Various Methods of Protein Quantification using Optical Density Measurements

Determination of protein in foods and the ability to monitor the effect of heat on protein levels in food products is important for making accurate nutritional claims. Optical density is a common method of measurement used to quantify protein levels from a variety of sources and can be achieved in various ways. Depending on the food choice or preparation method for processing, protein levels can fluctuate and quantification can change. Spectrophotometers can use optical density measurements to create a colorimetric assay of information to monitor these changes.

purple color optical density
Optical density measurements monitor color changes through chemical reactions and quantify these variations across the color spectrum.
Image Source: Flickr user Shakimba

Quantification methods

Animal proteins, such as meats, eggs, and dairy, are complete proteins and have essential amino acids that react to various chemical testing methods1 by changing color based on the level of protein available. Legumes, such as beans and nuts, also contain these amino acids and respond similarly when exposed to color altering chemicals. Raw foods and processed food products can utilize these various methods by measuring the optical density of the sample to create a quantification of the available protein levels. These measurements can then be used to create a colorimetric display of data to monitor variations and changes for creating specific protein food formulations and making process changes when needed.

protein bars optical density
Many high-protein processed foods utilize various ingredients during formulation. Monitoring the protein content and changes in each of these various raw food products is essential to developing a quality product.
Image Source: Flickr user Kesha Phillips

Change in protein “concentrations affect both the physical and nutritional characteristics of high protein foods and ingredients”2. Therefore, monitoring these changes is imperative to the final outcome and quality of the product. Many foods that claim to be high in protein undergo various stages of processing, which allow for more opportunities for these changes to occur. Food scientists utilize optical density measurements to help design processes that minimize protein loss.

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Effect of Heat on Protein

One of the major factors affecting protein concentration is heat. Changes in temperature during food processing can alter the protein content of raw protein ingredients such as eggs, milk, or legumes. Heating proteins denatures them3, causing them to lose some of their original properties. These changes can affect the texture and composition of many processed food products, so monitoring these changes can help inform the design of these processes. Protein interactions and storage stability are also concerns that affect protein changes and can be monitored with optical density measurement analysis.

eggs optical density
High-level protein foods such as eggs experience changes when exposed to heat. This process is call denaturation and can be monitored using optical density readings.
Image Source: Flickr user WillowGardeners

Optical Density and Instrumentation

The instrumentation used for optical density measurement varies according to the areas of the color spectrum that are utilized. Many of these optical density readings utilize spectroscopy to quantify color changes in nanometer increments. This method is not only simple and effective, but it is also extremely accurate for measuring even slight changes in protein content. Processed food manufacturers rely on this data to make changes to processing designs and quantify final product protein content for proper labeling and quantification.

Spectrophotometers offer color measurement analysis across the various areas of the spectrum. Ultraviolet/visible (UV/VIS), to infrared/near-infrared (IR/NIR), and even ultraviolet (UV) regions can be measured based on specific instrumentation capabilities. Each of these areas of the color spectrum serves a purpose in process monitoring and quality control in process food products.

HunterLab is a leading name in the food industry when it comes to spectral analysis. Our experience working with industry leaders have helped us develop instrumentation specifically optimized for the needs of this industry. For more information on optical density and color analysis in foods, please contact HunterLab today.

  1. “Amino Acids, Proteins, and Maillard Browning TA Documents,” last updated September 20, 2012,
  2. “Colorimetric analysis of protein sulfhydryl groups in milk: applications and processing effects,” 2005,
  3. “Muscle Mystery: Does Denatured Protein Still Make You Grow?” June 20, 2014,

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