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Now you know how protein values can be calculated based on Nitrogen values, it is time to learn how nitrogen in food can be measured. First study the different methods for nitrogen determination. After that there are some questions that help you to understand these methods.

The application of Kjeldahl methods can be manual, or semi- or fully automated. Fully unattended operations allow the analysis of 60 samples within 4 hours. Modern types of Kjeldahl apparatus allow the analysis of N in the range of 0.2 - 160 mg per sample. Modified Kjeldahl micro methods can detect N concentration in the range of 0.3 - 1.3 mg/ml. However. if a sample is small, be aware that the sample could be less representative for the food, because of heterogeneity of foods.

Advantages:

• Applicable to all foods.

• Not sensitive for interference with other food components (except non protein nitrogen).

• Internationally used standard method (e.g. ISO standards). Different modifications based on the Kjeldahl method are available for meat, milk and cereals.

The Kjeldahl method (or Kjeldahl-Wilforth-Gunning method) is the most universal technique for the determination of organic N in foods and it is internationally recognised for official and statutory purposes. The Kjeldahl method consists of three distinct steps.

1. Digestion: to break the intricate structure and chemical bonds in a food down to simple chemicals and ionic structures. Specifically, proteins and other forms of nitrogen are broken down and converted to ammonia.

2. Destillation: to separate ammonia–nitrogen from the digestate.

3. Titration: to determine released ammonia (NH3) by titration.

Finally, nitrogen values are converted to protein values using conversion factors. Play this slide show to learn more.