English 
Dutch 
French 
Nitrogen determination methods
Neither Kjeldahl nor Dumas measures protein directly.
The standard approach for measuring protein content in food
Nitrogen determination is the standard approach for measuring protein content in food because routine laboratory methods determine nitrogen rather than protein directly. The two internationally accepted methods are Kjeldahl digestion and Dumas combustion. Both measure total nitrogen, but they differ in analytical principle, speed, reagent use, sample throughput, and the forms of nitrogen they recover.
Total nitrogen vs. protein determination
Neither Kjeldahl nor Dumas measures protein directly. Both quantify total nitrogen in a sample, and protein content is then calculated by multiplying the nitrogen result by a conversion factor, most commonly 6.25, though food-specific factors exist and are discussed in more detail in our article on protein conversion factors.
This distinction matters analytically because total nitrogen includes contributions from non-protein nitrogen (NPN) sources, such as free amino acids, nucleic acids, urea, ammonia, and, in adulteration cases, nitrogen-rich compounds added specifically to inflate apparent protein content.
The 2008 melamine contamination incident in dairy and infant formula is the clearest illustration of this vulnerability, melamine’s high nitrogen content allowed it to artificially raise Kjeldahl and Dumas protein readings without contributing any actual protein. Neither method, on its own, distinguishes protein nitrogen from non-protein nitrogen. Where that distinction is analytically important, laboratories turn to complementary approaches such as amino acid analysis or protein precipitation techniques.
How is nitrogen measured in food laboratories?
Food laboratories rely on two established method families for total nitrogen determination. Wet chemistry, represented by the Kjeldahl method, uses acid digestion followed by distillation and titration. Combustion analysis, represented by the Dumas method, uses high temperature combustion followed by thermal conductivity detection.
Both are recognized across international standards bodies, ISO, AOAC International, and IDF (International Dairy Federation) all publish reference methods for each approach, and regulatory frameworks in the EU and beyond generally accept results from either, provided the method is validated for the relevant matrix.
Instrument choice at a practical level often comes down to sample throughput requirements, matrix compatibility, reagent handling and disposal considerations, and existing laboratory infrastructure, since Kjeldahl requires digestion and distillation equipment while Dumas requires a combustion analyzer and pure oxygen supply.
Wet chemistry vs. combustion analysis
The Kjeldahl method proceeds in three stages. Digestion breaks down organic nitrogen using concentrated sulfuric acid at high temperature, typically with a catalyst such as copper sulfate or titanium dioxide, converting organic nitrogen to ammonium sulfate. Distillation then releases the ammonia by making the digest alkaline with sodium hydroxide and steam distilling it into a boric acid receiving solution. Titration with a standardized acid determines the amount of ammonia captured, from which nitrogen content is calculated.
Dumas combustion takes a fundamentally different route. The whole sample is combusted at high temperature, generally above 900°C, in a pure oxygen atmosphere. Combustion converts nitrogen compounds to nitrogen oxides, which are then reduced to elemental nitrogen gas over a copper catalyst. Water and carbon dioxide are removed, and the remaining nitrogen gas is measured by thermal conductivity detection.
The practical contrast is significant. Kjeldahl uses corrosive reagents, generates acidic waste requiring disposal, and a single analysis typically takes on the order of one to two hours including digestion time. Dumas requires no wet chemical reagents, produces no hazardous liquid waste, and completes an analysis in roughly three to four minutes, making it considerably better suited to high sample throughput environments.
What is the combustion method?
Dumas combustion analyzers operate by weighing a small, homogenized sample into a tin or aluminum capsule, which is dropped into a combustion reactor and rapidly oxidized. Modern instruments detect nitrogen across a wide range, from around 0.003 mg to 50 mg of nitrogen depending on the instrument, and can typically accommodate sample sizes from a few milligrams up to 2 to 3 grams. Calibration relies on certified reference standards with known nitrogen content, commonly EDTA (ethylenediaminetetraacetic acid) or THAM (tris(hydroxymethyl)aminomethane), rather than the pH-based calibration and indicator systems used in Kjeldahl titration.
