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Nitrogen conversion factors for dairy products

Why is this research important?

For the last 140 years, the reference method for measuring protein in dairy products has involved measuring the product’s nitrogen content and multiplying it by 6.38 (i.e. the nitrogen – protein conversion factor for dairy protein). The size of the factor used directly influences the apparent quantity of protein present in a food. Despite being used for farmer milk payments, food labelling, nutritional assessments, global trade and the food sciences, the factor 6.38 has remained unchanged since its original 19th century calculation.

A few years ago, the United Nations (FAO / WHO) summoned a group of international experts to recalculate the conversion factor for dairy protein to better reflect modern nutritional and compositional knowledge. Surprisingly, this could only be done with a low – medium level of confidence, resulting in the recommendations remaining un-implemented.

What the research tells us

The low level of confidence reflects the difficulty in quantifying protein in complex foodstuffs in absolute terms – the accuracy of many modern methods is affirmed by comparison to the classical reference method (nitrogen x 6.38). The lack of confidence also reflects some ambiguity in the definition of “protein”. The nutritional sciences typically define protein as the mass of amino acids (i.e. the building blocks of protein), or sometimes the mass of dietary nitrogen. However, many proteins also contain carbohydrate portions as a fundamental part of their structure, whilst dairy proteins are unique by also containing phosphate – modified amino acids, which greatly enhance milk’s nutrition by binding calcium in a bioavailable “micelle” structure. Including or excluding these components alters the apparent mass of protein, and therefore requires altered conversion factors.

Another complicating aspect of dairy products is the presence of non – protein nitrogen – in raw milk, this typically comprises ~5% of total nitrogen. In many countries, this fraction is removed via a chemical precipitation procedure before nitrogen analysis, resulting in an apparent 5% reduction in total protein. However, this method ignores the presence of free amino acids and peptides in this “non protein” fraction, which can be considered protein under modern nutritional definitions.

Finally, 6.38 is used for all dairy products, encompassing products as diverse as whey, cheese, caseinates and lactoferrin. Unsurprisingly, the correlation between protein and nitrogen varies in these fractions – product – specific factors could greatly enhance measurement accuracy.

Outputs

This project aims to ensure accuracy in calculating conversion factors for milk, dairy ingredients and finished products. After critically reviewing the relevant literature (article currently in peer review), we have embarked on a method development project to improve the accuracy of protein measurement, which will require significant method validation.

Key Outputs

Presentations

  • International Dairy Federation Joint Symposium on Dairy Drying Technology and Recombined Milk Products 2025
  • 2024 Institute of Food Science and Technology of Ireland Conference
  • 2024 AEDIL COVE Copenhagen Dairy Symposium
  • Teagasc Daily article

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