The nitrogen/protein analysis according to Kjeldahl is up to date still the reference method in analytics. With its high degree of versatility, the method is suitable for many applications, e.g. food analysis, feed analysis, soil or water analysis. In addition to the versatility, the very high precision also speaks for the method. Particularly in case of highly inhomogeneous sample material, there is hardly any alternative to Kjeldahl due to the high weight of the sample. The Kjeldahl process was developed as early as 1883 by Johan Kjeldahl and is still used in this form today. It can be separated into 3 major process steps:
- Digestion of the samples with sulphuric acid
- Distillation of the digestion solution with steam
- Titration of the distillate and calculation of the result
In the classic application, manual laboratory heaters are used, as well as round bottom flasks for digestion and Erlenmeyer flasks for distillation. With the complete automation of the method using the digestion block and steam distillation system with integrated titration, the sample throughput can be increased significantly. At the same time, the repeatability of the analysis is increased and the entire work process is much safer for the laboratory staff. The diagram below shows how Kjeldahl analysis with automatic analysers works step by step.
Step 7 - Distillation
Addition of caustic soda to release ammonia, today this is done automatically in modern steam distillation units like VAPODEST
NH4+ + OH- → NH3↑ + H2O
Step 10 - Calculation
Calculation of nitrogen content:
% N = (1,4007 ∗ ceq ∗ (V - Vb)) / E
ceq - H+ Ion concentration of standard volumetric solution: hydrochloric acid c = 0,1 mol/l
alternativ: sulfuric acid ceq = 0,1 mol/l
V - Consumption volumetric standard solutions sample (ml)
Vb - Consumption volumetric standard solution blank (ml)
E - Weight (g)
Calculation of protein content:
% Crude protein = % N * PF
Examples of protein factors (PF):
| 6,38 | Milk, cheese, milk powder products, milk products |
| 6,25 | Meat, fish, poultry, egg, vegetable, fruits, different types of grain, corn, legumes, feed |
| 5,95 | Rice |
| 5,71 | Soy beans |
| 5,7 | Wheat and wheat flour |
| 5,4 | Oilseeds, nuts |
Note: The listed protein factors are often applied. Depending on regulation the analysis is based on, different protein factors must get applied. Especially in grain, nuts and oilseeds or single components of the above described sample types.
