Analytical methods for the determination of the urea content in milk

The unbalanced feeding of cows leads to a high urea content in milk, which in turn can affect milk production and fertility, leading to reduced profit margins for milk producers. A dependable and efficient method for the determination of the urea content in milk has yet to be adopted. This paper presents a comprehensive description of the methods currently in use for the determination of the urea content in milk, classified according to their principle: - direct methods, based on the measurement of a complex formed between urea and a reagent: essentially calorimetric methods; - indirect methods, based on the measurement of a byproduct from urea degradation, which reacts with a reagent: mainly enzymatic methods; - physical methods, which include infra-red measurement and combinations of enzymatic methods with the use of ammonium ion-selective electrodes. Both manual and (semi-)automated procedures for each of these methods are listed and compared according to recovery of added urea, linearity range, reproductibility and repeatability. For manual enzymatic determinations of urea contents, a preliminary protein precipitation is required, which is done according to several different procedures. The effect of the use of preservatives and of sample storage on the urea content in milk as discussed in different papers is also summarized, as is the influence of the original ammonia, uracic acid and uracil contents.