Manometric biosensor for on-line measurement of milk urea.

Performance of a prototype sensor for on-line measurement of urea in milk during milking was evaluated. The sensor was based on a manometric assay of the carbon dioxide generated by the enzymatic hydrolysis of urea. Temperature compensation of the sensor was described briefly, and was shown to be effective. The calibration of the sensor was described and resulted in a standard calibration error of about 0.15 mM of urea. The standard error of the sensor in milk was shown to be about 0.25 mM (given a physiological range of about 2-7 mM in cow milk). The sensor was simple, inexpensive, suffered from no interferences in raw milk, and completed a measurement cycle in about 5 min (less than the time to milk a typical cow). A custom made sampling device, whereby milk was passively collected from the milk line under vacuum, was shown to collect an ample volume within 10 s to run a test with the sensor. No measurable bubbles or foam were introduced from the sampling mechanism so that the milk sampled was not diminished in density compared to samples taken by other methods.