February 4, 2020

A Guide to Bean Temperature & The Coffee Roasting Curve

Roasting graphs are beautiful depictions of essential data – or alternatively, they’re really, really confusing. Until you understand exactly what’s happening, all those lines and data points can be overwhelming. Once you know what the lines and points refer to, however, they can help you improve your roasts, troubleshoot issues, and consistently repeat results.

There are a number of graphs used to represent different reactions during roasting, such as Rate of Rise (RoR) graphs, environmental rate of change, exhaust temperature graphs, and more. But today, the focus is on the bean temperature graph and how to understand it. This is the classic curve that looks like a check mark.

What does this roasting curve mean? How does it relate to what’s going on in the roaster? And how can it help you make decisions? Let’s take a look. 

Bean temperature (blue) and RoR (red) curves. Credit: Raul Juan Sorita

Lee este artículo en español Guía de la Temperatura Del Grano y la Curva de Tueste Del Café

The Drop in The Curve 

Let’s start from the very beginning. The graph begins at the charge temperature, which is the internal temperature of the roasting drum when you add the beans. This will be the first point represented on the graph. 

When you add the beans to the roaster, you will see a sharp and almost immediate decline in temperature. However, this is not because the bean temperature is decreasing. What you are actually seeing is the temperature changes recorded by the bean probe of the roasting machine. Unfortunately, the probes can’t actually measure bean temperature itself: they only record the temperature in the drum, which gives an indication of the bean temperature.

Before the beans are dropped into the drum, the air inside the roaster will have already heated up. The sudden addition of room-temperature coffee beans results in a decrease in the heat energy, reducing the temperature read by the probe. This explains the steep drop presented in the graph. Ever noticed that boiling water stops bubbling once you’ve thrown in your pasta, potatoes, or vegetables? It’s the same science.

The Turning Point

The temperature in the roasting drum is now reduced, while the beans need to heat up. They increase in temperature as they’re exposed to the heat in the drum, reaching a point where the temperature stops cooling and starts to increase. This is the moment that most roasters refer to as the turning point, represented on the bottom of the curve in the graph. It’s important to remember the coffee beans are getting hotter throughout the whole process, even though it might not look like that on the graph. 

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Roasting machine at a roastery in Ethiopia. Credit: Meklit Mersha

The Upwards Slope 

Beans keep losing moisture. They also continue gaining temperature at a variable rate, known as Rate of Rise (RoR), which depends on many factors. This includes the power at which the coffee is being roasted, the temperature chosen as the charge temperature, and the initial moisture content of the beans. Other factors can also have an impact, such as the technique of the roaster or even the origin of the beans and their processing method.

The angle of the ascending slope of the bean curve is directly related to the RoR. The higher this rate is, the more temperature the beans will gain per unit of time and the steeper the curve will be. 

RoR is not a constant number, and ideally, it should be decreasing as the roasting progresses. As the beans lose moisture, they also lose weight, which means that less heat is required to roast them. This explains why the bean temperature curve gradually becomes less steep throughout the roast. 

Checking on coffee beans for colour change during a roast. Credit: Meklit Mersha 

Chemical Reactions in The Upwards Slope 

There’s a lot going on inside the coffee bean during the roast. Certain chemical reactions are particularly important. They contribute to developing the aromas and flavors that we will savour when brewing coffee. 

There is a moment, usually a few minutes after the turning point, where the coffee beans have lost enough moisture to start showing a physical reaction: their natural greenish color turns yellow. Some people call the time before the color change “the drying phase.” This is correct in as much as the beans have been losing moisture until now. However, they will continue to lose moisture until the roast comes to an end and they have cooled down.

The next important chemical reaction is the the Maillard Reaction, which happens when beans have reached a temperature of about 302ºF/150ºC. A reaction occurs between the carbohydrates and amino acids in the beans due to the heat, which provokes changes in the color and flavor. 

Then we have Strecker Degradation. Here, amino acids react with carbonyl-grouped molecules which create compounds such as aldehydes and ketones. This is an important stage in developing aroma and flavor.

At about 338ºF/170ºC, you reach the caramelization stage, where the heat causes complex carbohydrates to break down into sugar molecules. This will contribute to the development of sweetness in the bean.

Coffee beans cool down after being released from the roaster. Credit: Julio Guevara

The Flattening of The Roasting Curve

Most of the roast is an endothermic process, meaning that energy or heat is absorbed by the coffee beans. They accumulate a considerable amount of energy. Because of this, the remaining water inside the beans’ cellular structure is put under a lot of pressure. This will build up until it creates the first – and very important – exothermic reaction, or release of heat, otherwise known as first crack

Iair Lemcovich, CEO of Spanish roastery Balnes Europe, says, “Even though first crack is not as easy to spot as the turning point when looking at a roasting curve, it very much serves as a reference.” It is key to determining when to end the roast and drop the beans.

The first crack marks the end of the second stage, where many chemical reactions occur, as well as the beginning of a third phase, often referred to as “development phase.” It’s important to remember – as Scott Rao says in The Coffee Roaster’s Companion – that beans are constantly developing. Like the drying phase, this name can be misleading. 

Some roasters will finish the roast shortly after first crack. Others will take the roast all the way through a second crack, which will happen because of the pressure built by an accumulation of CO2.

Let’s take a look at the coffee roasting curve at this final stage of the roast. After the first crack, the RoR has usually reduced and the increase in the bean temperature will have followed suit. This means the curve will show a flatter or less pronounced upwards slope toward the end of the roast. 

The curve will end when you remove the beans from the roasting drum, so you’ll also see the beans’ end temperature on the graph.

Person holding a handful of roasted coffee beans. Credit: Red Band Academy

The roasting curve or check mark tells you that your roast really has gone according to plan. Although other graphs and data points can add even more insight, starting with bean temperature will give you a good initial grasp on what’s happening inside your roaster, when, and why. 

Enjoyed this? Check out RoR, Bean Temperature, & More: How to Use Common Roast Data

Feature photo credit: Red Band Academy

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