Coffee roasting is both a physical and chemical process. Physically, we see the coffee beans change color, swell in size and become less dense as they lose about twenty percent of their original weight.
Simultaneously, there is a change in molecular chemistry that creates the aromas and flavor constituents we so appreciate in the cup.
One reason there is such great variation in coffee flavors is because we are dealing with a highly variable agricultural commodity that has an astounding number of fragile molecular compounds – more than any other food or beverage – and a heating process that directly contributes to its final flavor, more than any other cooking method.
The flavor of a coffee is determined by the natural potential within the beans and shaped by the way it is roasted. Green coffee traders have long been aware that not all beans are created equal. The best higher-grade coffees are rare and much sought after. Consequently they are more costly.
Regarding the how to roast coffee process, simply put, it is a time-temperature process that would be more straightforward if we were just baking a cake. A cake requires pre-heating the oven and baking for a prescribed length of time at a particular temperature. We can estimate when it’s ready after we check to see if it’s cooked throughout. Coffee, on the other hand is roasted to color. Without instrumentation to assess the color it must be roasted by eye or to a particular temperature or for a pre-determined amount of time.
The coffee roasting process is far more complicated than baking. Yet like baking, you can roast fast or slow. Normal roasting is based on the time it takes for the beans to absorb heat evenly throughout the beans. A gentle approach produces a normal roast with a common expected result. On the other hand, it is not uncommon to roast coffee beans as much as twenty percent faster or slower to produce a variation in the taste of a coffee.
Say a standard coffee roast takes twelve minutes for a standard taste. A faster coffee roast of maybe ten minutes could give a brighter and cleaner taste to the coffee. Such quality in a coffee is ideal for drip brewers. A slower roast of about fourteen minutes is possibly better for an espresso blend because it will likely give a smoother and heavier taste to the cup. This fine-tuning of time and temperature can be accomplished by controlling the heat input and airflow during the coffee roasting process.
Roasting of coffee beans is necessarily complex in its chemistry. This is because the myriads of natural compounds found in coffee beans, are considerably altered during the coffee roasting process. As coffee beans heat up, certain long chain molecules of complex compounds breakdown and are degraded into simpler compounds, some of which in turn, will re-combine into other compounds, creating the variation in the flavor and aroma components of the finished product.
The rate of heating the beans is but one of the ways the taste of coffees can be manipulated. The pre-heat temperature, rate of increasing or decreasing the temperature during the roasting process and alterations in the airflow can all have an effect on the final taste. A roastmaster will select these control parameters of time and temperature, which determines the profile of the roast.
All commercial roasting machines manufactured today are equipped with a computer controller that allows the operator to pre-select and control the heat and airflow throughout the roasting process. Some are able to track previous time/temperature profiles in order to duplicate previous roast results. Unfortunately all these systems only work on duplicating time and temperature routines without regard to the color whereas a laser reflectometer actually tracks the color development that encompasses all the other variables.
If all coffees were the same, one time/temp profile would suffice for producing consistency. Unfortunately this is not the case, so we roast to bean color which is a much more dependable approach to consistency in flavor.
There are several other factors that affect how a coffee roasts; moisture content, bean size, density and freshness. Some flavor notes are determined by the elevation at which the coffee is grown as well as climate conditions and soil composition. You have undoubtedly heard promotions touting the merits of “Mountain Grown” and “Volcanic Soil.”
This is why the country of origin and region makes a major contribution to the flavor of a coffee. A Sumatran coffee will taste dramatically different from a Colombian or Ethiopian. Experience has taught us that a Sumatran generally tastes better at a darker roast, Colombian at a lighter roast and Ethiopian usually at a medium roast.
We do not know in advance exactly what temperature will yield the desired degree of roast for each coffee. A series of taste tests “cuppings” of the same coffee at different degrees of roast “color” is conducted in pursuit of the optimum flavor results. The color is then assessed in the lab and entered into the roasting machine program to electronically read the developing roast color throughout the process and stop the roast automatically at the desired degree of roast regardless the temperature.
Color is a measure of consistency. The latest technology can produce the same color every time but a true artisan roaster can provide a roasting profile which considers not only the color but the development of the nuances of each coffee.