Protein analysis by the Kjeldahl method

How to determine the protein content of a bioproduct?

The total protein content of a bioproduct can be determined by the Kjeldahl method. This method is applicable to any type of environment

  • food (solids, liquids)
  • ingredients (liquid extracts, paste-like, powders)
  • vegetable and animal raw materials (cereals, insects)
This method will not take nitrates and nitrites into account.
Transcript : French English

What is the principle of the Kjeldahl method?

Nitrogen-containing organic compounds (proteins and nucleic acids in certain matrices) are decomposed under heating by sulfuric acid and a catalyzer. This catalyzer contains potassium sulfate (K2SO4), which allows an increase in the boiling temperature of sulfuric acid, and copper sulfate (CuSO4), which acts as a catalyst in the reaction. Nitrogen will quantitatively give ammonium sulfate: it’s the mineralisation step.

The ammoniac is subsequently displaced from its salt by sodium hydroxide, distilled by steam distillation and collected in a known quantity of excess hydrochloric acid. This step is called distillation.

The hydrochloric acid that has not reacted is dosed in return by sodium hydroxide. This is the dosage step.

How to calculate the potein content in a bioproduct?

Two samples will be prepared :
  • one sample containing the bioproduct, liquid or previously ground
  • one sample not containing the bioproduct, which will be the control

The following calculation will be then applied, considering a conversion factor of 6.25 (about 16% of nitrogen in the proteins):

This experiment requires:
  • suitable equipment (mineraliser, distiller, specific to the Kjeldahl method).
  • suitable individual and collective protections (laboratory coat, gloves, glasses, fume hood).
  • time: 2 half days in part-time (not necessarily successive).