the COLOR of COFFEE Using Color Analysis in Quality Control by Paul Songer OODS AND BEVERAGES caramelization and body. A medium quality control, but it is useful to have appeal to consumers through to full city roast has more complex instrumental confirmation to examine appearance, and color is an aromatics and a balance of acidity the roast in greater detail. While roasters Fimportant visual cue. Coffee is not as and body. Slightly darker roasts of the have flavor attributes on their mind bright or varied in color as many foods type used for Northern Italian espresso when they design a coffee, measuring and beverages, but coffee enthusiasts emphasize sweetness and body, and the the roast so these attributes can be do have preferences of roast degree darkest roasts are dominated by “roasty” consistently reproduced is essential for and can observe this by the color of the aromatics and burnt-sugar tastes. The quality control. There are a number of coffee. While offering a variety of coffee character of the green coffee can be either machines on the market that are capable flavor experiences is a key element emphasized or modified through the of performing this task. This article looks in marketing specialty coffee, when timing and final degree of roast, and this at how the color of roasted coffee can be customers find something they like, they can be measured in terms of color. measured and what one can conclude purchase it again. If what they purchase But coffee often looks better than it from the results. doesn’t provide the same experience as tastes. If it is known that the green coffee Instrumental measurement can give the previous purchase, they are likely to is of good quality, the problem is likely in clues as to what has occurred during the think less of the roaster. the roasting. What went wrong? roasting process, but other information is For roasters, the degree of roast is In production roasting, the goal is to needed as well. As with all lab methods, an essential consideration in balancing produce the intended roast degree and sample preparation and instrument flavor attributes. A “Nordic” light-roasted flavor development consistently. Sensory calibration will affect accuracy. coffee has a great deal of acidity but little determination is the primary method of continued on page 40 38 roast January | February 2015 39 THE COLOR OF COFFEE (continued) PART 1 What one sees or what a machine measures is the interaction between the colors of the illuminant and the color of the subject The Science itself. The capability to perceive this combination depends on the ability of the eye or sensor to “see” the resulting wavelengths of light Behind Color Analysis being reflected. With perception by eye, one’s ability to perceive is limited to the visible spectrum, but some instruments are not so limited. What to Measure? Instruments designed to measure degree of coffee roast differ in what they measure and how the results are reported. Instruments There are two crucial processes that occur during roasting: that measure visible color as perceived by human beings are (1) development of flavor attributes, mainly sugar-browning intended to measure the degree of sugar browning produced as reactions, and (2) degradation of compounds. Both processes occur the result of roast; other instruments measure different roasting as the result of application of heat over time. The final degree of processes, such as the degradation of compounds, using ranges of roast and the time in which it was produced is an indication of how color that cannot be perceived by the human eye. What is being much heat was absorbed by the coffee. From that, one can make a measured depends on the illuminant and sensor used. How the reasonable assumption about the processes that took place. Roast results are reported depends on how the machinery translates those measurement usually seeks to quantify either development or measurements into a standardized scale. degradation. What is observed or measured in terms of visual cues is the result of three phenomena: Measurement of Color The color of the light illuminating the subject by the light source One perceives different colors because of the innate human ability or emitter. This is the result of all different wavelengths of light to perceive light waves of specific wavelengths. A wave (Fig. 2) emitted by the source. is a vibration with a certain wavelength; different wavelengths result in different colors. Visible color is light in the area of 400 The optical properties of what is being examined, measured or to 700 nanometers (Fig. 3, pg. 42), but there are other non-visible perceived (the subject). The subject will either absorb or reflect wavelengths. While there are many methods of measuring roast, the light being emitted by the illuminant. they all seek to measure reflected wavelengths. The