How to be a crack at roasting coffee.

Steam. Pressure. Sound. And how to go from cool beans to hot stuff when roasting coffee.


Roasting coffee - A collage of coffee beans showing various stages of roasting from raw through to Italian roast


1-Minute NomNom

A good cup of coffee depends not only on the quality of the coffee beans but also how they were roasted. A big part of roasting coffee is to get the Maillard reactions and caramelization right (read all about it in the 1-Minute NomNom “Bean feeling bitter? You choose to feel butter!“).

roasting coffee has two cracks

And an important part of that has to do with sound. Wait… sound? Yup, specifically the sound of the beans cracking when we are roasting coffee. This crack sounds like popcorn popping. It happens twice during the process of roasting coffee and is a signal for the different stages of roasting.

The first time it happens at about 196°C/384F°, it signals the coffee beans have undergone their first phase of roasting. In the first phase, Maillard reactions (happens between 140°C/284°F to 165°C/329°F) and caramelization (160°C/320°F) have started and are ongoing.

roasting coffee has two cracks

The second crack happens at about 225°C/437°F. When we hear this second pop, it is because the bean structure is collapsing. It signals we should stop roasting the coffee. And to do it quickly as they can become burnt very quickly (matter of seconds). In fact, we might even want to cool the beans down deliberately with air or water.

Pop science

Where do these pops and cracks come from? In both cases, it has to do with the build-up of gases within the beans as we are roasting coffee. As the temperature rises further, the gases expand, creating a build-up in pressure at the same time. At some point, the pressure is so great that it breaks through the bean, and gases burst forth.

The difference in the two cracks has to do with the type of gases. The first crack comes from the build-up of steam. As the coffee beans are roasted to 100°C/212°F and beyond, the water inside the beans turns to steam. This builds up till 196°C/384°F before breaking forth.

The video above demonstrates what this first crack sounds like, followed by a dramatic slow-motion repeat of the cracks. So depending on how much drama you want in your life, you can watch it for as long or short as you like.

The second comes from the gases such as carbon monoxide (CO), carbon dioxide (CO2), nitric oxide and nitrogen dioxide (NOx). As the temperature rises to 225°C/437°F, these gases build up and eventually burst through the beans. You can hear these second cracks in the video below.

Amazingly, in roasting coffee, science and nature have given us two sound cues to help us get the coffee roasted right.


Screen Shot 2015-01-10 at 6.02.39 pm

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photos: in order – depositphotos/rixipixbelchonock


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