Protein analysis by the Kjeldahl method

An example for practicing


You have to blindly control two yogurts. You know that one of them is a classic yogurt, while the other is a protein-rich yogurt. To this end, you carry out a determination of total nitrogen for each of the products (A and B) by the Kjeldahl method (see experimental procedure).
  • the initial test sample mass (mproduct) is 1.035 g for each yogurt.
  • the titration volumes of NaOH 0.1 mol/L (Vsample) are 20.4 mL for yogurt A and 15.9 mL for yogurt B.
  • the titration volume of NaOH for the control (Vcontrol) is 25.1 mL.
Yogurt 1: Classic yogurt. Yoghurt B: High protein yoghurt. Initial test sample: 1035g for both products. The volumes of soda poured are 20.4mL for yogurt 1, and 15.9mL for yogurt B. Volume of soda poured for the control: 25.1mL.
The formula for determining the protein content in a product, in g for 100 g of product, is:
Yogurt A is the protein-rich yogurt.
The protein content in yogurt A, in g for 100 g of product, is:
The protein content in yogurt B, in g for 100 g of product, is:
If you are presented with a third yogurt (yogurt C), which is said to be half as high in protein as the classic yogurt, what volume of sodium hydroxide (0.1 mol/L) would you expect to pour to verify this information if the mass of the initial sample is 1.000 g?
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