How is protein content measured in animal feed?

Protein content in animal feed is measured using one of several established laboratory methods, with the Kjeldahl and Dumas combustion techniques being the most widely used in feed analysis. Near-infrared spectroscopy (NIR) is also commonly applied for rapid, high-throughput screening. The choice of method depends on the lab’s throughput requirements, accuracy needs, and regulatory context. This article walks through the key questions feed labs face when selecting and applying protein measurement methods.

Which laboratory methods are used to measure protein in animal feed?

The main laboratory methods used to measure protein content in animal feed are the Kjeldahl method, the Dumas combustion method, and near-infrared (NIR) spectroscopy. Kjeldahl and Dumas are both reference methods based on nitrogen determination, while NIR is a rapid screening technique calibrated against these reference methods.

Each method approaches protein quantification differently, and feed labs often use more than one depending on the stage of analysis:

Kjeldahl method: A wet chemistry technique that digests the sample in acid to release nitrogen, which is then quantified. Long established as a reference method in feed analysis.
Dumas combustion method: Combusts the sample at high temperature to release nitrogen as gas, which is measured directly. Faster and reagent-free compared to Kjeldahl.
NIR spectroscopy: A non-destructive, rapid technique that measures reflected or transmitted near-infrared light to predict protein content based on calibration models.
Wet chemistry alternatives: Methods such as the Bradford or Lowry assays exist but are rarely used in routine feed quality control due to their complexity and limited applicability for complex matrices.

For regulatory and compliance purposes in the Benelux and across the EU, Kjeldahl and Dumas are the accepted reference methods for crude protein determination in feed. NIR is widely used operationally but must be calibrated and validated against a reference method.

What is the difference between the Kjeldahl and Dumas methods?

The key difference between the Kjeldahl and Dumas methods lies in how they release and detect nitrogen from the sample. Kjeldahl uses wet acid digestion followed by distillation and titration, while Dumas uses high-temperature combustion to convert all nitrogen to gas, which is then measured by thermal conductivity detection. Both methods measure total nitrogen to calculate crude protein.

In practical terms, the two methods differ across several important dimensions:

Speed: Dumas is significantly faster, typically completing an analysis in a few minutes per sample. Kjeldahl digestion can take considerably longer per batch.
Reagent use: Kjeldahl requires concentrated sulfuric acid and other reagents, creating chemical waste. Dumas is essentially reagent-free, using only oxygen and helium or similar carrier gases.
Throughput: Dumas analyzers are well-suited to high-throughput environments. Kjeldahl can be run in parallel batches but involves more manual handling.
Result equivalence: When properly calibrated and validated, both methods produce comparable results for most feed matrices. Regulatory guidelines in the EU recognize both.
Sample size: Dumas typically requires a smaller sample weight, which can be advantageous when material is limited.

Feed labs transitioning from Kjeldahl to Dumas often do so to reduce analysis time and chemical waste without sacrificing accuracy. Both methods are accepted under EU feed legislation and relevant ISO and AOAC standards.

What does ‘crude protein’ actually measure in feed analysis?

Crude protein is not a direct measurement of protein molecules. Instead, it is a calculated value based on total nitrogen content in the sample, multiplied by a conversion factor, most commonly 6.25, which assumes that protein contains approximately 16% nitrogen on average. The term “crude” reflects that it captures all nitrogen, not just protein-bound nitrogen.

This distinction matters in practice. Non-protein nitrogen (NPN) compounds, such as urea, ammonium salts, or free amino acids, also contribute to the crude protein figure. In ruminant nutrition, some NPN sources can be utilized by the animal, but in monogastric nutrition, the distinction between true protein and NPN is more significant for formulation accuracy.

The conversion factor of 6.25 is a general approximation. Some feed ingredients, such as milk proteins or certain plant-based materials, have different nitrogen-to-protein ratios, and specific conversion factors are sometimes applied for these. Feed labs and nutritionists should be aware that crude protein figures are a practical, standardized measure rather than an absolute representation of digestible or functional protein content.

How accurate is NIR spectroscopy for feed protein analysis?

NIR spectroscopy can deliver high accuracy for protein analysis in animal feed, provided the instrument is calibrated with a robust, representative set of reference samples analyzed by a validated reference method such as Kjeldahl or Dumas. The accuracy of NIR is fundamentally dependent on the quality of its calibration model, not on the instrument alone.

