How do you measure the amount of protein in a food?

November 15, 2025 •

4 min read

According to the Food and Agriculture Organization (FAO), standardized analytical methods are crucial for accurately determining the protein content in food products. So, how do you measure the amount of protein in a food, whether for nutritional labeling, quality control, or just personal curiosity?

Quick Summary

Several methods exist for protein quantification in food, from precise lab techniques like Kjeldahl and Dumas to simple home tests. The choice depends on the required accuracy, with most lab methods relying on measuring nitrogen content, while consumers can use nutritional databases or food labels for estimations.

Key Points

Lab vs. Home Methods: Laboratory methods, including Kjeldahl and Dumas, are highly accurate for commercial and regulatory purposes, while home methods rely on nutritional labels or simple tests for estimation.
Crude vs. True Protein: The Kjeldahl and Dumas methods measure 'crude protein' by quantifying total nitrogen, including non-protein nitrogen, which can sometimes overestimate the true protein content.
Gold Standard Accuracy: Direct amino acid analysis is the most accurate method for determining 'true protein' content by measuring the specific amino acids, but it is also the most expensive and time-consuming.
Fast Lab Assays: Spectrophotometric methods like the Bradford assay offer a faster, lower-cost alternative for protein estimation in lab settings, though their accuracy can vary depending on the protein's composition.
Digital Tools for Consumers: For tracking dietary protein, consumers can rely on nutritional information from food labels or use digital tools and databases to log their intake based on standard food data.
Dumas Method Advantages: The modern Dumas method offers a significant advantage over the traditional Kjeldahl method due to its speed, automation, and elimination of hazardous wet chemicals.

Laboratory-Grade Protein Measurement

For precise and legally compliant nutritional labeling, food scientists rely on highly accurate, standardized laboratory procedures. The most common are the Kjeldahl and Dumas methods, both of which measure crude protein based on the food's nitrogen content.

The Kjeldahl Method: The Historical Standard

Developed in 1883, the Kjeldahl method was for decades the international reference for determining protein content. It is a wet-chemistry technique that measures total Kjeldahl nitrogen (TKN), including both protein-bound nitrogen and non-protein nitrogen (NPN).

The process consists of three main steps:

Digestion: The food sample is heated with concentrated sulfuric acid and a catalyst, which breaks down organic matter and converts nitrogen into ammonium sulfate.
Distillation: The digested sample is neutralized with a strong alkali, releasing ammonia gas. The ammonia is then distilled and collected in a trapping solution.
Titration: The amount of collected ammonia is quantified through acid-base titration. The resulting nitrogen content is converted into a crude protein value using a specific conversion factor, typically 6.25 for many foods.

Advantages: Widely used and considered highly reliable for its precision and reproducibility. Disadvantages: It is a time-consuming method that requires the use of hazardous chemicals. It also measures non-protein nitrogen, which can lead to overestimation of true protein content.

The Dumas Method: The Modern Alternative

The Dumas combustion method is a more modern, rapid, and automated technique. It is increasingly replacing the Kjeldahl method due to its speed and safety.

Combustion: The sample is burned at a high temperature (around 900°C) in an oxygen-rich atmosphere, converting all nitrogen into nitrogen gas (N₂), carbon dioxide, and water.
Detection: After removing the CO₂ and H₂O, the nitrogen gas is measured by a thermal conductivity detector.
Calculation: As with Kjeldahl, the measured nitrogen content is converted to a crude protein percentage using a conversion factor.

Advantages: It is significantly faster than the Kjeldahl method, fully automated, and does not require hazardous chemicals. Disadvantages: It has a higher initial cost for equipment and still relies on conversion factors, meaning it measures total nitrogen rather than true protein.

Direct Amino Acid Analysis

For the most accurate measurement of true protein content, direct amino acid analysis is the gold standard. This method does not rely on nitrogen conversion factors. It involves hydrolyzing the food protein into its constituent amino acids, which are then separated and quantified using techniques like High-Performance Liquid Chromatography (HPLC).

Advantages: Provides the most accurate measurement of true protein by quantifying the sum of individual amino acids, removing issues with NPN. Disadvantages: Requires highly sophisticated and expensive equipment and is a time-consuming procedure.

Rapid and Spectrophotometric Assays

Other common lab methods provide rapid estimations, often used in biochemistry research or quality checks.

