The determination of non-protein nitrogen in milk is a routine procedure for quality assurance. A simple and fast procedure for non-protein nitrogen determination in milk according to the Kjeldahl method, as described in ISO 8968-4:2001(E), is introduced in this study. The protein of the sample is precipitated by addition of trichloroacetic acid and after filtration the filtrate is digested with sulfuric acid using the SpeedDigester K-436 or K-439, followed by distillation and titration with the KjelFlex K-360.
The filtrate requires digestion with sulfuric acid to convert nitrogen into ammonium sulfate. After conversion to ammonia through the alkalinization with sodium hydroxide, the sample is distilled into a boric acid receiver by steam distillation, followed by a titration with hydrochloric acid solution. The non-protein nitrogen content is calculated using the volume from the hydrochloric acid and the weight from the sample, trichloroacetic acid solution and filtrate.
There are no differences between the results obtained with the K-436 and the K-439 respectively and correspond to the values from literature. The determined non-protein nitrogen contents are reliable and reproducible with low relative standard deviations.