Determination of total proteins in cow milk powder samples: a comparative study between the Kjeldahl method and spectrophotometric methods

Abstract

In the present paper, a comparative study between the Kjeldahl method and several spectrophotometric methods was carried out for the determination of total proteins in a range of milk powder samples (skim milk powder, whole milk powder, whey protein powder, buttermilk powder). The most important finding of this paper was that the Bradford method could be used for the determination of total proteins in skim milk powder and whole milk powder samples (without extraction of lipids) instead of the Kjeldahl method. The Bradford method showed the highest sensitivity of the spectrophotometric methods. Using casein and BSA as standard proteins, the Lowry method showed the lowest variation of specific absorbance indicating either casein or BSA could be used as a standard. The UV-220 nm method with previous extraction of lipids showed the best results for the determination of total proteins in all the samples; all the results were not statistically different ( P>0.05) from those obtained by the total protein nitrogen (TPN) without extraction of the lipids. The Bradford method (without extraction of lipids) showed the best results for the determination of total proteins in all the samples whose results were not statistically different ( P>0.05) from those obtained for total nitrogen by the Kjeldahl method. However, when these results were compared to TPN they were statistically different from each other ( P<0.05) for the buttermilk powder and whey protein powder samples. This means that the high sensitivity of the Bradford method was enough to overcome the turbidity of solutions due to the fat material in the samples. The determination of total proteins using the Bradford method for the whole milk powder and whey protein powder samples with and without the extraction of lipids was not statistically different ( P<0.05) from each other.