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 (A(340)) 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. (C) 2007 Elsevier Ltd. All rights reserved.