To the untrained eye, a roast curve can seem like a daunting muddle of lines and axes. However, for roasters, learning to interpret a roast curve is a useful tool for understanding the technical aspects of the roasting process in greater detail.
This is particularly important for specialty coffee roasters, for whom cup quality is always a focus. Using a roast curve can help them fine-tune a roast profile and maximise the distinct flavour notes within each coffee.
To learn more about how roast curves help specialty coffee roasters unlock the potential within their coffee, I spoke with Carlos Núñez, owner of Boconó Specialty Coffee Roasters in Madrid.
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What Influences The Shape Of A Roast Curve?
To successfully roast coffee, roasters need to meticulously monitor a number of different variables. The better roasters get at controlling these variables, the more precisely they will be able to dial in a roast profile.
Today, thanks to the use of dedicated software, the roasting process can now be translated into a graph where the temperature of the coffee inside the roaster’s drum is plotted against time. The data points collected by these software forms what we know as a roast curve.
On a typical roast curve, the horizontal axis represents the roast time, while the vertical axis represents temperature. Each line within the curve is a graphical representation of what is occurring during the roasting process.
A roaster can use roast curves to make modifications and refine the flavour and final taste of the coffee they are roasting. As a general rule, all changes on a roast curve should be “smooth”, as raising or lowering temperature too sharply can disrupt the process and cause scorched or underdeveloped flavours.
There are three main variables that influence the shape of a roast curve.
At the start of the roasting process, cold coffee beans enter the roaster set at a very high temperature. As the cold beans absorb thermal energy, a hot probe inside the roaster cools.
This is represented by an initial drop on the curve, which lasts until the beans and the probe reach the same temperature. You can see this until roughly the 01:00 mark on the graph above.
However, once the beans and the probe reach the same temperature, they will then rise in sync with each other. Once reached, this equilibrium is known as the “turning point”.
Airflow, as the name would suggest, is the flow of air within the roaster.
If there is not enough airflow throughout the roast, the rate of rise will “stall” and the beans will end up taking on a baked, doughy flavour.
Similarly, if there is too much airflow, the beans will effectively be repeatedly superheated, leading to what is known as “scorching”; when the beans are consistently exposed to temperatures that are too high.
Airflow is used to control the rate of rise throughout the duration of the roast; this is how “sharply” the curve will bank and increase as seen on the graph.
Throughout the roast, the coffee beans in the drum will lose their moisture. This occurs as they are heated, and happens in tandem with the development of sugars within the beans.
However, as the beans are heated, and release more and more moisture, less temperature will be required to heat them. As a result, the temperature line on a roast curve becomes “flatter”; it starts steep and gradually plateaus as less heat is required to remove moisture from the beans.
What Is Rate Of Rise (RoR)?
Rate of rise (RoR) is a measurement of how the temperature in your coffee changes during the roast. RoR is usually measured over a specific window of time, most often between 30 and 60 seconds.
As an example, a RoR of 5 in 30 seconds translates as your beans increasing by 5°C every 30 seconds. It is effectively a measurement of how “aggressively” you are roasting your coffee.
If your RoR is too high, it can result in coffee that doesn’t have enough time to develop, leaving sour flavours in the cup. However, if it is too low, your beans can “stall”, resulting in baked and doughy flavours.
Carlos Núñez is the owner of Boconó Specialty Coffee Roasters in Madrid, Spain. He says that the RoR is one of the most important variables in roasting because it provides early indications of temperature development, allowing roasters to “predict” their desired profile and consequently achieve it more effectively.
During the roast, it’s important that you keep RoR as controlled as possible. This means if certain variables suddenly change during the roast, you can react quickly enough that your beans don’t bake, scorch, or ultimately end up being ruined.
How Do You Use Roast Curves To Stay Consistent?
One of the reasons customers return to any particular brand of specialty coffee is because they know what to expect. For roasters, it is vital to maintain consistency when roasting, so customers receive the same flavours each time they purchase a new bag of the same coffee.
Carlos says that roasters can take advantage of roast curves to be more consistent in their final product. “In my experience, the best way of repeating roast curves is defining which parameters should be fixed during the majority of the roasting process,” he says.
By charting and fixing these parameters, you will be more able to replicate the same profile time and time again.
“Ultimately, making fewer modifications is the key,” he tells me. “For me, defining the initial and maximum temperature of the air during the roasting process is of fundamental importance.”
By using roast curves, you will be able to chart and create a repeatable profile for your coffee that you can dial in time and time again. You will also be prepared if something does go wrong, allowing you to adapt and respond to any potential problems.
While fully appreciating every variable throughout roasting takes time, practice, and effort, visually representing these variables will help you become a better, more efficient roaster in the long term.
At MTPak Coffee, we understand how important it is for specialty coffee roasters to create the perfect product for their customers. For that reason, we offer a range of innovative packaging options that reflect all the hard work that has gone into roasting your coffee.
For more information on our range of coffee packaging solutions, contact our team.
Photo credits: Cris Flores
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