Refinement of the Pressure Assay for Milk Urea Nitrogen

¹ Department of Biological and Agricultural Engineering, University of California at Davis, 95616
² Department of Animal Science, University of California at Davis, 95616
³ School of Veterinary Medicine, University of California at Davis, 95616

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).

Key Words: biosensor • bronopol • cysteine • urease

Submitted on October 19, 1999
Accepted on March 6, 2000