Short Note: A simple and fast procedure for casein determination in milk according to the Kjeldahl method. The casein is precipitated by adding acetic acid and, after filtration, the filtrate is digested with sulfuric acid using the SpeedDigester K-436 or K-439, followed by distillation and titration with the KjelFlex K-360. Request Short Note 051/2010 at www.mybuchi.com for all the details and read below for some additional information.
Caseins account for nearly 80% of proteins in cow milk that is not attained into the whey and so will be processed to e.g. cheese. Casein is not only used in food but also in medicine for additive and in technical use as a binder, cosmetics, paints and adhesives.
The casein is precipitated from a test portion by the addition of acetic acid solution. The filtrate requires digestion with sulfuric acid to convert nitrogen into ammonium sulfate. After conversion to ammonia through the alkalinization with sodium hydroxide, the ammonia of the sample is distilled into a boric acid receiver by steam distillation, followed by a titration with hydrochloric acid solution.
The casein content is the difference between the total nitrogen content and the non casein nitrogen content (ncn is calculated using the titrant volume and the sampleweight).
Instrumentation: SpeedDigester K-436, K-439, KjelFlex K-360
Samples: Whole milk UHT and partial skimmed milk UHT, casein contents 2.66% and 2.71%, respectively .
Conclusion:The determination of casein contents in milk according to Kjeldahl using SpeedDigester K-436, K-439, and KjelFlexK-360 provides reliable and reproducible results with low relative standard deviations and correspond to the values from literature.
References:  Souci Fachmann Kraut, CRC Press, 7th edition, 2008