How is protein measured in food?

Protein in food is commonly measured by determining the total nitrogen content and then multiplying by a conversion factor. For many years, the standard method for determining nitrogen content has been the Kjeldahl method (or similar). (AOAC, 2000).

Kjeldahl Method Explained

The Kjeldahl method is a widely used technique that involves several steps:

1. Digestion: The food sample is digested in strong acid (typically sulfuric acid) to convert all nitrogen present into ammonium ions.
2. Neutralization: The solution is neutralized with a strong base (e.g., sodium hydroxide).
3. Distillation: The ammonia is distilled and collected in a receiving solution.
4. Titration: The amount of ammonia is quantified by titration, which then allows for calculation of the total nitrogen content.

Calculating Protein Content

Once the total nitrogen content is determined, it is multiplied by a conversion factor to estimate the total protein content. This factor varies depending on the type of food because different amino acids (the building blocks of protein) have varying nitrogen content.

• Why a conversion factor is needed: Not all nitrogen in food comes from protein. Some may come from non-protein nitrogenous compounds.

• Common Conversion Factors:

  • 6.25: A general factor, often used when the specific nitrogen-to-protein ratio is unknown. This assumes that protein contains 16% nitrogen.
  • Specific factors: More accurate factors exist for different food groups (e.g., wheat, dairy, meat) based on their typical amino acid profiles.

Example

Let's say a food sample is analyzed using the Kjeldahl method and found to contain 2 grams of nitrogen. If we use the general conversion factor of 6.25, the estimated protein content would be:

2 grams Nitrogen * 6.25 = 12.5 grams of protein

Summary

Measurement Type	Method	Process
Total Protein Content	Kjeldahl (or similar)	Determine total nitrogen content using acid digestion, then multiply by an appropriate conversion factor (e.g., 6.25) based on the food type to estimate protein content.