Metrolab Blog

NPN and NCN Determination in Milk according to the Dumas combustion method

Milk is a source of animal protein of high nutritional quality. The nitrogen compounds of milk can be divided in: casein proteins and non-casein nitrogen (NCN). Among this, there are soluble whey proteins (WP), enzymes and non-proteins nitrogen substances (NPN), composed of urea, amino acids, uric acid, creatine, creatinine and ammonia.
Casein is composed of different single proteins, which are linked together with other substances, such as water, enzymes, calcium and phosphorus, and take on a spherical form, named micelle, which have important digestive and also industrial functions, especially in the dairy sector.
Whey contains milk proteins called alpha-lactalbumin and beta-lactoglobulins, which provide a great contribution in terms of branched chain amino acids, much higher than in casein. They are easily digestible by the body, and enter the circulation much faster.
Furthermore, adulterants as NPN are added in milk to increase protein content, thereby increasing the milk quality in fraudulent way. Commercial urea (Sharma et al. 2012) or melamine was added to milk to increase protein content falsely (Liu et al. 2012). So, the determination of NPN is important to discriminate the “true protein content” from “crude protein content”.
Using NDA 702, it’s possible to easily and quickly determine the nitrogen content of all the N fractions obtainable with the cited reference method and in accordance with the cited literature.
Although Kjeldahl is nowadays the most common method for determining nitrogen and protein contents in foods and feeds, Dumas method, shows a high level of precision and reproducibility compared to Kjeldahl method.

Download the full application note here.