What is Kjeldahl analysis?
The Kjeldahl method is the reference wet-chemistry procedure for determining organic nitrogen — and, via a conversion factor, crude protein — in food, feed, dairy, soil, fertilizer and water samples.
In short: Kjeldahl analysis determines nitrogen in three steps — digestion, distillation, titration — and converts it to crude protein. A fully automatic Kjeldahl analyzer performs all three steps unattended in 3–8 minutes per sample.
Three steps, fully automated.
Digestion
The sample is digested in concentrated sulphuric acid with a catalyst on a digestion block (Smart, X, S or Gr series), converting organic nitrogen to ammonium.
Distillation
Alkali is added and steam distillation releases the ammonia, which is absorbed in boric acid — automated on every K-series analyzer.
Titration
The absorbed ammonia is titrated to a colorimetric end point detected by a 16-bit tri-primary color sensor (built in on K06, K12, K18 and K1306; external on K1305/K1301).
Data handling
Results are calculated, stored and exported — K12/K18 store up to 5,000,000 test results with USB, Wi-Fi and host-control output; K18 adds LIMS connectivity.
Semi-automatic vs fully automatic.
The defining difference is the titration system. A semi-automatic analyzer (K1301) automates reagent addition, distillation and waste handling, while the end-point titration happens off-line. A fully automatic analyzer (K06, K12, K18, K1306) adds the titration system in-line, so the result comes out directly without manual operation during the determination. All K-series models share the 0.1–240 mg N measuring range and a listed 3–8 minutes per sample.
Sonnen’s K12 and K18 additionally provide an auto-sampler interface — the A20 AutoSampler is the current recommended solution for unattended batches.
The chemistry, in numbers.
| Step | What happens | Typical parameters |
|---|---|---|
| Digestion | Organic nitrogen is converted to ammonium sulphate in concentrated H₂SO₄; K₂SO₄ raises the acid boiling point, CuSO₄/TiO₂ catalyses the oxidation. | Block temperature around 420 °C; about 30–60 minutes to a clear digest, matrix-dependent. |
| Distillation | NaOH (~40%) liberates ammonia; steam carries it into boric acid (2–4%), where it is trapped as the ammonium-borate complex. | 3–8 minutes per sample on an automated K-series analyzer. |
| Titration | The trapped ammonia is titrated with standard acid (0.05–0.1 M HCl or H₂SO₄) to a colorimetric end point. | Result is calculated automatically as %N, or as %protein after applying the conversion factor. |
Reagent concentrations and digestion times vary with the standard applied — always follow the method sheet for your matrix.
From nitrogen to protein: the factor matters.
Kjeldahl measures nitrogen, not protein directly. Crude protein is nitrogen multiplied by a conversion factor — and the factor is not universal. It is fixed by the standard for each matrix:
| Sample | Factor | Basis |
|---|---|---|
| Most foods and feeds | 6.25 | Default factor in ISO and AOAC Kjeldahl methods |
| Milk and dairy products | 6.38 | ISO 8968 |
| Wheat and flour | 5.70 | ISO 20483 |
| Soybean and soy products | 5.71 | AOAC practice — see our soybean protein workflow |
Reporting protein without stating the factor is a common audit finding. Every Sonnen analyzer lets you set the factor per method, so the printed result already carries it.
Where the method is used.
Kjeldahl remains the reference procedure wherever regulations define protein by nitrogen. Typical workloads we see across installations:
Milk & dairy products
Raw milk runs about 3.0–3.5% protein (factor 6.38, ISO 8968). High daily throughput is the norm in dairy labs — autosamplers pay back fastest here.
Cereals & flour
Wheat typically tests at 9–15% protein (factor 5.70, ISO 20483). Trading and inspection labs run to national and contract specifications.
Feed & oilseed meals
Soybean meal trades at 44–48% protein. Feed mills test every batch, in and out — the highest sample volumes we serve.
Soil, fertilizer & water
Total nitrogen in soils usually falls between 0.02–0.5%; water labs report TKN. Low-level work puts the emphasis on blank control and recovery.
What “unattended” means in practice.
Automation level decides how much of the working day the analysis consumes. A fully automatic K-series analyzer returns a result every 3–8 minutes once the digests are ready. Pair a K12 or K18 with the A20 autosampler and a 20-position batch runs roughly 1–2.5 hours with nobody at the bench; the A35 extends that to 35 positions.
The smoothest workflow we ship starts one step earlier: the Smart 20 digester finishes twenty samples in the same 20-position rack, and the whole rack lifts straight onto the A20 — no tube-by-tube transfer between digestion and analysis. With a distillation unit (K1301, K1305) the operator returns for the titration step instead.
Instruments for the method.
Standards built on Kjeldahl.
Questions labs ask us.
Is the Kjeldahl method still the reference method in 2026?
Yes. The ISO, AOAC and national standards that define protein in food, feed, dairy and cereals are built on Kjeldahl nitrogen determination — ISO 8968 for milk, ISO 20483 for cereals, ISO 1871 as the general guideline. Faster combustion methods (Dumas) are standardized for many matrices too, but Kjeldahl remains the reference method specified in trade contracts and regulations in most markets.
How long does a complete Kjeldahl determination take?
Digestion takes about 30–60 minutes on a block digester, depending on the matrix. Distillation and titration on a fully automatic analyzer add 3–8 minutes per sample. In batch workflows the two stages overlap: while one rack of digests is being analyzed, the next rack is already on the digestion block.
How much sample does a Kjeldahl test need?
For food and feed, typically 0.5–2 g of a homogenized sample; high-protein materials need less, low-nitrogen matrices such as soil or water use more sample or a larger volume. The standard for your matrix specifies the exact test portion.
What is the difference between Kjeldahl and Dumas?
Kjeldahl is a wet-chemistry method: acid digestion, then distillation and titration of the released ammonia; it measures organic nitrogen plus ammonia. Dumas combusts the sample at high temperature and measures total nitrogen by thermal conductivity. Both are standardized; the choice depends on your matrix, the standard you must report against, and throughput needs. A full side-by-side comparison is in preparation in our resources section.
Can the Kjeldahl method measure nitrate and nitrite?
Not directly — the standard procedure covers organic nitrogen and ammonia. Nitrate and nitrite require a reduction pre-treatment (for example with salicylic acid or a reducing agent) before digestion, as specified in the applicable standard.
Choosing between K06, K12, K18 and K1306?
Tell us your sample type and daily throughput.