Refinement of the pressure assay for milk urea nitrogen

We report improvements in the application of a pressure-based assay for urea. The assay involved the enzymatic hydrolysis of urea and subsequent measurement of CO2 partial pressure. Effects of the preservative bronopol on the assay and their implications for laboratory applications are discussed. A method of remediating these effects with cysteine is described. A method is also described wherein these additives can be used to prepare standards of known urea concentration in milk. The improved assay can be used to measure urea N in unadulterated milk or in bronopol preserved milk with an accuracy of +/-0.7 mg/dl (0.25 mM) in the range from 0 to 30 mg/dl (0 to 10.7 mM).