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The Coffee Bean Roasting Curve

  • by Justin Lim
  • 5 min read

Roasting graphs are gorgeous representations of critical information, or alternatively, they're extremely perplexing. Until you know what's going on, all those lines and data points might be overwhelming. They may help you improve your roasters, troubleshoot difficulties, and repeat results once you know what the lines and points represent.

Coffee beans in a roasting machine

Rate of Rise (RoR) graphs, environmental rate of change, exhaust temperature graphs, and others are just a few of the graphs employed to display roasting reactions. However, today's topic is bean temperature graphs and how to interpret them. This is the standard curve that looks like a check mark.

What does the roasting curve signify? What is the significance of this graph, and how does it impact what's happening in the roaster? And, more importantly, how can you use it to make good judgments? Let's have a look.

Bean temperature chart | Credit: Raul Juan Sorita

Blue - Bean Temperature | Red - RoR Curves


Drop in the Curve

Let's begin at the beginning. When you add the beans to the roasting drum, it reaches its internal temperature, which is known as the charge temperature. The first point on the graph will be this position.

When you put the beans into the roaster, you'll notice a steep drop in temperature almost immediately. This isn't because the bean temperature is going down, though. What you're seeing is the result of changes in temperature recorded by the roasting machine's bean probe. Unfortunately, the thermocouples can't actually measure bean temperature; they just record a temperature in the drum, which is an indication of bean temperature.

The air within the roaster has already heated up before the beans are dropped in. The addition of room-temperature coffee beans causes a drop in heat energy, causing the probe reading to decrease. The graph's steep drop can be explained in this way. Have you ever noticed that once you add your pasta, potatoes, or other veggies to boiling water, the bubbling stops? It's the same physics at work.


Turning Point

The temperature in the roasting drum is now decreased, but the beans require to warm up. They heat up as they are exposed to the heat in the drum, reaching a point where the temperature no longer cools and starts to rise. This is the point that most roasters consider to be the turning point, as shown in the graph's bottom curve. It's vital to remember that, while the coffee beans are becoming hotter throughout the whole process, it may not seem like it on the chart.


Upwards Slope

Beans lose moisture at a rapid rate. They also continue to increase in temperature at a variable rate, referred to as Rate of Rise (RoR), which is influenced by many things. This includes the roast power, the charge temperature, and the beans' initial wetness level. Other elements may also play a role, such as the roaster's technique or even the beans' origin and processing approach.

The angle of the bean curve's ascending slope is directly proportional to the RoR. The faster the beans heat up, the steeper and more vertical the curve will be.

Ovens are not only useful for baking, but they can also be used to roast. When you remove the roast from the oven after a few hours, it will be extremely dry and light. This causes your roast to lose weight during the roasting period. As a result, less heat is required as it becomes increasingly dry. The rate of reduction in RoR is determined by production variables and can range from 20% to more than 20% each hour. This explains why the bean temperature curve becomes flatter as the roast progresses.


Upwards Slope: Chemical Reactions

During the roast, there's a lot going on inside the coffee bean. Chemical reactions are quite essential. They help to generate the aromas and tastes that we enjoy while drinking coffee.

The coffee beans are allowed to dry. After the turning point, there is a period, usually about a few minutes, when the coffee beans have lost enough moisture to start displaying a physical reaction: their natural greenish color fades and becomes yellow. The time before the color change is known as "the drying phase." This is true as long as the beans have lost moisture thus far. They will, however, continue to lose moisture until the roast reaches its conclusion and the beans have cooled down.

The Maillard Reaction, which takes place when beans reach a temperature of approximately 302°F/150°C, is the next major chemical reaction. Due to the heat, carbohydrates and amino acids in the beans react with each other, resulting in changes in color and flavor.

Then there's Strecker Degradation. Amino acids combine with carbonyl-grouped molecules to create aldehydes and ketones, which are important aroma and taste components.

When the temperature reaches around 338°F/170°C, complex carbohydrates in the bean break down into sugar molecules as a result of the heat. This will enhance the sweetness of the bean.


Flattening the Roasting Curve

The majority of the roast is an endothermic process, which means that coffee beans absorb heat or energy. They gain a lot of energy in the process. Because of this, the remaining water within the beans' cellular structures is put under a lot of strain. This will increase until it leads to the first - and quite crucial – exothermic reaction, often known as first crack.

“Even though the initial crack isn't as simple to detect as the turning point when examining a roasting curve, it plays an important role,” Iair Lemcovich, CEO of Spanish roastery Balnes Europe, explains. It's critical to know when to end the roast and toss the beans.

The first crack signals the conclusion of the second stage, which includes numerous chemical reactions as well as the start of a third phase known as "growth phase." Beans, like anything else, continue to develop as they mature. Similar to the drying stage, this term might be deceptive.

After first crack, many roasters will stop the roast. Others will finish it after a second crack is formed as a result of the pressure generated by an accumulation of CO2.

After the first crack, the RoR is likely to be lower and the bean temperature will have risen as a result. The term "logarithmic curve" refers to a graph that is generated when the rate at which you add heat to a coffee bean changes over time. When this happens, the curve will show a flatter or less pronounced upwards slope as it approaches the end of the roast.

When you remove the beans from the roasting drum, the curve will come to an end, so you'll be able to read the beans' final temperature on the graph.

The roasting curve or check mark indicates that your roast has been completed as intended. Despite the fact that other graphs and data points may provide even more information, beginning with bean temperature will give you a good start in understanding what's going on inside your roaster, when, and why.

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