Johan Gustav Christoffer Thorsager Kjeldahl (1849 – 1900), was a Danish chemist who developed a method for determining the amount of nitrogen in certain organic compounds using a laboratory technique which was named after him: the Kjeldahl method. He also derived the nitrogen-to-protein conversion factor of 6.25.

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When nitrogen values for a food are known, the protein contents is estimated by multiplying the nitrogen contents by a nitrogen-to-protein conversion factor; this factor is often set at 6.25. This factor (dating back to the 19th century) assumes that all organic nitrogen originates from protein and that the nitrogen contents of protein is 16 %. (Note that 100/16 = 6.25). Protein values based on these calculations

(N x 6.25) are called ‘crude’ protein.

On the next page you will find some exercises that will help you to understand how to use the nitrogen-to-protein conversion factor.