The Kjeldahl method: nitrogen and protein determination, standards and the FOSS Kjeltec 9 workflow

In 1883 Johan Kjeldahl, head of chemistry at the Carlsberg Laboratory in Copenhagen, published a new method for nitrogen determination in organic substances. His practical problem concerned barley protein during malting and its effect on beer quality.

Catalysts and potassium sulphate later accelerated digestion, while boric acid simplified ammonia collection. The core logic remains: convert organic nitrogen into a measurable form and quantify it by titration. Today the principle is embedded in industry standards and supports verification of rapid NIR and FTIR methods.

The analytical result is first obtained as a nitrogen mass fraction. Protein is calculated as Protein (%) = Nitrogen (%) × F, where F is the conversion factor prescribed by the standard or accepted method for the matrix. A common general factor is 6.25; 6.38 is widely used for dairy and 5.70 for wheat.

The factor must not be selected simply to obtain a convenient result. The report should identify the standard, sample type and factor used. Kjeldahl also includes non-protein nitrogen, so “crude protein” is often the more precise term unless the standard defines otherwise.

The test portion is heated with concentrated H₂SO₄ in the presence of a catalyst and K₂SO₄. The organic matrix is destroyed; carbon is mainly converted to CO₂, hydrogen to H₂O and organic nitrogen to ammonium, usually represented as (NH₄)₂SO₄ in solution. Simplified: organic N + H₂SO₄ → (NH₄)₂SO₄ + CO₂ + H₂O.

Kjeltec 9 does not perform this stage. A separate Digestor is required: manual DT 208/220 blocks for 8 or 20 tubes, or automated Digestor 2508/2520 units. Corrosive fumes require the matching exhaust manifold and consideration of a scrubber, together with a fume hood and an appropriate safety procedure.

After digestion the solution is made alkaline with NaOH. Ammonium is converted to volatile ammonia: NH₄⁺ + OH⁻ → NH₃↑ + H₂O. For ammonium sulphate, the molecular form is (NH₄)₂SO₄ + 2NaOH → 2NH₃↑ + Na₂SO₄ + 2H₂O.

Steam carries the ammonia into a boric-acid receiving solution. Kjeltec 9 controls reagent addition and distillation; the SAfE mode helps reduce the risk of a violent reaction with difficult samples. The exact programme must follow the approved method and expected nitrogen level.

Ammonia captured in the receiving solution is titrated with a standardised acid to the endpoint. With hydrochloric acid the simplified reaction is NH₃ + HCl → NH₄Cl. Titrant volume and concentration, corrected by the blank, provide the nitrogen amount. In general: N (%) = ((V − V₀) × c × 14.007 × 100) / (m × 1000), where V and V₀ are sample and blank titrant volumes, c is molarity and m is the test portion in grams. Protein is Protein (%) = N (%) × F.

Kjeltec 9 Analyser uses built-in colorimetric titration, automatic LED calibration and automatic endpoint-colour setting. Automation reduces manual operations but does not replace titrant standardisation, blanks, control materials or periodic nitrogen-recovery checks.

There is no single universal standard number for every product. The method and protein factor must be selected for the matrix, the scope of the standard and the laboratory procedure. Relevant CIS standards include GOST 23327-98 and GOST 34454-2018 for milk, GOST 25011-2017 for meat, GOST 10846-91 for grain and GOST 13496.4-2019 for feed. International references include ISO 8968-1 / IDF 20-1 for dairy and ISO 937 for meat.

Before implementation, the laboratory must verify the current edition, scope, sampling and preparation rules, permitted reagents, blank correction, accuracy control and reporting format. Owning a Kjeltec does not by itself create compliance: compliance belongs to the complete validated procedure.

Kjeltec 9 Distillator performs steam distillation and can work with an external titrator. Kjeltec 9 Analyser adds built-in colorimetric titration and automatic calculation. Analyser with Autosampler accepts 20 or 60 samples for batch work. FOSS states up to 240 samples per 24 hours and more than four hours of unattended operation with the autosampler.

For Kjeltec 9 Analyser, FOSS specifies a 0.1–210 mg N range, 3.5 minutes at 30 mg N and 6.5 minutes at 200 mg N, with recovery above 99.5% for 1–200 mg N. These are manufacturer specifications under stated conditions and do not replace performance verification for the laboratory matrix.

A working system has at least two process modules: digestion in a Digestor and distillation/titration in Kjeltec. The digestion block needs racks and tubes, an exhaust manifold and a fume-removal or neutralisation solution. The analyser requires reagent vessels, reagents, burette and consumables; an autosampler is added for higher throughput.

For lower throughput, DT 208/220 with Kjeltec 9 Distillator or Analyser is a practical setup. Medium and high throughput favours Digestor 2508/2520 with Kjeltec 9 Analyser, optionally with a 20- or 60-sample autosampler. LabLog clarifies matrices, standards, sample flow, utilities and traceability needs, then configures supply, consumables, commissioning and training for projects in Uzbekistan.