Kjeldahl Method for Determining Nitrogen

Proteins have a major role in the growth and maintenance of the human body and are, along with carbohydrates and lipids, the energy giving nutrients in the diet. In addition, proteins also pose a wide range of other functions in the body, such as enzymatic activity and transport of nutrients and other biochemical compounds across cellular membranes.

The Kjeldahl method was developed in 1883 by a brewer called Johann Kjeldahl. A food is digested with a strong acid so that it releases nitrogen which can be determined by a suitable titration technique. The amount of protein present is then calculated from the nitrogen concentration of the food.

The same basic approach is still used today, although a number of improvements have been made to speed up the process and to obtain more accurate measurements. It is usually considered to be the standard method of determining protein concentration. Protein is determined by the analysis of the nitrogen content. From this, the protein content is calculated. Protein consists of amino acids which contain nitrogen (N) in the amino group.

The Kjeldahl method has three different steps: digestion, distillation, and titration. In this method, most organic nitrogen-containing samples are digested with sulfuric acid to ammonium sulfate; the ammonium is then liberated by raising the pH and measured by titration.

The Kjeldahl method was performed according to method 981.10 of the AOAC International. Approximately 1 g of raw material was hydrolyzed with 15 mL concentrated sulfuric acid (H2SO4) containing two copper catalyst tablets in a heat block at 420 ◦C for 2 h.

After cooling, H2O was added to the hydrolysates before neutralization and titration. The amount of total nitrogen in the raw materials were multiplied with both the traditional conversion factor of 6.25 and species-specific conversion factors in order to determine total protein content. The species-specific conversion factors were 5.6 for fish and shrimp, 5.4 for flours and 4.59 for seaweed, respectively.
Kjeldahl Method for Determining Nitrogen

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