Because combustion captures essentially all nitrogen in the sample, including some forms that Kjeldahl digestion can under recover, results between the two methods are not always numerically identical for every matrix, which is one reason regulatory methods specify which approach applies to which product category.
Which foods are suitable for Dumas?
Dumas performs well on cereals, oilseeds, animal feed, and dry, homogeneous matrices where rapid throughput is valuable, this is reflected in:
- ISO 16634-1, which governs oilseeds and animal feeding stuffs
- ISO 16634-2, which covers cereals, pulses, and milled cereal products
- OAC 992.15 specifies Dumas combustion for meat and meat products
- ISO 14891 covers Dumas determination of nitrogen in milk and milk products.
Dumas is also widely used for plant-based protein ingredients and meat analogues, where its speed suits routine quality control across large sample batches.
Matrices with very low nitrogen content, or samples that are difficult to homogenize into the small portions Dumas requires, can be more challenging, since representative sampling becomes critical when working with only a few milligrams of material.
Which foods are better analyzed using Kjeldahl?
Kjeldahl remains the reference method specified for a wide range of matrices where its long analytical history and regulatory acceptance carry weight:
- AOAC 981.10 specifies Kjeldahl for meat products
- AOAC 991.20 and ISO 8968-1/IDF 20-1 for milk
- AOAC 979.09 for grains
- AOAC 976.05 for animal feed and pet food.
- ISO 20483 governs the Kjeldahl determination of nitrogen in cereals and pulses specifically
- ISO 1871 provides general guidelines for Kjeldahl nitrogen determination across food and feed products more broadly
Kjeldahl is often favored for complex or fatty matrices, such as certain meat products, confectionery, and starches, where digestion can more reliably break down the full sample matrix, and it continues to serve as the referee method against which newer techniques, including Dumas and near infrared spectroscopy, are validated in many regulatory contexts.
What is recovery in nitrogen analysis?
Recovery in nitrogen analysis is the percentage of the true nitrogen content that an analytical method successfully detects. High recovery demonstrates that digestion or combustion is complete and that nitrogen results accurately represent the sample.
ISO 20483 specifies that Kjeldahl systems must demonstrate a recovery rate of at least 99.5 percent for acetanilide and 99.0 percent for tryptophan before the acid quantity or method parameters are considered acceptable. These reference compounds are chosen because acetanilide represents a straightforward nitrogen source while tryptophan, an amino acid with a nitrogen containing ring structure, is more resistant to digestion and therefore a stricter test of digestion efficiency. Recovery testing on both compound types helps confirm that a method performs reliably not just on simple nitrogen sources but on the more complex nitrogen structures found in real food proteins.
How to validate nitrogen results
Validating nitrogen results in a food laboratory typically follows several parallel checks. Repeatability is assessed by running duplicate or triplicate analyses on the same homogenised sample and confirming the results fall within the repeatability limit specified by the relevant standard, ISO 20483, for example, defines specific repeatability and reproducibility limits for Kjeldahl nitrogen determination in cereals and pulses.
Recovery is verified using certified reference materials or spiked samples with known nitrogen content, run alongside the routine batch, and compared against the method’s specified acceptance range. Blank determinations are run with each batch to confirm reagent and system background nitrogen remains stable and within expected limits. Cross-method verification, running a subset of samples by both Kjeldahl and Dumas, is a common practice when switching methods or introducing new instrumentation, since discrepancies between the two can reveal matrix-specific digestion or combustion issues that single-method validation would miss.
Laboratories operating under ISO/IEC 17025 accreditation additionally maintain documented method validation records covering linearity, limit of detection, limit of quantification, and measurement uncertainty, ensuring nitrogen results remain traceable and defensible over time.
