How do they calculate protein content in food for nutrition labels and beyond?

October 14, 2025 •

4 min read

The average human requires between 0.8 and 1.0 grams of protein per kilogram of body weight, but how is the exact protein content in our food determined? The answer lies in scientific methods that measure nitrogen and convert it to protein, revealing the complex process behind how they calculate protein content in food.

Quick Summary

This guide explains the primary lab methods for determining food protein content, including the Kjeldahl and Dumas techniques. It covers the use of nitrogen conversion factors, distinguishes between crude and true protein, and discusses the importance of accurate labeling for informed dietary choices.

Key Points

Nitrogen Basis: Protein content is primarily calculated by measuring the nitrogen amount in food, as nitrogen is a key component of all proteins.
Kjeldahl and Dumas Methods: The two main methods used are the wet-chemistry Kjeldahl method and the faster, automated Dumas combustion method, both of which measure total nitrogen.
Universal Conversion Factor: The standard conversion factor of 6.25 assumes protein is 16% nitrogen, but this can lead to inaccuracies because different foods have varying nitrogen content.
Crude vs. True Protein: Nitrogen-based methods measure crude protein (total nitrogen), which can include non-protein nitrogen (NPN) and potentially overestimate true protein.
Amino Acid Analysis: The most accurate way to determine true protein is through amino acid analysis, which quantifies the actual protein components.
Improved Accuracy: The Digestible Indispensable Amino Acid Score (DIAAS) is a modern metric that assesses protein quality more accurately than older methods, focusing on ileal digestibility.

Measuring Crude Protein: The Role of Nitrogen

At its core, the most common methods for calculating protein content rely on a fundamental principle: all proteins contain nitrogen. For over a century, food scientists have used techniques to measure the total nitrogen in a sample, and then multiply that number by a conversion factor to estimate the total protein. While this approach has been the industry standard for years, it has notable limitations, which led to a deeper understanding of what is actually being measured and how it impacts nutritional accuracy.

The Kjeldahl Method

Developed in 1883 by Johan Kjeldahl, this wet-chemistry technique was considered the gold standard for protein analysis for many years. The Kjeldahl method measures total nitrogen in a sample, which is then used to estimate the crude protein content.

The process involves three main steps:

Digestion: The food sample is heated with a strong acid, typically concentrated sulfuric acid, along with a catalyst. This process breaks down the organic material and converts the nitrogen into ammonium sulfate.
Distillation: The digested mixture is neutralized with sodium hydroxide (NaOH), which causes the ammonium sulfate to release ammonia gas (NH₃). This gas is then distilled into a trapping solution.
Titration: The captured ammonia is quantified through an acid-base titration, which determines the nitrogen content. This nitrogen value is then converted into crude protein using a specific conversion factor.

The Dumas Method

An increasingly popular alternative to the Kjeldahl method is the Dumas method, also known as combustion analysis. This technique is favored for its speed and automation, and it avoids the use of hazardous chemicals involved in the Kjeldahl process.

The Dumas method follows these steps:

Combustion: A food sample is combusted at a very high temperature in an oxygen-rich environment, which converts all nitrogen-containing compounds into nitrogen gas (N₂).
Detection: The liberated nitrogen gas is then detected and measured using a thermal conductivity detector (TCD).
Calculation: Similar to the Kjeldahl method, the total nitrogen content is converted to protein using a conversion factor.

Understanding the Nitrogen-to-Protein Conversion Factor

The biggest potential source of inaccuracy in both the Kjeldahl and Dumas methods comes from the conversion factor used to translate nitrogen content to protein. The assumption is that protein contains approximately 16% nitrogen, leading to the widely used universal conversion factor of 6.25 (100 / 16 = 6.25).

However, this assumption is flawed because:

Nitrogen isn't just in protein: The measurement includes non-protein nitrogen (NPN), which can come from compounds like free amino acids, nucleotides, creatine, and urea. This can cause an overestimation of the true protein content. This loophole was infamously exploited during the 2008 Chinese milk scandal, where manufacturers added nitrogen-rich melamine to boost apparent protein levels.
Nitrogen content varies by protein type: The actual nitrogen percentage differs based on a protein's specific amino acid composition. Some proteins are nitrogen-rich, while others are not. This has led to the development of specific conversion factors (known as "Jones factors") for different foods, such as 5.7 for wheat flour and 6.38 for milk.

