Have you ever wondered how coffee roasters turn green coffee into magical little morsels of deliciousness? Well, the secret lies in perfecting the roast curve so you can consistently replicate that roast in the future. We have several coffee experts here at Bruvi ® that geek out over these charts, so we wanted to share a bit more about where the art of roasting meets science.
What Is a Roast Curve?
Roast curves are graphical representations of essential data from the roast process. There are many graphs used to capture important data from the process, each that serves a unique purpose. Of those, one is always front and center, the temperature graph.
Critical Moments on The Roast Curve
Inspired by shutterstock: pongpinun traisrisilp
A probe is used to measure the temperature of the roasting drum itself which is reflected on the temperature curve. As the drum starts to heat, before you put the green beans in, the highest temperature point is called the charge temperature. The moment the beans are added to the roasting drum, you will notice an immediate drop in the curve as the beans are placed in the drum and colder air enters the opening. Appropriately called “the drop”, this represents the beginning of the actual coffee roasting process.
As the beans begin to heat up you will notice the temperature curve start to go up rather quickly which is defined as the turning point. As coffee continues through the roasting process, the beans gain temperature at a variable rate which is called the Rate of Rise (RoR). This rate is the result of many factors including the starting temperature (charge temperature) and the amount of moisture in the coffee itself.
With the coffee continuing to rise in temperature, there are physical changes that happen to the coffee as transforms from green coffee into roasted coffee. Not only does the coffee change in appearance but chemical reactions happening inside will ultimately shape the coffee flavor.
Coffee absorbs a lot of energy as it heats up during the roasting process. Ultimately, there becomes a point when the water inside the beans is under so much pressure that it releases heat, this is known as the first crack. It can be heard during the roast process and is reflected by a flattening of the curve (RoR declines). From this point on, the roast is in the developmental phase where some roasters opt to end the roast quickly producing a lighter roast, or continue to let the roast progress into a darker roast.
When the beans are removed from the roaster and go through a cooling process, the curve ends marking the completion of what we hope is a great roast!