Colour changes

Let’s start nice and easy. This is one of the more well-known things about coffee roasting: green beans go in and brown come out. Luckily, the explanation for this is simple (ish). Effectively, the coffee roasting process is broken up into stages, which we’ve looked at before, but today we are looking at the science! The first stage is known as the “drying phase”, where the beans start losing moisture.

The chlorophyll starts to break down in the first few minutes, changing from green to yellow, then light brown. This is all due to the Maillard Reaction, where a series of chemical reactions cause the amino acids and sugars in the beans to react with heat, creating new flavours and colours. Not long after this, you hear the first crack as the brown turns to deep caramelisation. This will be where you’ll find a majority of light roasts being pulled to cool.

They will remain in the heated drum for longer for medium and darker roasts. They eventually go through a second crack, which results in dark (burnt) beans that have a shiny appearance. You’ll typically find this with commodity coffee, as they purposely over-roast the to hide any sour flavours from poor quality beans.

The different types of coffee roasting

What are the ‘cracks’?

This is a word used to describe the sounds during the roasting process. It sounds pretty similar to popcorn popping, but not quite as loud. This indicates that the beans have lost most of their moisture and have expanded in size, which literally causes them to crack!

The second crack is when the beans have become so dry they are brittle and breaking apart. This tends to be louder than the first one as the beans physically break apart rather than popping. We never roast to this stage as we think it causes the natural flavours of our speciality beans to be lost, replacing incredible notes with bitter ones. Yuck!

What are roasting degrees?

So you’ve heard of light & fruit and medium roast styles, but what about cinnamon and city? These are some other names given for other specific roast styles. A cinnamon roast style, for example, is a name given to coffee that is released very early after the first crack. This brew is described as very acidic and ‘peanutty’, with a grassy and floral aroma.

For a city roast, these are what we would call a light and fruity roast. These are released just after that first crack and have a juicy, floral, fruity flavour. A full city is medium to medium-dark and has caramel, chocolatey notes.

You can even get a roast style called Viennese! This would be classed as a popular roast style and is a little darker than we go at Two Chimps. It has that bittersweet, nutty coffee you get at a coffee shop chain.

Density and weight loss

Coffee loses about 12-24% of its weight during roasting, depending on several variables like initial moisture content, etc. Light and fruity roasts, as they are roasted the shortest, only lose about 14-16% of their weight compared to darker roasts, which lose 22% or more. So, what does this mean? In terms of experience, you may find a fuller body from beans with a higher density, but the flavour comes down to the roast style rather than the density.