The Chemistry of Roasting Coffee
Nothing affects coffee favor more than how the green, “raw” beans have been roasted. If to brew raw beans, the result would not possess any flavors recognizable as coffee.
Roasting is the key stage, whereby the characteristics taste, aroma and final flavor of the beans is developed.
The length of time of the roasting process also determine whether the coffee will end up being a “cinnamon,” “city,” espresso,” or “dark French roast.”
In simplified terms, coffee is describes as “light,” “medium,” or “dark” roasted.
In the roasting process, heat from and external source is applied to the raw coffee beans in large vats or drums, spinning and heating them evenly at temperature reaching up to 550 degree F (290 degree C).
The heat essentially creates chemical changes in the physical structure and composition of the beans.
Water evaporates from the beans, starches convert to sugars and the sugars caramelize. The beans increase in size by 25 to 35 percent. They begin to pop, much like popcorn.
They lose 18 to 22 percent of their weight, mostly through this evaporation. The caffeine content, however is not affected by these changes.
Gradually, the green beans turn a yellowish color, then darken to a deep rich brown. During this color change, a number of chemical reactions occur, causing the beans’ sugar and proteins to interact with each other.
It is these changes, and the release of caffeol, or coffee oil, that are essential in bringing out the flavor and aroma of the beans.
The darker the beans, the more oil they produce.
Great care must be taken as the process nears completion, to ensure that the beans are not burnt.
Flavorful acids from as the beans turn into a medium dark roast. As the roasting progresses toward a darker roast, these same acids will now begin to break down, and the sugar components will start to caramelize.
A darker roast has more body and an intense richer flavor to the palate, that is why espresso beans are characteristically low in acidity, rich in body, and sometimes caramel like.
After this monitored roasting process, the coffee beans are rapidly cooled down by jets of cold air, thereby seeing in all flavor and aroma that the heated air has brought to life from the dormant green beans.
The lighter the roast, the more flavor acids, resulting in interesting flavors and sparkle. Lighter roasts are lighter in body because the roast has not produced caramelized sugars or caffeol.
Medium roasts have less acidic snap: they are richer, with a more rounded flavor. Here, coffee oils begin to appear.
At the dark roast stage all acidic tones disappear; the beans are oilier, there is a definite bittersweet, chocolate flavor, the brew is rich, and full in body and texture.
An interesting note on roasting as it pertains to caffeine content: The darker the roast, the less caffeine content it will have. Higher, longer roasting temperatures eliminate more caffeine from the beans than will a brief, cooler roast.
The Chemistry of Roasting Coffee
