Determination of N/Protein in Cheese by Kjeldahl Method (CAT#: STEM-EA-0038-ZJF)

The Kjeldahl method is a method for the quantitative determination of nitrogen contained in organic substances plus the nitrogen contained in the inorganic compounds ammonia and ammonium (NH3/NH4+). It is an important method for analyzing proteins based on the principle that the nitrogen content in the protein is constant.
The co-heating of an organic sample with concentrated sulfuric acid (H2SO4) digests the sample by oxidation to liberate the reduced nitrogen as ammonium sulfate. Catalysts like selenium, Hg2SO4 or CuSO4 are often added to make the digestion go faster. Na2SO4 or K2SO4 is also added to increase the boiling point of H2SO4. Then the sample solution is distilled with a small amount of sodium hydroxide (NaOH) to convert the ammonium salt into ammonia. Ammonium ion concentration in the acid solution, and thus the amount of nitrogen in the sample, is measured via titration. Indirect back titration is used instead if strong acids were used to make the standard acid solution: strong base of known concentration (like NaOH) is used to neutralize the solution. In this case, the amount of ammonia is calculated as the difference between the amount of HCl and NaOH.

Food processing, feed production, beverage, tobacco, livestock, chemicals, environmental analysis, pharmaceutical, cosmetics, soil fertility, agriculture, scientific research, teaching, quality control, etc.

1. Digestion: The goal is to break down the bonds that hold the polypeptides together and convert them into simpler molecules (such as water, carbon dioxide and ammonium sulphate). These reactions can be speeded up by the temperature used during digestion (the higher the temperature used, the faster the digestion can be obtained) and by the presence of acid, salt and catalysts (selenium, copper, mercury, titanium).
2. Distillation: The ammonium sulphate present in the digested sample are converted into ammonia gas, heated and distilled. The ammonia gas is led into an acid trapping solution where it dissolves and becomes a trapped ammonium ion once again. Using the Kjeldahl method, nitrites and nitrates are not detected. In order to quantify these elements, a reduction of the sample is necessary before the digestion stage.
3. Titration: The goal is to determine the amount of ammonia distilled off from the digested solution and hence calculate the nitrogen or protein amount, as %. During the Kjeldahl distillation process, the ammonia content is condensed and collected in a boric acid solution to avoid loss of gaseous NH3.

• Kjeldahl Nitrogen Analyzer
• Sample material: Food & feed (grain, meat, fish, milk, dairy, fruit, vegetables), beverage, environmental (agriculture, oilseeds, soil, fertilizers, water, wastewater, sludge), chemical and pharmaceutical industries (paper, textiles, rubber, plastic, polymer), etc.