The Kjeldahl method is the internationally recognized reference technique for determining nitrogen content and calculating crude protein in food, feed, and pharmaceuticals. Developed in 1883, it remains the gold standard for nutritional labeling and quality control. Ovalab provides accredited protein determination using this validated method.
Key Facts About Protein Determination (Kjeldahl)
- Principle: Three steps: digestion (sulfuric acid converts nitrogen to ammonium sulfate), distillation (ammonia released and steam-distilled), and titration (acid-base quantification).
- Conversion factors: 6.25 (general foods), 6.38 (dairy, ISO 8968-1:2014), 5.70 (wheat/cereals), 5.71 (soybeans).
- EU labeling: Protein must be declared under EU Regulation 1169/2011. Daily reference intake: 50 g (Annex XIII, Part B).
- Claims: “Source of protein” requires ≥ 12% of energy from protein; “high protein” requires ≥ 20% (EU Regulation 1924/2006).
- ISO standards: ISO 8968-1:2014 for milk/dairy. For general foods: AOAC 2001.11 and EN ISO 20483.
Testing Methods at Ovalab
Ovalab determines protein using the Kjeldahl digestion method with automated distillation and titration, applying appropriate nitrogen-to-protein conversion factors per matrix. All testing is under ISO/IEC 17025:2018 accreditation (ČIA, Certificate 537/2025), supporting EU nutritional labeling compliance.
Frequently Asked Questions
Why is Kjeldahl the reference method for protein?
The Kjeldahl method has been the internationally accepted reference for over 140 years due to its reliability. It measures total nitrogen, converted to crude protein using established factors. Recognized by ISO, AOAC, and EU frameworks.
What is the nitrogen-to-protein conversion factor?
The general factor is 6.25. Specific factors: 6.38 for dairy (ISO 8968-1:2014), 5.70 for wheat, 5.71 for soy products.
How does Kjeldahl differ from the Dumas method?
Kjeldahl uses wet chemical digestion with sulfuric acid; Dumas (combustion) burns the sample and measures nitrogen gas. Kjeldahl is the regulatory reference; Dumas is faster but calibrated against Kjeldahl results.
What sample types can be tested?
Ovalab performs Kjeldahl protein determination on food products, dietary supplements, animal feed, and raw materials. The method is adaptable to virtually any organic matrix.
How long does protein determination take?
Typically 3–5 working days from sample receipt, including digestion (several hours), distillation, titration, and reporting.