The best way to make sure you are roasting consistently is by using a data logging program and tracking your coffee through a roast curve. But what is a roast curve? And how do you use it to better improve your coffee?
Tracking the Temperature
All the lines on the graph of the data logging software are simply temperature readings that are tracking the heat inside the roaster. Temperatures are read by a thermocouple, a small electrical sensor that produces a temperature-dependent voltage which can be used to accurately track the temperature inside your roaster. In our roastery and classrooms, we use our data logging software, RoastPATH.
There are typically three or more thermocouples inside your roaster positioned in different locations to gather multiple data points while you roast. The primary temperature reading that you will be looking at when evaluating a roast curve is the bean temperature. This temperature reading comes from a probe located at the bottom of the drum that's covered by the bean pile during your roast. This is the temperature most roasters reference when marking the landmarks of their roast.
The line produced from the bean temperature thermocouple is what makes the green checkmark-type shape of an average roast curve.
Looking for landmarks
Throughout the roast there are different landmark events that happen at different time and temperature points. These landmarks track meaningful changes in the roast or the coffee. First is the charge point, which is the temperature that the roast begins, sometimes referred to as "pre-heat temperature". This can vary depending on the roaster/batch size and type of coffee, but many roasters will use the same charge temperature for all of their roasts. In our production roastery at Mill City Roasters, we charge at 465° on our 20-kilogram machines.
The next landmark is the turning point. Because you have just added room-temperature green coffee to a hot roaster, the overall temperature seems to drop consistently for about 1-2 minutes before the temperature starts to “turn” or rise again. When the temperature stops decreasing and begins increasing, you've reached the turning point.
The next event that happens is color change, where the green coffee changes from green to yellow in color. This is the start of the Maillard reaction, and many roasters consider it the start of when the coffee starts to develop flavor and body.
The last major landmark(s) are first and second crack. These occur when the remaining moisture and vegetable oil in coffee start to release, causing a crackling sound. All of these landmarks help to separate the roast curve into three main phases of the roast: dry phase, mid phase, and development phase.
Phases of the roast curve
A roast curve is segmented into three main phases. The first phase occurs between the charge point and the green-to-yellow color change. This is called the drying phase. The main purpose of this phase is to expel most of the water content and to build momentum for the rest of the roast. While no flavor is being created during this phase, it's crucial for setting up the success of the roast.
Phase two begins once the green to yellow landmark has been reached. This is called the dry phase, or the Maillard phase, where our first major chemical reaction takes place. Here, the coffee is beginning to brown, and the heat is causing the proteins and sugars in the green coffee to form new flavor compounds. This is where the coffee is starting to develop its flavor and body.
The development phase is the final phase of the roast. It begins at first crack. This phase is where a lot of our taste perception of coffee starts to change. When the development stage is drawn out, we notice more “roasty” flavors in our coffee
Putting it all together
Now that we know the basics of the roast curve, what is the most helpful thing when reading our data and applying that to how the coffee we just roasted tastes? Despite what you may hear from other roasters, having a “nice looking” roast curve doesn’t always mean the coffee tastes good. Cupping and evaluating your roast along with the data is key to seeing how the coffee turned out the way it did.
In my experience, keeping track of the three phases is the best way to use the roast curve to evaluate coffee. For example, maybe your light roast coffee doesn’t taste as light as it should even though you might have dropped the coffee at a “light” finish temperature. This could be because the roast spent too much time in the development phase. Now that you know that, you can go back and adjust your fuel and airflow settings to move your landmarks, and when you find a profile that works best for that coffee, you can use that roast curve again as a reference.
Ready to create your own roast curve? Download RoastPATH and sign up for a free account to get started.
