Protein Content in Food and Feed

Protein content represents the total nitrogen-containing compounds in a sample, expressed as crude protein using a defined nitrogen-to-protein conversion factor. In food and feed analysis, protein is a primary nutritional parameter and a central component of proximate composition.
Analytically, most routine protein determinations measure total nitrogen and convert this value to protein using a factor. It is therefore important to recognise that reported crude protein reflects total nitrogen rather than true protein exclusively, as non-protein nitrogen (NPN) compounds are also included.

Protein levels vary significantly depending on raw material origin, agricultural practices, and processing conditions. In feed systems, major protein sources include:
‣ Soybean meal
‣ Oilseed cakes and meals
‣ Fishmeal
‣ Animal by-products
‣ Legumes and cereal grains
In food commodities, protein is a key nutritional declaration parameter in flours, dairy products, plant-based ingredients, pet food, and processed foods.
Protein concentration directly influences formulation accuracy, animal performance, and cost efficiency. In compound feed production, underestimation or overestimation can affect ration balancing, amino acid supplementation, and regulatory compliance.

Protein is essential for growth, tissue repair, enzyme production, and metabolic function. In animal nutrition, inadequate protein intake reduces productivity, while excessive levels may increase nitrogen excretion and environmental burden.
From a regulatory perspective, declared protein values must reflect actual composition. Historical adulteration incidents involving nitrogen-rich compounds have demonstrated the importance of robust total nitrogen testing to detect fraudulent inflation of protein values.
Because crude protein includes non-protein nitrogen, interpretation must consider the matrix. In certain feed materials, elevated NPN may be intentional (e.g., urea in ruminant feed), whereas in others it may indicate quality concerns.

Protein content is commonly included in:
‣ Nutritional labelling requirements (food and pet food)
‣ Feed registration specifications
‣ Raw material trading contracts
‣ Quality control release criteria
Declared values must be supported by validated analytical methods. Laboratories operating under ISO 17025 accreditation must demonstrate method validation, ongoing quality control, and estimation of measurement uncertainty.

Protein is routinely determined by measuring total nitrogen using either combustion (Dumas method) or wet chemical digestion (Kjeldahl method), followed by application of a conversion factor.

The Dumas method involves high-temperature combustion of the sample (typically >900 °C) in an oxygen-rich environment. Nitrogen-containing compounds are converted to nitrogen gas, which is measured using thermal conductivity detection.
Key characteristics:
‣ Rapid analysis with high throughput
‣ No use of concentrated corrosive chemicals
‣ Automated operation
‣ Measures total nitrogen directly
Because combustion is complete, all nitrogen, including nitrate, nitrite, and other nitrogenous compounds, is detected.

The Kjeldahl method involves the digestion of the sample in concentrated sulphuric acid with a catalyst, converting organic nitrogen to ammonium sulphate. Following digestion, the solution is neutralised, distilled, and the released ammonia is titrated to quantify nitrogen.
Key characteristics:
‣ Long-established reference method
‣ Widely recognised in regulatory frameworks
‣ Measures organic nitrogen and ammonium nitrogen
‣ Does not detect nitrate or nitrite without modification
The process involves hazardous chemicals and is more labour-intensive compared to combustion methods.

Both Dumas and Kjeldahl determine crude protein indirectly by measuring nitrogen and applying a conversion factor. However, the critical distinction lies in the forms of nitrogen detected, which can influence reported protein values in certain matrices.
The Dumas combustion method measures total nitrogen, regardless of its chemical form. This includes:
‣ Organic nitrogen present in proteins
‣ Ammonium nitrogen
‣ Nitrate and nitrite
‣ Other inorganic nitrogen compounds
Because all nitrogen is detected, the method can produce higher apparent crude protein values in materials containing inorganic nitrogen. In most conventional feed and food matrices, this difference is negligible. However, in samples where non-protein nitrogen is present — whether through intentional inclusion (e.g. urea in ruminant feed) or contamination — Dumas reflects total nitrogen rather than protein-specific nitrogen.
The Kjeldahl method, by contrast, quantifies organic nitrogen and ammonium nitrogen, but does not detect nitrate or nitrite unless additional reduction steps are incorporated. As a result, it excludes certain inorganic nitrogen forms from the reported value.
For this reason, Kjeldahl is often regarded as providing a measurement more closely aligned with protein-bound nitrogen, particularly in matrices where inorganic nitrogen may be present. This characteristic historically contributed to its status as a reference method in regulatory frameworks.
In practice, the magnitude of difference between the two methods depends entirely on matrix composition. In clean, well-characterised raw materials, results are typically comparable. In matrices containing measurable inorganic nitrogen, Dumas may yield slightly higher crude protein values due to its broader nitrogen detection scope.
Understanding this distinction is important when interpreting results, comparing historical data, or evaluating compliance with specification limits.

Routine protein monitoring supports:
‣ Verification of raw material specifications
‣ Feed formulation accuracy
‣ Nutritional labelling compliance
‣ Detection of adulteration
‣ Supplier quality control
Trend analysis of protein results assists in identifying variability in incoming materials and helps prevent formulation drift. Because protein is calculated from nitrogen, the correct application of conversion factors is essential for accurate reporting.
Accurate and reproducible nitrogen determination remains fundamental to nutritional assurance, regulatory compliance, and commercial integrity in both food and feed production systems.