Kjeldahl Method for Determining Percent Protein from Percent Nitrogen

In the Kjeldahl method, a food’s protein content is calculated by multiplying the reduced nitrogen content in a sample by a characteristic factor. This Demonstration does the calculation using several commonly accepted factors. It also illustrates how the conversion factor's choice affects the calculated percent protein.


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Snapshot 1: protein contents in evaporated milk
Snapshot 2: protein contents in defatted soybean flour
Snapshot 3: protein contents in cooked brown rice
Determining the protein contents by the Kjeldahl method requires the digestion of a food sample by hot concentrated sulfuric acid, thus converting the reduced nitrogen (such as in a protein’s and groups) into ammonium sulfate. Later addition of concentrated sodium hydroxide releases the in the form of ammonia, which is captured and its quantity determined [1]. The amount of nitrogen so determined is converted into protein content by multiplying by a factor in the range of 5.30–6.38, which represents the different typical reduced nitrogen content of proteins in foods [2].
This Demonstration does the calculation with fixed conversion factors arranged in a list, which is a slightly modified version of a table suggested by the FAO [2] and with continuous factor and percent nitrogen values entered and altered by sliders.
The graphical displays are of the percent protein versus percent nitrogen plot for the chosen factor and the percent protein versus the chosen factor plot for a chosen fixed percent nitrogen. They are accompanied by the numerical values of the chosen percent nitrogen, factor, and corresponding calculated percent protein.
[1] W. Horwitz (ed.), Official Methods of Analysis of AOAC International, 17th ed., Gaithersburg, MD: Association of Official Analytical Chemists, 2000.
[2] K. Tontisirin et al., "Food Energy: Methods of Analysis and Conversion Factors," Report of a Technical Workshop, Rome, Dec 3–6, 2002, Rome: Food and Agriculture Organization of the United Nations, 2003 pp. 7–11. www.fao.org/uploads/media/FAO_2003_Food_Energy_02.pdf.
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