Chemical Assay of Urea for Automated Sensing in Milk

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

Results of a new chemical assay for urea involving enzymatic hydrolysis to ammonium and carbonate and subsequent measurement of CO₂ partial pressure are presented. The assay is simple to implement in an automated version, and the hardware used is not prone to fouling and damage by raw milk. The assay sensitivity at 24°C is about 0.367 kPa per milligram per deciliter of urea N. The assay has no dependence on milk fat in the sample, and effects of milk proteins and lactose are slight (less than 2% change in sensitivity per change in w/v percent). Observed sensitivities to urea in spiked milk samples were not significantly different from each other or from standards in distilled water or 0.1 M phosphate-buffered saline. The standard error of the assay is about 0.3 mg/dl of urea N (0.1 mM) in standard solution, and about 1 mg/dl of urea N (0.3 mM) in milk in a range of 0 to 30 mg/dl of urea N (0 to 11 mM). The assay holds promise for use in an on-line sensor to measure milk urea N in the milking parlor.

Key Words: milk urea nitrogen • nutritional management • on-line biosensor

Submitted on December 7, 1998
Accepted on April 12, 1999