NIR works by measuring how feed samples absorb near-infrared light at different wavelengths. Statistical models then correlate these spectral patterns to known protein values from reference analyses. When calibration is well-developed and the sample matrix is consistent, NIR can achieve accuracy comparable to reference methods for routine quality control purposes.

However, NIR has limitations that feed labs should account for:

Calibrations are matrix-specific: a model developed for soybean meal may not perform reliably on a different ingredient without recalibration.
NIR cannot detect adulteration or unusual nitrogen sources that fall outside the calibration range.
Regular validation against reference methods is essential to maintain accuracy over time.
Sample preparation (particle size, moisture) affects spectral readings and must be consistent.

In Benelux feed labs, NIR is widely used for incoming material screening and in-process checks, while Kjeldahl or Dumas remain the reference methods for official reporting and regulatory compliance.

What sample preparation steps are required before protein analysis?

Sample preparation before protein analysis in animal feed typically involves drying, grinding, and homogenization to produce a representative, consistent sub-sample. The specific steps depend on the analytical method being used and the physical form of the feed material.

For Kjeldahl and Dumas analyses, the feed sample must be dried to a known moisture content and ground to a fine, uniform particle size. This ensures accurate weighing of the sub-sample and complete digestion or combustion. Moisture content is usually determined separately so that results can be expressed on a dry matter basis.

For NIR analysis, particle size consistency is particularly critical, as spectral readings are sensitive to surface texture and uniformity. Pellets, mash feeds, and raw ingredients may each require different preparation approaches before scanning.

Common sample preparation steps across methods include:

Representative sampling from the batch using appropriate sampling tools
Drying at a controlled temperature to reduce moisture variability
Grinding using a mill to achieve a defined particle size
Homogenization to ensure the sub-sample reflects the whole batch
Accurate weighing of the sub-sample within the instrument’s specified range

Inadequate sample preparation is one of the most common sources of variability in feed protein analysis. Investing in proper sample preparation support can meaningfully improve the reproducibility of results across all downstream analytical methods.

Which protein measurement method should a feed lab choose?

The right protein measurement method for a feed lab depends on throughput requirements, regulatory obligations, budget, and the type of feed matrices being analyzed. For most routine feed quality control labs in the Benelux, Dumas is increasingly the preferred reference method due to its speed and lower reagent burden, while NIR is used alongside it for rapid screening.

A practical decision framework looks like this:

High throughput, regulatory reporting: Dumas combustion offers the best combination of speed and compliance with EU feed standards.
Existing Kjeldahl infrastructure: Kjeldahl remains a fully valid and accepted method; upgrading is worthwhile when throughput or waste reduction is a priority.
Rapid at-line or in-process screening: NIR is the practical choice, provided calibrations are maintained and validated against a reference method.
Limited sample availability or complex matrices: Consider which method’s sample size requirements and matrix tolerance best fit the material being analyzed.

Many feed labs operate a combination: NIR for fast, high-volume screening and Dumas or Kjeldahl for confirmation and regulatory reporting. This layered approach balances efficiency with analytical rigor.

How SciSPX supports protein analysis in feed labs

SciSPX works with feed quality control and analytical labs across the Benelux to select, install, and validate the right instrumentation for protein analysis in animal feed. Whether a lab is evaluating a move from Kjeldahl to Dumas, building out an NIR screening workflow, or looking to improve sample preparation consistency, SciSPX’s team of application specialists and service engineers provides hands-on support at every stage.

Application-level guidance on method selection and instrument fit for your feed matrices
IQ/OQ installations and validations in line with GMP and GLP requirements
Training for lab technicians and QC managers on instrument operation and method performance
Ongoing service and preventive maintenance to keep instruments performing to specification

SciSPX operates exclusively in the Benelux, with offices in the Netherlands and Belgium, ensuring local expertise and fast response times. If you are reviewing your feed protein analysis setup or selecting new instrumentation, get in touch with our team to discuss your lab’s specific requirements.

Frequently Asked Questions

Can the Dumas method fully replace Kjeldahl in a feed lab, or are there situations where Kjeldahl is still necessary?

