A protein figure on a specification sheet looks like a direct measurement. In most cases it is not. It is a nitrogen measurement multiplied by a conversion factor, and the factor chosen can change the result by several percent.
That matters when comparing two protein ingredients, checking a result against a specification, or working out whether a product can support a protein claim.
How protein is usually measured
The FAO report Food energy: methods of analysis and conversion factors describes the standard approach. For many years the protein content of foods has been determined from total nitrogen, with the Kjeldahl or a similar method almost universally used to measure the nitrogen. That nitrogen figure is then multiplied by a factor to give protein.
The general factor, 6.25, comes from early findings that proteins contain about 16% nitrogen on average: 1 divided by 0.16 is 6.25.
Why one factor does not fit every food
The FAO report identifies two problems with a single factor:
- Not all nitrogen is protein. Foods also contain nitrogen in other compounds.
- Proteins differ. Depending on their amino acid make-up, proteins contain from about 13% to 19% nitrogen, which corresponds to factors from 5.26 to 7.69.
In 1941, Jones proposed replacing 6.25 with factors specific to each food. These Jones factors have been widely adopted, and for commonly eaten foods they range from 5.18 for nuts and seeds to 6.38 for milk.
| Food | Jones factor |
|---|---|
| Milk | 6.38 |
| Eggs | 6.25 |
| Meat | 6.25 |
| Maize | 6.25 |
| Sorghum | 6.25 |
| Rice | 5.95 |
| Millets | 5.83 |
| Oats | 5.83 |
| Wheat, whole kernel | 5.83 |
| Soybean | 5.71 |
| Wheat, endosperm | 5.70 |
| Peanuts | 5.46 |
Selected factors from Table 2.1 of the FAO report.
How much the factor moves the result
The FAO report quantifies the effect. Using 6.38 rather than 6.25 increases apparent protein content by 2%. Using a specific factor of 5.7 rather than 6.25 decreases it by 9% for the foods concerned.
An illustrative sample with 13.6% nitrogen shows how far apart the results land:
The same analysis can be reported as anything from 77.5% to 86.8% protein, depending only on the factor. Each figure is arithmetically correct for the factor used, which is exactly why the factor has to be known before two figures are compared.
Standards name their factor
For this reason, compositional standards often state the factor to use. The Codex standard for whey powders, for example, defines protein as total Kjeldahl nitrogen multiplied by 6.38.
The factors are still debated
A review in Critical Reviews in Food Science and Nutrition argues that when 6.25 is applied whatever the food, the resulting protein figure is really just nitrogen expressed in another unit. It also argues that Jones' specific factors are scientifically flawed, even though the ratio of nitrogen to protein genuinely varies between foods.
The FAO report recommends measuring protein as the sum of its amino acids wherever possible. Where total nitrogen is used instead, it recommends a specific Jones factor when one is known, and the general factor of 6.25 when it is not.
What to check when comparing protein figures
- The factor. Two specifications using different factors cannot be compared directly.
- The method. Total nitrogen by Kjeldahl or a similar method, or amino acid analysis.
- The basis. As is, or on a dry basis. A result of 85% as is, on a sample with 6% moisture, is 90.4% on a dry basis.
- The rule that applies to your label. For labelling and claims, the factor is set by the regulations for the product and market, so confirm the current requirement rather than relying on the factor a supplier happened to use.