Crude Protein vs. True Protein

This brings us to the crucial distinction between crude protein and true protein.

Crude Protein: The protein amount calculated by multiplying the total nitrogen by a conversion factor. This is the value commonly found on nutrition labels.
True Protein: The actual amount of protein present, excluding non-protein nitrogen. Measuring this requires more sophisticated techniques.

More Accurate Methods: Amino Acid Analysis

The most accurate method for determining true protein is amino acid analysis. This method involves breaking down the protein into its constituent amino acids and then quantifying each individual amino acid. While more complex and expensive, this technique provides the most precise measure of a food's actual protein content.

The Future of Protein Quality: DIAAS

Recognizing the limitations of older methods, the Food and Agriculture Organization (FAO) introduced the Digestible Indispensable Amino Acid Score (DIAAS) to replace the older PDCAAS (Protein Digestibility Corrected Amino Acid Score). DIAAS measures the digestibility of individual indispensable (essential) amino acids at the end of the small intestine, providing a more accurate score of a protein's contribution to human amino acid needs.

Method Comparison: Kjeldahl vs. Dumas


Feature	Kjeldahl Method	Dumas Method
Speed	Slow (over an hour)	Rapid (minutes)
Automation	Can be automated, but more steps are manual	Fully automated
Hazardous Chemicals	Uses concentrated sulfuric acid	Uses no hazardous chemicals
Cost	Lower initial equipment cost	Higher initial equipment cost
Precision	Very reliable and precise	High repeatability
NPN Measurement	Measures non-protein nitrogen (NPN), leading to potential overestimation	Measures all nitrogen, including NPN, leading to potential overestimation
Acceptance	International reference standard for many applications	Widely recognized and standardized

The Impact on your Nutrition Diet

For most people tracking their macronutrients, the values on a nutrition label based on the standard nitrogen-to-protein calculation are a reliable enough estimate. The differences between crude and true protein are often minor in common food items and mixed diets. However, for specific applications, like infant formula or performance supplements where protein quality is paramount, the distinction becomes more significant. Knowing that most labels rely on an indirect nitrogen-based method empowers consumers to make more informed choices, particularly when considering different protein sources or products.

Understanding the methods behind protein calculation is a fascinating glimpse into food science and the complexities of translating lab results into everyday nutritional information. As science advances, newer methods like DIAAS provide more nuanced insights into protein quality, pushing the industry toward greater accuracy and transparency. For more on protein quality, a resource from the Food and Agriculture Organization is an excellent starting point: Dietary protein quality evaluation in human nutrition.

Frequently Asked Questions

The 6.25 conversion factor is the historically standard multiplier used to estimate a food's crude protein content based on its total nitrogen. It assumes all protein is approximately 16% nitrogen, so multiplying the measured nitrogen by 6.25 (100 divided by 16) yields the protein value.

Foods have different specific conversion factors, known as Jones factors, because the actual percentage of nitrogen varies depending on a food's unique amino acid composition. For example, wheat flour uses a factor of 5.7, while milk uses 6.38.

Non-protein nitrogen (NPN) refers to nitrogen from sources other than protein, such as free amino acids, nucleotides, and urea. Standard nitrogen measurement methods cannot distinguish NPN from true protein, which can lead to overestimation.

Nutrition labels provide a reliable estimate of protein content, but they are based on crude protein, which can include non-protein nitrogen. This can result in a slight overestimation of true protein. While usually acceptable for general consumption, more precision may be needed for specialized diets or nutritional tracking.

Just like human food, protein content in animal feed is often determined using the Kjeldahl or Dumas method. However, since feed can contain significant levels of non-protein nitrogen, separating true protein from NPN is particularly important.

The most accurate method for determining true protein is amino acid analysis, which breaks down the protein and quantifies each individual amino acid. This technique avoids the errors associated with nitrogen-to-protein conversion factors and non-protein nitrogen.

DIAAS, or Digestible Indispensable Amino Acid Score, is a modern protein quality metric recommended by the FAO. It measures the digestibility of individual essential amino acids, offering a more precise evaluation of a protein's nutritional value for humans than older methods.