In most routine feed analysis scenarios, the Dumas method can fully replace Kjeldahl — both are accepted reference methods under EU feed legislation and relevant ISO and AOAC standards. However, some labs may need to retain Kjeldahl capabilities if their specific accreditation scope, customer contracts, or legacy reporting requirements explicitly mandate it. Before decommissioning Kjeldahl equipment, it is worth reviewing your lab’s regulatory obligations and any third-party audit requirements to confirm that Dumas results will be accepted across all reporting contexts.

How often should NIR calibrations be validated against a reference method, and what triggers a recalibration?

As a general best practice, NIR calibrations should be validated against a reference method such as Kjeldahl or Dumas at regular intervals — typically monthly or quarterly depending on sample volume and matrix variability — and whenever a significant change occurs in the feed matrix, ingredient sourcing, or sample preparation workflow. Triggers for recalibration include systematic bias detected during routine validation checks, introduction of new raw materials or ingredient origins, instrument servicing or lamp replacement, and seasonal variation in ingredient composition. Keeping a validation log helps identify drift early before it affects reported results.

What are the most common mistakes feed labs make when interpreting crude protein results?

The most common mistake is treating the crude protein figure as an accurate representation of digestible or functional protein, when in reality it reflects total nitrogen — including non-protein nitrogen (NPN) sources such as urea or ammonium salts. This can lead to overestimation of the nutritional value of certain ingredients, particularly in monogastric feed formulation. A second frequent error is applying the standard 6.25 conversion factor universally, without accounting for ingredients like milk proteins or specific plant-based materials that have different nitrogen-to-protein ratios and warrant ingredient-specific conversion factors.

How do feed labs handle protein analysis for mixed or compound feeds compared to single raw ingredients?

Mixed and compound feeds present greater analytical complexity than single raw ingredients, primarily because their heterogeneous composition makes representative sampling and homogenization more challenging. For Kjeldahl and Dumas analyses, thorough grinding and homogenization are especially important to ensure the sub-sample accurately reflects the batch composition. For NIR, compound feeds often require dedicated calibration models distinct from those used for individual raw materials, as the spectral profile of a blend differs significantly from any single ingredient. Labs analyzing both raw materials and finished compound feeds typically maintain separate NIR calibration sets for each matrix type.

Is it possible to detect protein adulteration — such as nitrogen spiking — using standard feed protein analysis methods?

Standard nitrogen-based methods like Kjeldahl and Dumas cannot distinguish between protein-bound nitrogen and nitrogen from non-protein sources, which is precisely what makes nitrogen spiking — the deliberate addition of cheap NPN compounds like melamine or urea to inflate crude protein readings — an ongoing concern in feed and food supply chains. NIR is similarly unable to detect adulteration that falls within the spectral range of the calibration model. Detecting adulteration requires complementary techniques such as amino acid profiling, wet chemistry for specific NPN compounds, or targeted screening methods. Labs with adulteration risk in their supply chain should consider a tiered testing strategy that goes beyond routine crude protein determination.

What should a feed lab consider when purchasing a new Dumas or NIR instrument for the first time?

Beyond the instrument’s technical specifications, labs should evaluate the supplier’s ability to provide application support specific to their feed matrices, local service coverage and response times, and the availability of installation qualification and operational qualification (IQ/OQ) documentation aligned with GMP or GLP requirements. For NIR, it is also important to assess the quality and breadth of the calibration models available out of the box and the supplier’s support for developing or expanding custom calibrations. Requesting a demonstration or trial analysis using your own representative feed samples is a practical way to evaluate real-world performance before committing to a purchase.

How should feed labs manage protein analysis results when sample moisture content varies significantly between batches?

Moisture variability is a significant source of inconsistency in feed protein results if not properly controlled. The standard approach is to determine moisture content separately — typically by oven drying — and express all protein results on a dry matter basis, which allows meaningful comparison across batches regardless of moisture fluctuations. For NIR analysis, moisture also directly affects spectral readings, so consistent sample drying or the use of NIR models that simultaneously predict both moisture and protein is essential. Reporting protein on both an as-received and dry matter basis is considered best practice in feed quality control, as it supports both commercial transactions and nutritional formulation accuracy.

This content was generated with the help of AI and it may contain mistakes