Roasting Coffee Part 2: Technical Aspects
Aug 26, 2019
Roasting coffee! Is it art or science? Some say both, but a basic understanding of the science of roasting is critical if one is to make an art of it. Fundamentally, heat application is at the heart of roasting coffee. How and when to do this, are the art and science of roasting coffee.
At Desert Sun we have a drum roaster. The green coffee beans drop into a large rotating metal drum, which has heat applied on the outside. Desert Sun’s roaster has an on board computer which allows us to control the amount of heat applied and the amount of airflow inside the drum. The computer also tracks the temperature and time that beans have been in the roaster, creating a temperature vs time “curve” that defines a specific roast. For more about roast level go back and read Roasting Coffee Part 1: Roast Level.
Green beans will get dropped into the roaster at a specific temperature. For Desert Sun this is usually between 375 and 400 degrees F. Since the beans are at room temperature, the temperature inside the drum will drop dramatically at first until it reaches a “turn around” point where the temperature begins to rise again. As the temperature rises, the beans will change in color and smell. When the internal bean temperature rises to 212 degrees F., water inside the beans starts evaporating. Around 370 – 380 degrees F. this vapor is released and what is called “first crack” occurs. First crack is the first audible reaction from the roast process; it sort of sounds like popcorn cracking in the roaster. From this point the beans start turning from dark yellow to light brown to a darker brown color. As this happens, the pressure inside the coffee bean will increase, due to the forces of carbon dioxide gases, and the bean expands in size.
From first crack until second crack, coffee beans will develop their specific aromas and flavors. Once first crack occurs, how and when heat is applied will determine how the flavors develop in the coffee bean. For example if I were to do two roasts of the same bean with the same finish temperature, I could get two results by changing the time spent in the roaster. If I let the first batch roast for 3 minutes after first crack and the second roast for 5 minutes after first crack, both ending at the same finish temperature, they would develop different flavors. Every bean has different tolerances for heat application and flavor development. Trying multiple ways to roast the same bean will reveal different flavors. Which one is right? It’s up to you. There is no right or wrong answer and that is the art of roasting coffee.
As you try different ways to roast the same bean, keep in mind “How did nature intend this bean to taste?”. This way you’re searching for the maximum potential of the bean. How much heat can these beans take without losing acidity? How little heat can these beans take so that they develop fully? This may take many attempts at roasting the same bean different ways; but that’s the art of roasting. Finding the sweet spot where not too little nor too much heat was applied. This process is what we consider the art and passion of creating masterful coffees.
Most importantly though, is taking both roasts and cupping them for quality to determine which exhibits the best taste! Cupping is a professional technique for evaluating the coffee’s fragrance, aroma, taste, body and aftertaste. By trying different roast profiles and cupping them for quality, we custom develop the coffee flavor profiles that one experiences when drinking a cup of Desert Sun Coffee. Once we find that perfect way to roast an individual coffee, we can easily recreate that roast because we keep detailed records of how it was done. I’ll talk about cupping in the next issue. As always, thanks for reading and feel free to email with any questions.