Comparison of the Dumas and Kjeldahl method
Feature | Kjeldahl Method | Dumas Method |
Principle | Wet chemistry (digestion–distillation–titration) | High‑temperature combustion |
Nitrogen detected | Organic nitrogen only | Total nitrogen (organic + inorganic) |
Typical analysis time | 2–4 hours | 3–5 minutes |
Chemicals required | Sulfuric acid, sodium hydroxide, catalysts | None (only gases) |
Automation | Limited | Fully automated |
Safety & environment | Chemical waste, corrosive reagents | Low waste, safer process |
Instrument cost | Lower | Higher |
Operating cost | Moderate (reagents, waste disposal) | Low (mainly gases) |
Best suited for | Regulated food/feed protein | High-throughput labs, total N |
Learn more about nitrogen and protein
Discuss your food analysis requirements
Our Xperts are happy to discuss your application and help you find the right solution.
FAQ
What is the difference between the Kjeldahl and Dumas methods?
The Kjeldahl and Dumas methods are the two internationally accepted techniques for nitrogen determination and protein analysis. Both methods measure total nitrogen rather than protein directly. The Kjeldahl method uses acid digestion, distillation, and titration, while the Dumas method uses high-temperature combustion and thermal conductivity detection. Dumas is significantly faster, whereas Kjeldahl remains a widely used reference method in many regulatory applications.
Do Kjeldahl and Dumas measure protein directly?
No. Neither Kjeldahl nor Dumas measures protein directly. Both methods determine total nitrogen content, and protein content is calculated by multiplying the nitrogen result by an established protein conversion factor, commonly 6.25.
Why is nitrogen used to determine protein content?
Proteins contain nitrogen as a key structural component. Because direct protein measurement is often more complex, laboratories determine total nitrogen and convert it into protein content using standardized conversion factors.
What is total nitrogen analysis?
Total nitrogen analysis is the measurement of all nitrogen present in a sample, including protein nitrogen and non-protein nitrogen. Results are commonly used to calculate protein content in food, feed, dairy, grain, and agricultural products.
What is non-protein nitrogen (NPN)?
Non-protein nitrogen refers to nitrogen-containing compounds that are not proteins, such as free amino acids, urea, nucleic acids, ammonia, and certain additives. Both Kjeldahl and Dumas measure these compounds as part of total nitrogen.
Why is non-protein nitrogen important in protein analysis?
Non-protein nitrogen can artificially increase calculated protein values because nitrogen-based methods cannot distinguish between protein nitrogen and non-protein nitrogen. This is why additional analytical techniques may be required in some applications.
What was the melamine contamination incident?
The 2008 melamine contamination incident involved the addition of melamine to dairy products and infant formula to artificially increase apparent protein content. Because melamine contains a high concentration of nitrogen, both Kjeldahl and Dumas methods reported falsely elevated protein levels.
How does the Kjeldahl method work?
The Kjeldahl method measures nitrogen through three main steps. The sample is first digested in concentrated sulfuric acid, converting organic nitrogen into ammonium sulfate. Ammonia is then released through alkaline distillation and captured in a receiving solution. Finally, the ammonia is quantified through titration to calculate nitrogen content.
What is the Dumas method?
The Dumas method is a combustion-based technique for nitrogen determination. Samples are combusted in pure oxygen at temperatures above 900°C, converting nitrogen compounds into nitrogen gas that is measured using a thermal conductivity detector.
Why is the Dumas method faster than Kjeldahl?
The Dumas method eliminates digestion, distillation, and titration steps. Most Dumas analyses are completed within three to four minutes, whereas Kjeldahl analysis often requires one to two hours.
Which analytical method is better: Kjeldahl or Dumas?
Neither method is universally better. Dumas offers speed, automation, and no hazardous liquid waste, making it ideal for high-throughput laboratories. Kjeldahl remains highly accepted by regulators and is often preferred for certain complex food matrices and reference testing applications.
