Spectrophotometric Analysis: The Nuances of Coffee Bean Color During the Roasting Process

The process of roasting coffee begins with heat and a swelling of pastel green beans. First shedding their silver skins, cycles of bursts and darkenings soon follow, transforming the color of the beans from green to tan to the deepest browns. Eventually, the beans shrink, ending their life cycles in a quick, smooth blackening and hardening of their skins.

Coffee beans undergo a series of changes from the green beans, observed at the top right, to the darker colors that occur upon roasting.
Image Source: Flickr CC user Jessica Spengler

The physical changes that occur during the roasting process might seem straightforward; however, hundreds of chemical reactions are occurring in tandem. Proteins, polysaccharides, lipids and organic acids undergo decarboxylation and dehydration events as Maillard browning and caramelization occur among redox and polymerization reactions. Together, these events result in the disappearance of some reactants and the appearance of many more, which themselves add flavor and aroma to the beans. Importantly, roastmasters have used and still use color to determine when a roast has undergone the multitude of chemical changes described above and is considered “finished.”

Roasting begets color and flavor

Relevantly, the numbers and varieties of chemical reactions coffee beans might undergo largely depend on when a roastmaster decides to extract the beans from the roasting process, a decision that strongly influences the beans’ final flavors and aromas when ground and brewed. Indeed, the most time-advanced roast levels have lost the natural flavors of the coffee beans and are mostly characterized by the tastes and aromas of the roasting process.

The extent to which coffee beans are roasted is, therefore, an essential step in the final production of coffee, and given the relationship between bean color and roast progression, shifts in bean color can serve as a powerful means of predicting coffee’s final flavor.

Coffee flavors are influenced by a variety of chemical reactions, which themselves depend on how the coffee beans were grown, where, etc. The roasting process and subsequent color measurements can also provide a good estimate of flavor.
Image Source: Flickr CC user sarahemcc

The challenge of accurate color measurement

The use of bean color to determine when to end the roasting process, however, is not without difficulty. Part of the challenge lies in determining if the color you see is truly what you’ve seen before because of three phenomena, namely the optical properties of the light source (illuminant), what’s being examined (coffee bean), and the observer (roastmaster). These properties together affect the colorfulness, saturation, and hue of coffee beans, and any real or perceived changes could lead to inaccuracies in bean color detection and subsequently, the “wrong” coffee for consumers.

Realizing the challenge of color interpretation, in 1913 the International Commission on Illumination (abbreviated CIE for its French name) developed a system based on standard illuminants and a person of “average visual ability,” known as the CIE L*a*b* system. However, this metric it is not ideal for measuring the color of roasted coffee given that the CIE L*a*b* brown color spectrum is quite small, thus limiting its usefulness for a product that is mostly brown. In 1995, the Specialty Coffee Association of America (SCAA) developed its own standards for classifying coffee roasts, which are still in use today. Still, there are a number of opportunities for wrongful color detection: coffee bean colors are often not homogenous and outside factors such as packaging and density can affect what an individual sees.

There can be differences between how color is perceived based on the context. These differences, however, can be devastating for coffee suppliers who rely on consistency.
Image Source: Flickr CC user david pacey

Standardize quality with spectrophotometric analysis

Despite these challenges, it is crucial that coffee roasters standardize their desired roast progression in order to maintain the integrity of a coffee’s flavor. As businesses grow and a roastmaster’s responsibilities are delegated to others, color as well as other indicators (e.g. temperature, time, the presence of cracks and other sounds) serve as powerful methods for approximating the final product. Once a roastmaster has identified the “standard coffee” to be produced, color measurements via instrumentation is an essential step for the continued correct identification of the progression of the roasting process.

The HunterLab ColorFlex EZ Coffee spectrophotometer is an instrument that allows such precise identification. Using a light source within the device to approximate the color of normal daylight, the HunterLab spectrophotometer determines coffee bean color or roast progression by recording a sample’s color measurements from various angles. The results of the analysis are easily downloaded onto a computer and provide individuals with the SCAA number, roast classification, and the HunterLab Coffee Color Index (HCCI), for an accurate and comprehensive measure of bean color and roast progression. By also using a Direction 45°/0° reflectance instrument, which closely mimics how the human eye actually sees, HunterLab ensures the color of coffee produced will meet consumers’ expectations.

Our recent version also includes Hunter L,a,b and CIE L*a*b* color data analysis, enabling coffee suppliers to use the ColorFlex EZ Coffee to measure a range of non-coffee samples including liquids, semi-solids, and powders. Given our machine’s unique ability to measure coffee at each stage of development, we ensure coffee suppliers that the ColorFlex EZ Coffee will support them in their production of the highest quality and most consistent coffee products. For more information about coffee color measurements or any of our other spectrophotometers that can be modified to serve the needs of coffee suppliers, please contact HunterLab today.