The art of roasting coffee is concerned with balancing the flavor attributes that are available in the green bean to create a pleasantly tasting roasted coffee. The "pleasantly tasting" part of that definition is somewhat subjective -different individuals have different preferences. As seen on the green coffee page, different green coffees have different characteristics. Depending on personal preference, one may want to emphasize certain flavors or mask them. Understanding the process of roasting and how flavors reveal themselves at different stages is the means to creating one's ideal roasted coffee.
When coffee is roasted, a number of processes are occurring simultaneously.
- The coffee is going through physical changes mainly related to its moisture content and density. This is discussed the in greater detail on the roasting process page about what can be observed during an actual roast, but these physical changes also affect flavor.
- Some flavor-causing chemicals are being formed as the result of the heat being applied.
- Some flavor-causing chemicals are being degraded.
- The degradation of some flavor-causing chemicals causes by-products that are also flavor active.
The most obvious difference between roasted coffees is the degree (lightness/darkness) of roast. Also important is the timing of the roast; how quickly the coffee goes through the various stages of roast. But first one must understand the basics of what happens during the roasting process as the green bean progresses through the different stages from light to dark. This will help the home roaster develop a "target" roast.
In the chart below, the myriad changes that occur during the roasting of coffee are summarized into 4 major categories. At different stages, certain flavor attributes are emphasized and more apparent.The categories are:
- Acidity is at its most intense early in the roast. Besides the fruit-like and other and other acids present in the green bean, the sugar browning processes that are initiated add to the level of acidity.
- The "unique flavors" are also mostly the product of sugar browning. They include the aromatics reminiscent of certain familiar fruits, flowers, nuts, earth, and many other qualities. Some of these are only found in coffees grown under certain conditions or from certain origins. This is where the variability between green coffees is most notable.
- Body is the feeling of heftiness or visocity of the liquid. In coffee roasting, this mainly due to the cell walls of the coffee loosening so they will more soluble when the coffee is brewed, but the impression of body is also influenced by the presence of oils and sweetness.
- The category listed as "Roasty, dark" is also a product of sugar browning. It is not as apparent in the earlier stages of roast, but increases in intensity throughout the roast until at the darkest stage it dominates all other flavor attributes.
By roasting to different levels, the balance of flavor attributes are changed. In the chart at the right, click on the lines to see a radar chart of each level of roast illustrated.
- At the phase marked "Light Roast", the emphasis is on acidity, though some of the more unique flavors are developing and a small amount of body is present. Early in the roast, some "green, vegetal" flavor from the green bean may still be noticeable.
- Moving to "Medium Roast", the acidity has begun to degrade slightly, the unique flavors are playing the strongest role in the flavor profile, but the body is also coming up and the "roasty" flavors are playing more of a role with almost no "green, vegetal" flavors still perceptable.
- As the roast progresses darker to "Medium dark",the body and "roasty" flavors are emphasized, the "unique flavors" are stll playing a prominant role, but the acidity is much less perceptible.
- Finally, at the "Dark Roast" stage, the degree of sugar browning and charring of the coffee bean cell walls result in the "Roasty, dark" flavors dominating all others.
Besides the final darkness of roast, flavor is affected by the timing of the roast. If the roast progresses too quickly, the outside of the bean may charr and the dark-roast flavor will dominate, while at the same time the interior of the bean will not have been penetrated by the heat, resulting in little flavor development, more "green vegetal" qualities, and considerable bitterness. If the roast is too slow, the sugar browning process that contribute to the "unique flavors" will not occur and the acidity will degrade (professional roasters call this "baked", indicating the bean did not heat up enough to be fully roasted).
The roaster brings the coffee through different stages to their ideal final degree of roast by manipulation of heat parameters. These stages, what is happening in the coffee, and what can be observed by the roaster is discussed on the roasting process page.