Determining Protein By Analysis

Establishing protein levels in foods is complicated and accuracy can be limited. As protein is such a complex mix of amino acids, the only effective way to establish the protein content is to calculate from the nitrogen content.

In line with the EU labelling regulations EU1169/2011, NutriCalc uses a global factor for this conversion of nitrogen x 6.25. 

The legislation is intended to simplify the production of food labels, but it is not the best factor for some types of food:  For grain products, a factor of 5.7 would be more accurate.

Also, applying a global factor from nitrogen can create some strange looking results where there’s a high nitrogen content.

For example, guanylic acid has an apparent protein content of 120% and gelatin typically has a protein content of 95% even though the moisture content is 13%.

In the laboratory, there are two main methods to establish nitrogen content from which protein can be calculated:

  • the Kjeldahl method (considered to be the ultimate standard), which involves essentially digesting the sample with acid, distilling the ammonia, and measuring it chemically

and

  • the Dumas method, in which the sample is incinerated, the nitrogen collected, detected, and measured by instrumentation

Both methods will pick up ‘non-protein nitrogen’, i.e. nitrogen presence that is not in the form of protein, resulting in protein values which may be too high. Kjeldahl is slightly better than Dumas in this respect as Dumas will pick up compounds such as nitrite and nitrate that are not even organic.

Dumas has the advantage of not requiring any acids for processing. And it is generally preferred for routine use these days. 

Its main limitation is that the largest sample that can be analysed is about 1g, which makes it very difficult to get a representative sample of any food that is not very homogeneous.  So precision can be slightly reduced.

Because the methods determine nitrogen rather protein directly, it is actually possible to raise the apparent protein content of a food by adding a chemical high in nitrogen. 

There have been reported examples of urea being added to meat products and melamine to milk powders.  These practices are of course illegal!

David F. Bartley PhD

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