Coffee Roasting is a Chemistry

Coffee roasting is a chemistry: the chemical process by which aromatics, acids, and other flavour components are created, balanced, or changed in a way that would enhance the flavour, acidity, aftertaste and body of the coffee.   

Many of these reactions are sensitive to variations     in temperature and length of exposure to heat, meaning a small change in roasting technique can have a significant impact on the roasting profile.

Understanding what happens during roasting and knowing why these changes occur can help you adjust your roasting methods.  Once you have a general overview of how chemical compounds are created and changed during the process, you can better understand what went wrong, or right, with your batch and use the information to make your next one better.

There are 8 main roast stages, which we will briefly discuss in chronological order.  According to these, you could formulate your preferred roasting profile with the various origins of green beans. 

  1. Charge Temperature

Charge temp is the temperature of the drum when you first add your beans.  Keep in mind, this is not a roast stage with many home roasting machines, you can’t really control this well with a pan, popcorn machine or many of the home roasters on the market.  However, you may want to experiment with it as you get more into your roasting as it can have a dramatic impact on the rest of your roast.

  1. Rate of Rise (RoR)

This is the speed at which heat increases inside your roaster.  It helps you to control acidity, body, and more.  Again, most home roasters won’t be able to directly control this but it’s a key term in the industry and therefore worth knowing.

  1. Yellowing

An early colouring during the roast, when your beans start to dry.  Only moments away from browning.

  1. Maillard Reaction/Browning

The Maillard reaction is a chemical transformation.  This browns the beans and creates its delicious flavour compounds, specifically the savoury ones.

  1. Caramelisation

Caramelisation also creates many flavour and aroma compounds.  However, the chemical process is different.  You can recognise it by the subtle caramel notes in the warm air.  It follows soon after the Maillard reaction.

  1. First Crack

The first crack is a special moment.  A literal auditory crack, a sign that your beans are nearly ready. 

    Many roasters do not continue much longer (than 5               minutes) after first crack.  The second crack, as it                     borders the first stage of dark roasts, risk losing                       flavour characteristics they would typically pursue in               some beans. 


  1. Second Crack

The second set of audible cracks during roasting.  If you are going for a darker, espresso-like roast, you’ve arrived!

  1. Resting

After the roast is completed, allow them to rest and de-gas for a few days before grinding and brewing them.  (Or grind and brew some on the spot to have and compare notes in a few days.  Who knows, you might prefer one to the other.)

Now, let’s have a look at what you taste and why (not in chronological order): 
The Flavour Analysis

Acidity & Sourness
Acidity is when a coffee creates a similar sensation on your tongue as eating something sour.  There are different types of acidity in coffee.  This can be manipulated through roasting but challenging with equipment that doesn’t allow for much control over the various conditions applicable during roasting.

This is once brewed, the cup’s texture.  Sometimes described as rounded, silky, full, etc.  Body is a factor that can also be enhanced during roasting.
A desirable trait, this can be highlighted during roasting.

Generally seen as a negative trait in specialty coffee but with exceptions according to preferences.  Over-developed and overly dark roasts tend to be on the bitter side.

Nutty, Spiced, Floral, & Fruity
Common in coffee characteristics, the more you taste and try the more you’ll find bean origins have their unique flavour profiles.

This is a trait most common in darker roasts. 

This suggests that the coffee was under-developed.

The flavours and aromas are less vibrant.  This often indicates that the green coffee was old.