Determination of protein content

Protein content in food varies widely. Foods of animal origin and legumes are excellent sources of proteins. 

Determination of total protein can accomplished by a number of different techniques. Some of these include gravimetric, nitrogen determination, amino acid analysis, colorimetric, methods, spectrophotometric and fluorometric methods.

A nitrogen determination is the most commonly used protein assay. It is generally assumed that a mixture of pure proteins will contain 16% nitrogen.

Determination of protein concentration by micro-Kjehdahl method gives accurate and reliable results, providing that a correlation is made for non-protein nitrogen in the sample.

Various modifications have been devised to improve its accuracy and speed.

This method is very suitable for most commercial food proteins and feed proteins, since low amounts of lipid and even high amounts of carbohydrate and other contaminants do not interfere with the procedure.

The Kjehdahl procedure can be basically divided into three parts:
*Digestion
*Distillation
*Titration

In the Kjeldahl procedure, proteins and other organic food components in a sample are digested with sulfuric acid in the presence of catalysts. The total organic nitrogen is converted to ammonium sulfate.

The result of the analysis represents the crude protein content of the food since nitrogen also comes from non-protein component.

While the Kjeldahl method has been used widely, the availability of automated instrumentation for the Dumas method in many cases is replacing used the Kjeldahl method. Both methods are official for the purposes of nutrition labeling of foods.
Determination of protein content