Colour is a parameter that you can use to assess the degree of your home roast. When coupled with the sounds of first and second crack, plus the changing aromas as your roast develops, you will start to get a greater insight into the stages the beans go through and what are your preferred tastes. Coffee beans become drinkable once they have undergone first crack and some beans are divine as lighter city roast. Whereas certain beans suit a darker full city roast. This again, is all part of the fun of roasting.

The illustration opposite simplifies the different stages a green coffee bean goes through to reach something worthy of putting in your cup.

  • Green Unroasted Bean - this doesn’t need much further explanation. You would not want to drink this! Green coffee beans have other uses but that doesn’t need to be elaborated on here.

  • Yellow Bean Stage - your roaster has been tumbling your beans for a few minutes now and gradually you will notice the beans start to take on a more yellow shade. The coffee has a ‘hay’ like smell at this stage. This warm up stage leading up to first crack is known as an endothermic process. After a few more minutes the ‘hay’ like smells will be replaced by something more reminiscent to baking bread.

  • Light Brown Bean Stage - The bean is now approaching first crack and will have started to expand. It will also start to shed some paper thin skin, known as chaff.

  • 1st Crack Begins - the coffee has now taken more of a brown tone, this occurs due to browning from sugars and also another browning reaction known as a Maillard Reaction. At around 7-8 minutes you will start to hear some cracking noises, that are very similar to popcorn popping. The bean will start expanding further and show deeper cracks down the centre. First crack is an exothermic reaction as the beans are giving off heat, but they quickly become endothermic, which means you must maintain the heat on your hob for your roaster at this point and you can afford to turn the heat up a fraction.

  • City Roast - this is where first crack is complete. This should be anywhere between 10-11 minutes. The beans are noticeably darker and have now gained a smoother surface texture. At this stage the beans begin to vent off C02

  • 2nd Crack Begins - the process between first crack ending and second crack beginning is quick, anywhere between 15-30 seconds. Second crack is less pronounced, the noises are more like rice crispies snapping, as oppose to popcorn popping. During second crack you will start to notice some oil appear on the surface of the beans.

  • Full City Roast - this where more audible snaps are heard and more oil will appear on the surface of the bean. This should be around 12-13 minutes.

  • The further you now start to take the roast on the more you loose the flavour of the bean origin and start to develop more flavours from the roast itself. These roast profiles are often referred to as Vienna or full French roast.

This is a brief introduction to the different stages of roasting. If want to get more complex go for it. There are many roasting books available that explain the chemical reactions in more detail. The choice is yours. The fun is in the experimenting with different beans and flavours.