observer or, in the case of an instrumental measurement, the sensor. The observer perceives the light reflected from the subject and, if the wavelengths of the source are quantified, can Figure 2 :::: A wave. The wavelength (λ) is the distance conclude or measure which wavelengths have been absorbed. between two points at the same height (amplitude) Figure 1 :::: Aspects of color perception Light Source (illuminant) Observer Like most sensory perceptions, not everyone sees the same wavelengths; some cannot distinguish between red and green and some cannot perceive different colors at all (color blindness). All wavelengths of light combined together are perceived as white; absence of any wavelength of light is black. Subject (object being observed) continued on page 42 40 roast January | February 2015 41 THE COLOR OF COFFEE | Using Color Analysis in Quality Control (continued) Figure 4 :::: Spectral curve for natural Figure 5 :::: Spectral curve Figure 3 :::: Visible spectrum from infrared to ultraviolet. Wavelengths to the left A Scale of Different Colors A major challenge in the history of outdoor light, which varies according to for incandescent light and right of the infrared and ultraviolet lines cannot be seen by most humans. The science has been to develop scales that location on Earth from a tungsten bulb spectrum shows color bands according to specific wavelengths, but in the real world are consistent enough for scientific A raw color measurement consists of several 300 colors are made up of different combinations of these wavelengths. quantification, analysis and comparison. individual measurements of the different 100 250 This becomes more difficult when wavelengths of light being reflected. This is 200 the scale also should correspond 90 often illustrated in the form of a histogram in a meaningful way to a sensory 60 150 showing the intensity of reflected light at experience, such as taste, as one’s 40 100 various wavelengths. The different spectra Gamma Rays X-rays Ultraviolet Infrared Microwaves Radio Waves senses do not always operate in a linear 20 50 can then be used to illustrate a “spectral or predictable way. Developers of color 0 0 (µW/10nm/Lumens) Radiant Power curve.” This measurement of the reflection Equal Lumens for Power Relative 400 450 500 550 600 650 700 300 350 400 450 500 550 600 650 700 750 measurement scales sought to simplify depends on the wavelengths of light Wavelength (nm) Wavelength (nm) the complex interaction between light illuminating the sample (Figures 4–7). Figure 6 :::: Spectral curve for a Figure 7 :::: Spectral curve for a and the perceiver into something As color analysis evolved, different cool white fluorescent tube warm white fluorescent tube that was consistently measurable and scales for reporting colors were developed. corresponded to human experience. 300 300 Most of these were directly related to the The discovery of the spectrum of 250 250 graphic arts or areas where precision and light is credited to Sir Isaac Newton 200 200 consistency of visible color were important, Opticks (1643–1727) in his work . He believed 150 150 such as in the clothing industry. However, different colors were the result of subtle 100 100 as the technology developed and became particles that had mass and followed the more precise, color analysis was applied 50 50 laws of physics. J. C. Maxwell (1831–1879) 0 0 to other disciplines in ways that did not (µW/10nm/Lumens) Radiant Power (µW/10nm/Lumens) Radiant Power theorized that color was made up of 300 350 400 450 500 550 600 650 700 750 300 350 400 450 500 550 600 650 700 750 depend only on the visible spectrum. continued on page 44 Wavelength (nm) Wavelength (nm) 42 roast January | February 2015 43 THE COLOR OF COFFEE | Using Color Analysis in Quality Control (continued) waves. Earlier, Thomas Young (1773–1829) At this point, the two major aspects of could be mapped in three dimensions. The be combined to produce all colors in the Similarly, Agtron developed two widely This scale is used as a reference had proposed a theory that all color was color perception were quantified: Newton three-dimensional representations are visible spectrum. used scales of coffee roast degree: gourmet throughout the coffee industry, regardless a combination of the primary colors red, had discovered that color was light (no called tristimulus color spaces. To get closer to the actual difference and commercial. On the gourmet scale— of the type of instrument. However, not all yellow and blue, and Maxwell’s work also light equals no color) and Helmholtz had However, these models had their in sensory perception, the CIE L*a*b* the one most widely used in specialty instruments measure the Agtron number in focused on this theory. It was Hermann determined that the observer was the final limitations. Each color has the properties system was developed.