Protein is an important component of food for several different reasons. It is the main source of energy and contains essential amino acids such as lysine, tryptophan, methionine, lysine, isoleucine, and valine, which are essential for human health and that the body cannot fail to manufacture.

Protein is also a major structural component of many natural foods and often determines their overall texture, for example, the tenderness of meat or fish products.

Protein isolates are often used in foods as ingredients due to their unique functional properties, i.e., their ability to provide the desired shape, texture, or stability.

As a rule, protein is used as an automatic factor, emulsion, foam factor, and concentrated material. Many nutritional proteins are enzymes that can improve the speed of some biochemical reactions.

These interactions can have a positive or detrimental effect on the general characteristics of the food. Food analysts want to know the total concentration, type, molecular structure, and functional properties of the proteins in food.


The Kjeldahl method was developed in 1883 by a brewer named Johann Kjeldahl. The food is digested by a strong acid so that it releases nitrogen, which can be determined by the proper titration technique.

The amount of protein present is then calculated from the nitrogen concentration in the food. The same basic approach is still used today, although some improvements have been made to speed up the process and achieve more accurate measurements.

It is often a standard way to determine the concentration of protein. The Kjeldahl method does not directly measure the protein content, so the transfer factor (F) is necessary to convert nitrogen levels measured in the concentration of protein into the Kjeldahl unit.

A conversion factor of 6.25 (equivalent to 0.16 g of nitrogen per gram of protein) is used for many applications.

However, this is only an average and each protein has a different conversion factor depending on its amino acid composition. The Kjeldahl method can be divided into three steps: digestion, neutralization, and titration.

The food sample to be analyzed is weighed in a digester and then decomposed by heating it in the presence of sulfuric acid (an oxidizing agent that digests food) and anhydrous sodium sulfate (to speed up the reaction via boiling point). and catalysts, such as copper, selenium, titanium, and mercury (to speed up the reaction).

The digestion process converts any nitrogen in food (not nitrogen in the form of nitrate or nitrite) into ammonia and other organic matter into carbon dioxide and water.

Ammonia is not released into an acidic solution because ammonia is in the form of an ammonium ion (NH4+) bound to a sulfate ion (SO42-) and thus remains in the solution:


When digestion is complete, the digester is connected to the receiving vessel using a hose. Then the solution in the digester is converted to an alkaline by adding sodium hydroxide, which converts ammonium sulfate to ammonia gas.

The ammonia gas formed from the solution is released from the digester into the receiving vessel, which retains the excess boric acid. The lower pH of the future solution converts ammonia gas into ammonium ions and simultaneously converts boric acid into borate ions.


The nitrogen content is then estimated by titrating the ammonium borate of standard sulfuric acid or hydrochloric acid with an appropriate indicator to determine the endpoint of the reaction.

The hydrogen ion concentration (in moles) required to reach the endpoint is equivalent to the nitrogen concentration in the original food (Equation 3).

The following equation can be used to determine the nitrogen concentration in one gram of sample using HCl Mx titration solution:

where vs and vb represent the volumes of the sample titrated and blank, and 14 g is the molecular weight of nitrogen N. The blanks are usually run by the assay to calculate the potential residual nitrogen in the drug. The test is used to perform the analysis.

Once the nitrogen content has been determined, it is converted to protein content using the appropriate conversion factor: % protein = F % n.

Merits and demerits

The Kjeldahl method is widely used all over the world and remains the standard method for comparison with all other methods. Its ubiquitous presence, high accuracy, and good reproducibility have made it the leading method for protein determination in food. flaw.

It does not give a true measure of protein, as not all nitrogen in food is in the form of protein. Different proteins require different correction factors because they have different amino acid sequences.

The use of concentrated sulfuric acid at high temperatures poses significant risks, as does the use of some potential catalysts. This technique is far from being implemented.


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