Proteins are large organic compounds that are formed by chaining amino acids together. Proteins are polymers consisting of amino acid building blocks. Each protein has different amino acid sequences, which give the proteins unique properties. To understand the function of proteins, it is necessary to determine the properties of the amino acids that make up them. There are 22 types of amino acids that are effective in the formation of proteins.

For living things, a protein-free lifestyle is unthinkable. Although fats and carbohydrates are the most important energy sources of living things, these nutrients cannot replace proteins for the formation of new tissues in the organism and for the repair of existing tissues.

Kjeldahl method is used to determine the amount of crude protein. This method is used for the analysis of plant and animal foodstuffs and for the determination of nitrogen content. However, although this method is applied in different ways and using different devices, it generally gives the same results.

In the structure of proteins formed by chemical bonding of amino acids, carbon, oxygen, hydrogen, sulfur and nitrogen elements are present. The total amount of organic nitrogen in food products is indicated directly as nitrogen or as protein.

There are different amounts of protein in food products. Nutritional value and quality of food products are determined by the amount of protein. Many methods are used to determine the amount of protein in food products of animal and vegetable origin. However, the most commonly used method is the Kjeldahl method, which was developed by a Danish chemist and named after him.

In the authorized laboratories, digestible crude protein analyzes are also performed within the scope of chemical tests. A few standards referenced in these studies are:

• TS EN ISO 8968-1 Milk and dairy products - Determination of nitrogen content - Part 1: Kjeldahl principle and calculation of crude protein
• TS EN ISO 5983-1 Animal feed - Determination of nitrogen content and calculation of crude protein content - Part 1: Kjeldahl method
• TS ISO 1871 Food and feed products - General rules for the determination of nitrogen by the Kjeldahl method