Metrolab Blog

Casein Determination in Milk according to the Kjeldahl method

The milk proteins are the oldest and most widely consumed food proteins. There is currently a large interest in these substances, both in nutritional field and in technological application. Casein proteins are a family of proteins involved in the production of cheese and fermented milk. They are very important nitrogen compounds not only for the dairy products production, but they are also additives in medicine, and they have a technical use in cosmetics, paints and adhesives. Casein content is determined by the difference between the total nitrogen (Ntot) content and the non-casein nitrogen (NCN), obtained by the milk, following the procedure indicated in this document.

Kjeldahl is nowadays the most used method for determining nitrogen and protein contents in foods and feeds, thanks to the high level of precision and reproducibility and to its simple application. The modern Kjeldahl method consists in a procedure of catalytically supported mineralization of organic material in a boiling mixture of sulphuric acid and sulphate salt at digestion temperatures higher than 400 °C. During the process the organically bonded nitrogen is converted into ammonium sulphate. Alkalizing the digested solution liberates ammonia which is quantitatively steam distilled and determined by titration.

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