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Applying the Right Heat for the Bean

In coffee roasting, there isn’t a single formula or method that you can call “tried and true”. The variables are numerous, and knowing how to address each variety of coffee is what makes a roastmaster. In How Roasting Style Changes the Flavor of Coffee, I discussed how a faster application of heat can lead to sweeter flavors with more clarity. While this might be desirable for some varieties of coffees, for others it can ruin them!

How quickly a coffee bean will absorb heat is partly determined by its density. Beans that are grown at higher altitudes with wider variations in temperature between daytime and nighttime tend to be more dense than beans grown at lower altitudes and in more steady climates. Heat absorption is also influenced by the way a coffee bean is processed. “Wet Processing” results in a higher consistency from coffee bean to coffee bean because it’s possible to use the process to cull out beans that are of lower density (often due to being less fully developed) from those that are more dense. A more consistent bag of beans will roast more evenly, even when heat is applied rapidly.

The example roast shown in this post is a lower altitude Yellow Bourbon from the Mogiana region of Brazil. This is a wonderful-tasting coffee but for the roaster it presents two challenges. It is both lower in density and natural processed. These beans are too sensitive and inconsistent to apply the “fast heat for sweetness” approach as demonstrated by the two example roasts shown. Both were “finished” at 415°F. The roast on the left uses a lower charge temperature and a slower application of heat with higher airflow. The total roast time was 14:30. As a result, the roast is very even and consistent, even though there is a certain amount of variability in the coffee beans. The roast on the right had a faster application of heat with lower airflow, meaning more heat was absorbed through conduction (physical contact with the roasting drum) than convection (heated air bathing the beans) and finishing at a faster 13:10.

You can see in the example that the faster roast, which would have been a winner for a Colombian or Kenyan coffee, produced a terrible result. Some beans are under-roasted, others show burnt spots and oil emerging on the surface. By appearance, you would think that someone blended two or three very different roasts, but this is not the case. It is simply a excess of convection heat, applied too rapidly, has stretched the inconsistencies in this particular batch of beans to the point of making a coffee that might go over well with those who like the Starbuck’s flavor, but would be considered garbage by third-wave coffee fans. In truth, it’s an interesting taste, combining the smokey qualities of a dark roast with sweeter notes of a light-medium roast. I’m sure it will have some fans were I to share this batch, but I would never produce a roast like this intentionally, other than to demonstrate how to produce an inconsistent roast.

One of our goals at Evansville Coffee Company is to create the best possible single origin and blended coffees from some of the best coffees available. We won’t diminish that goal with a less-than-remarkable coffee, so when you try our Yellow Bourbon, you can be sure we will use the slower, high-airflow roasting profile we developed for this coffee.


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