Author: Nick Day

Before diving into this article, please have a read of the previous two prerequisite sections, Part 1 (Understanding the roast process….Heat vs. Mass) and Part 2 (What happens during the coffee roast process…).

So much like in the 1st part of this article, understanding heat vs mass, is critical to understanding how to properly roast.  Roasting coffee is exactly like boiling a pot of water, except instead of a pot, we have a drum and instead of water, we have beans with water in them.  If you understand the pot-water-boiling experiments in the 1st article, then you can understand easily what is happening inside the bean.  

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Let’s look at what coffee roasting is, and should be…

What happens during the ideal roast…

First of all coffee is a seed, of the arabica or robusta tree….  it generally has between 9-14% water inside of it.  Yes, the same kind of water in our pots in the 4 experiments above.  When we roast the coffee, at a certain point, that water reaches the boiling point and begins to escape the bean with an audible low-volume “POP”.  This commonly sounds the same as popcorn in the microwave popping, just lower volume.  This is commonly what people refer to as 1st crack.  It’s generally one of two indicators of the status of our roast.  As the water boils away from the bean, and as we keep heating it, the bean begins to caramelize and the structure of the bean begins to expand, and stretch as the internal sugars begin to caramelize.  This builds up stress in the bean to at which point the bean’s shell fractures and cracks.  This cracking can be

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“As simple as boiling a pot of water.”

One of the most common subjects we teach new roasters about is understanding Heat vs. Mass.  A common approach for folks excited to start producing coffee on their new roasting toys within minutes of the roaster being assembled, is to throw a pound of coffee inside, crank up the temperature and roast away until something happens.  More often than not, it results in something drinkable, but not necessarily exceptional, and in some cases a fire whence the coffee has gotten too hot and catches flame.

In order to get the whole process under precise and predictable control, I find it necessary to go back to basics on something that we all understand very well.

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The reason why your roast profile temperatures may be different than other people’s… or even the profiles suggested here.

One of the key concepts that we need to understand is that we are all building a custom roaster, as unique in many ways as our own personality.  We all use a different grill from one make to another model, to another year version, with different BTU’s and so forth.  

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