N as a percentage of NPN varied from 33 to 79 (average 56·5%).It is suggested that values for ‘protein content’ of milk should refer only to ‘true’ protein ((TN – NPN) × 6·38) and that in the absence of an analytical result for NPN the use of an average value such as that above would be justified.