Kjeldahl Method for Determination of Total Nitrogen Content of Milk: Collaborative Study

Abstract A macro-KJeldahl procedure using a copper catalyst for determination of milk total nitrogen was developed for both traditional and block dlgestor/steam distiller equipment, and the performance was evaluated by collaborative study. In the first trial of the collaborative study, 9 pairs of blind duplicate milk samples were analyzed for total nitrogen and total nitrogen was converted to "protein" by using a factor of 6.38. Protein content of milk samples ranged from 3.086 to 3.610%. In the first trial, sR and R values for the block dlgestors were influenced significantly by protein concentration; 8R and R values were not Influenced by protein concentration for traditional equipment. It was hypothesized that total digestion time for some block dlgestors In the first trial was not sufficient for high protein milk samples. Thus, a second trial was undertaken with boiling time after clearing Increased by 0.5 h. In the second trial, none of the parameters for reproducibility with either type of equipment were Influenced by protein concentration. It was concluded that laboratory-to-laboratory differences In line voltage may require different total digestion times in different laboratories, particularly those using block dlgestors. The Kjeldahl method using a copper catalyst and either traditional or block digestor equipment for determination of milk total nitrogen has been adopted official first action by AOAC to replace method 920.105.