Bradford Assay: This dye-binding assay uses Coomassie Brilliant Blue G-250, which changes from a reddish-brown to a blue color when bound to proteins. The absorbance is measured with a spectrophotometer at 595 nm. It is fast and sensitive but can be influenced by the amino acid composition of the protein.
Biuret Method: A simple colorimetric test where copper ions react with the peptide bonds of proteins in an alkaline solution to produce a violet color. It has low sensitivity but is generally not affected by amino acid composition.
Lowry Assay: A modification of the biuret method that is more sensitive but more complex and sensitive to interfering substances.

Practical Ways to Estimate Protein Content at Home

For the average consumer, laboratory analysis is impractical. Fortunately, there are simpler methods to estimate protein intake.

Reading Nutritional Labels: Packaged foods are required to list their protein content per serving, typically determined by Kjeldahl or Dumas testing. This is the most reliable way for consumers to measure protein in processed foods.
Using Nutrition Apps and Databases: Digital tools like MyFitnessPal and nutrient databases contain vast information on the protein content of various foods. Users weigh their food and enter the quantity to get an approximation of protein intake.
Simple Home Tests: Basic science experiments, such as the Biuret test using copper sulfate and sodium hydroxide, can be used to detect the presence of protein at home, although they do not provide accurate quantification. A simpler, chemical-free method is heat coagulation; heating foods like egg whites or milk causes proteins to curdle or coagulate.

Comparison of Protein Measurement Methods


Feature	Kjeldahl Method	Dumas Method	Amino Acid Analysis	Bradford Assay
Accuracy	Good (Measures crude protein)	Good (Measures crude protein)	Excellent (Measures true protein)	Variable (Affected by amino acid type)
Speed	Slow (1-2 hours)	Very Fast (<5 minutes)	Slow (Time-consuming hydrolysis)	Fast (<10 minutes)
Cost	Low operating cost, but higher manual labor	High initial cost, low operating cost	Very High	Low to Moderate
Safety	Uses hazardous, concentrated sulfuric acid	Uses no hazardous wet chemicals	Requires careful handling of lab chemicals	Generally safe, but uses chemicals
Principle	Measures total nitrogen via digestion/titration	Measures total nitrogen via combustion	Measures individual amino acids after hydrolysis	Measures protein via dye binding

Conclusion

The choice of method for measuring protein content depends heavily on the intended application and required level of accuracy. For legally mandated and highly precise measurements in a lab setting, the Kjeldahl and Dumas methods remain the standard, with amino acid analysis providing the most accurate 'true protein' value. For everyday dietary tracking, relying on accurate nutritional labeling or using reputable nutrition databases is the most convenient and practical approach. Understanding these different techniques helps appreciate the science that underpins the nutrition information we rely on daily.

For more detailed information on food analysis methods recommended by the FAO, see their technical report on food energy.

Frequently Asked Questions

Crude protein is an approximation based on total nitrogen content, multiplied by a conversion factor. This includes both protein-bound nitrogen and non-protein nitrogen (NPN). True protein, on the other hand, is the actual protein content measured by quantifying the individual amino acids present, a more precise method.

The Kjeldahl method is indirect because it does not measure the protein molecules themselves. Instead, it quantifies the total nitrogen in a food sample, which is then used to infer the protein content based on the assumption that protein contains a relatively consistent amount of nitrogen.

The Bradford assay is useful for estimating protein in purified samples but is not ideal for complex food matrices. It is prone to interference from other substances in food and its accuracy can be influenced by a protein's specific amino acid composition.

The Dumas method, a combustion-based technique, is significantly faster than the traditional Kjeldahl method, with analysis taking only a few minutes per sample.

Food manufacturers typically use the standardized Kjeldahl or Dumas methods to measure the crude protein content of their products. This data is then used to generate the nutritional information found on the packaging.

Yes, for simple detection, you can observe heat coagulation. For example, heating egg white causes the protein to solidify. The Biuret test is another simple chemical method, but it requires handling corrosive chemicals and is not for accurate quantification.

Accurate protein measurement is essential for ensuring product quality, meeting regulatory and nutritional labeling requirements, and preventing food adulteration. For certain products, like milk and wheat, protein content can also determine economic value.