Nitrogen & Protein Determination in Milk By Digestion

Protein Determination in Milk

Protein Determination in Milk

Nitrogen and Protein Determination in Milk by Digestion with Hydrogen Peroxide and Sulfuric Acid

Application Note #54 introduces a simple and very fast procedure for protein determination in milk and provides detailed steps for completing the process. BUCHI’s Kjeldahl application database lists more than 100 Kjeldahl and Non-Kjeldahl applications using dedicated BUCHI equipment: Download #54 and all our other Kjeldahl Application Notes online.

The sample is digested with hydrogen peroxide and sulfuric acid using the SpeedDigester K-436 or K-439, followed by distillation and titration with the Kjeldahl Sampler System K-370/K-371. The determined protein contents correspond to the results obtained with the Kjeldahl method.

Introduction:
The digestion with hydrogen peroxide 30% instead of Kjeldahl tablets for nitrogen and protein determination is a very fast (30 min instead of 85 min (K-439) or 100 min (K-436)) and environmentally friendly (free of any heavy metal) alternative to the classical Kjeldahl methods.

Instrumentation:
SpeedDigester K-436, K-439, with H2O2 suction module, Kjeldahl Sampler System K-370/K-371

Samples:
Whole milk UHT, partially skimmed milk UHT, and chocolate milk beverage: labeled protein contents 3 g/100ml (whole milk) and 3.5 g/100ml (partially skimmed and chocolate milk). Protein determinations according to the classical Kjeldahl method, measure 3.15% for the whole milk, 3.19% for the partially skimmed milk, and 3.30% for the chocolate milk beverage.

Determination:
Approx. 5 g of the homogenized sample were weighed directly into a sample tube. A portion of 20 ml of sulfuric acid was added, and the digestion was started using the parameters specified (See complete note for data). Initially, 15 ml of hydrogen peroxide 30% was added to the capillary funnel 2 minutes after the start of the digestion. Then, another 15 ml of hydrogen peroxide 30% was added to the capillary funnel 10 minutes after the start of the digestion. The method was verified by using 0.18 g tryptophan as the reference substance.

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