Direct and Indirect Determination of True Protein Content of Milk by Kjeldahl Analysis: Collaborative Study

Abstract

Currently, the reference procedure for determination of the "protein" content of milk is based on measurement of the total nitrogen content of milk by the Kjeldahl method (AOAC method, 920.105). About 6% of the total nitrogen content of milk Is nonprotein nitrogen. Therefore, total nitrogen multiplied by the conversion factor 6.38 overestimates the true protein content of milk on average by about 6%. In the present study, new direct and Indirect methods were developed for measurement of the true protein content of whole milk by Kjeldahl nitrogen determination. Both new methods are sample preparation procedures used to fractionate the nitrogen-containing compounds In milk prior to measurement of the nitrogen content of these fractions by Kjeldahl analysis. The collaborative study consisted of 9 pairs of blind duplicate milk samples that were analyzed for total nitrogen, nonprotein nitrogen, and protein nitrogen by each of 10 laboratories. Both methods for true protein measurement (direct and Indirect) gave acceptable statistical performance characteristics and good agreement between methods. The new direct method requires about half the laboratory analysis work of the indirect method (i.e., total minus nonprotein nitrogen). The methods have been adopted official first action by AOAC as (1) a new method for nonprotein nitrogen determination in milk, (2) a new method (direct) for determination of protein nitrogen content of milk, and {3) an alternative method (indirect) for determination of protein nitrogen content of milk.

Total Protein Methods and Their Potential Utility to Reduce the Risk of Food Protein Adulteration

“…Milk analyses using total nitrogen based methods were not adequate for use in this system because the variable content of nonprotein nitrogen in milk introduced significant error ( Barbano and Lynch 1991). This led to the development of a true protein method for fluid milk (Barbano and Lynch 1991;Kuzyk and others 1994). The method was adopted as final in 1994 as an AOAC International method (991.22) and also IDF-ISO-AOAC method (Barbano and Lynch 1991;AOAC International 2005).…”

“…This led to the development of a true protein method for fluid milk (Barbano and Lynch 1991;Kuzyk and others 1994). The method was adopted as final in 1994 as an AOAC International method (991.22) and also IDF-ISO-AOAC method (Barbano and Lynch 1991;AOAC International 2005).…”

“…These results are similar to those reported for normal Kjeldahl analysis for this matrix (see Table 3). An indirect variation of this method measures the total nitrogen and nonprotein nitrogen contents of milk separately using the Kjeldahl method but shows lower precision (Barbano and Lynch 1991).…”

Chemical Pretreatment and Microfiltration for Making Delipidized Whey Protein Concentrate

“…Triplicate samples were accurately weighed into separate aluminum dishes, dried in a convection oven for 1618 hr at lOO"C, cooled in a desiccator and reweighed. True protein content of WPC was determined in triplicate by the macro-Kjeldahl method according to Barbano et al (1991), using a conversion factor of 6.38 (AOAC, 1984). Total lipid and phosphohpid contents of WPC were determined by the method of Morr and Seo (1988).…”

Gelation of Calcium‐Reduced and Lipid‐Reduced Whey Protein Concentrates as Affected by Total and Ionic Mineral Concentrations

“…True protein was determined by the macro-Kjeldahl method according to Barbano et al (1991) using a conversion factor of 6.38 (AOAC, 1984). Total lipids and phospholipids were determined by the method of Morr and Seo (1988) as described by Laye (1993).…”