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| * Note that the Jones’ factors are based on protein nitrogen only and do not include NPN. However, in food composition databases, the protein contents in a food is based on the total amount of N measured in that food. For research purposes, a few laboratories may correct for the NPN fraction. ** The value of 16gN/100 g protein is only an example. This value differs between food as was explained on the previous page. |

The figure below shows how the Jones factors have been determined.

Some scientists have criticized those Jones' factors and have published factors for a wide range of foods based on detailed amino acids analysis. As opposed to the Jones' factors these new factors also included the NPN. You can read more about this in F.Mariotti et al 'converting Nitrogen into Protein - beyond 6.25 and Jones' factors' (Critical reviews in Food science and nutrition (2008): 48: 177-184). However these “improved “factors are not used in food composition data bases.

Summarized, it is only possible to interpret protein values in food composition databases correctly if the conversion factor is reported, including information on the source of this conversion factor. Using 6.25 as conversion factor for all foods can lead to considerable over- or underestimation of protein content.

In the next part you will learn how nitrogen content of foods is measured.