An automated instrumental technique has been developed which is capable of rapidly measuring the crude protein concentration of food samples and is beginning to compete with the Kjeldahl method as the standard method of analysis for protein content for some foodstuffs.
The method consists of combusting a sample of known mass in a high temperature (about 900°C) chamber in the presence of oxygen. This leads to the release of carbon dioxide, water and nitrogen. The gases are then passed over special columns(such as potassium hydroxide aqueous solution) that absorb the carbon dioxide and water. A column containing a thermal conductivity detector at the end is then used to separate the nitrogen from any residual carbon dioxide and water and the remaining nitrogen content is measured. The instrument must first be calibrated by analyzing a material that is pure and has a known nitrogen concentration. The measured signal from the thermal conductivity detector for the unknown sample can then be converted into a nitrogen content. As with the Kjeldahl method, conversion of the concentration of nitrogen in a sample to the crude protein content is performed using conversion factors which depend on the particular amino acid sequence of the measured protein.
Advantages and limitations
The Dumas method has the advantages of being easy to use and automated. It is also considerably faster than the Kjeldahl method, taking a few minutes per measurement, as compared to the hour or more for Kjeldahl. It also does not make use of toxic chemicals or catalysts. One major disadvantage is its high initial cost. Also, as with Kjeldahl, it does not give a measure of true protein, as it registers nonprotein nitrogen, and different correction factors are needed for different proteins because they have different amino acid sequences. Finally, the small sample size raises the risk of obtaining an unrepresentative sample.
- Kjeldahl method, another nitrogen analysis method
- Bicinchoninic acid assay, a colorimetric assay for protein-nitrogen