Improvement of the ammonia measurement using a clarifying reagent and application to evaluate heat damage in commercial milk samples

The enzymatic Boehringer procedure for ammonia measurement in milk was improved to be rapid and simple using a clarifying reagent without any pretreatment of the sample. A good correlation (r=0.99, n=36) existed between the proposed enzymatic method (EM) and the specific electrode method (SEM) when applied on raw and commercial milk samples. The repeatability of the EM applied to UHT milk was satisfactory with a relative standard deviation of 5.4% (n=26). The EM was used to evaluate heat damage in commercial half-skimmed pasteurised, direct or indirect or unlabelled UHT and in-bottle sterilised milk. The EM was well correlated with absorbance at 340 nm (A340) of transparent modified milk (r=0.97, n=42), with lactulose determined by HPLC and by capillary electrophoresis (r=0.95, n=17 and r=0.95, n=26, respectively). All parameters differentiated significantly (P<0.001) between pasteurised, UHT- and in-bottle sterilised milk, and also direct-UHT and indirect-UHT samples. More intensive heat treatment produced higher contents of ammonia, lactulose and A340 values.