We already know that roasting green coffee turns it into something we want to drink. We also know that how one roasts the coffee makes a difference. It shouldn’t come as much of a surprise, then, that the final color of the coffee is relevant to our experience. The final color is really a function of the roast profile, and it is best thought of in that way. However, just referencing the roast color can be valuable as it often correlates to some bean characteristics and sensory experiences. Beware, though, sometimes, the roast profile can have an influence that beguiles the expectation of a particular roast level.
Coffee roasting is a function of temperature, as is cooking any food using heat. As the temperature of the bean increases and roasting progresses, some chemical reactions continue to occur while new ones come and go. The bean is continuously undergoing chemical changes. Thus, a lighter roast is chemically different than a darker roast; this is well researched by scientists and I’ll spare you the gory details. The only general category of reactions worth mentioning is the Maillard reaction.
A Maillard reaction is one in which an amino acid (a component of protein) reacts with carbohydrates (often sugars). There isn’t a specific end product from this reaction, especially as the reactions continue to occur; compounds formed from the reaction can react with each other, creating a dizzying array of complex molecules. Maillard reactions are common in cooking and are responsible for much of the browning we’re familiar with.
Think seared meat and the crust of bread. And of course, think brown in coffee. The brown compounds resulting from this reaction, called melanoidins, are significant in coffee; they can comprise some 25 percent of the solid material in a cup of coffee. They are also the likely source of any antioxidant behavior in coffee. While they likely contribute to the flavor of coffee in some way (no research exists on it), we can only guess at it in a roundabout way. Melanoidin content increases as roasts get darker (no surprise, there!). So, it isn’t unfair to guess they may contribute to our sense of the difference between lighter and darker roasts.
Recent research on a compound called N-methylpyridinium (N-MP, a degradation product of trigonelline) is also worth mentioning. It seems to be a significant inhibitor of gastric acid secretion in the stomach, potentially preventing nausea or indigestion—something that happens to some unfortunate coffee drinkers. As its occurrence is directly related to the destruction of trigonelline, its concentration in coffee increases as roasting progresses. In other words, darker roasted coffees may make for fewer upset stomachs.
For most of us, what we most want to understand about coffee roast levels is how they differ in taste. Coffee geeks have strong feelings about the roast levels they think are best and consumers are no different. However, to anyone wanting to try something new, a little guidance might be helpful. The literature repeatedly shows that as the roast level darkens, acidity, fruity/citrus, grassy/green/herbal, and aromatic intensity decrease. Concurrently, roasted, ashy/sooty, burnt/smoky, bitter, chemical/medicinal, burnt/acrid, sour, and pungent flavors all increase. That’s a pretty grim picture but only because some of the research examined extreme roast cases. What must be realized is that these flavors occur on a continuum, with the intensity changing as the roast darkens.
Underroasted coffee is not very coffeelike. It tastes leguminous, herby, and nutty. This taste happens just after first crack (see the section on coffee as a test tube) and lasts for a brief time. Once it is roasted just past that, all the coffee’s soul is laid out for the palate.
All the nuance, complexity, and acidity that could be in the taste exist at this point. Very light roasts are like puppies—full of verve and energy and spunk and sometimes just as annoying. As the roast progresses, those flavors might disappear or mature or become tempered. Coffee has many faces between very light roasts and approximately second crack. When the second crack happens, the process of roast begins to creep in. Thus, roasted, woody, smoky flavors begin to develop. From there, the process of roast becomes more and more dominate, approaching an end result of a black, charred bean that closely resembles charcoal.
There’s no right answer for how light or how dark any given coffee should be roasted.
Ultimately, the person roasting gets to decide, and she’ll likely make that decision based on her personal belief of what best exemplifies the coffee in combination with what she thinks her market desires. Give the same coffee to ten roasters, and you’ll get ten somewhat different coffees.
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