Just to summarise what you might already know: roasting coffee beans, in an absolute nutshell, is the process of transforming the physical and chemical properties of coffee cherry seeds, through the application of heat, to create flavour, and solubility.
That’s a bit of a mouthful!
Let’s dive a little deeper and understand the different stages beans go through.
The taste characteristics of different roasts
First, as you may already know, different roast degrees bring out different tastes out of green coffee beans.
Light roast coffees are typically sweet, with complex and lively perceived acidity. Most of the origin and processing characteristics will be easily discernible.
Light roasts are also referred to as “city roast”. They are usually more mottled on the outside of the bean and have a light coloured chaff. Generally a light roast is most successful for very high quality coffees. If you want to have a great filter experience, this is often the best bet.
A good light roast is sweet with lots of good acidity and features the inherent origin characteristics of the coffee. They usually taste best with at least a few days rest and stay fresh longer than medium and dark roasts.
Roasts are usually still in first crack when they finish or they end at the same time as first crack.
Proper development is more difficult to achieve and coffees can easily taste vegetal, sour, papery and weak if the roast doesn’t fully develop. Often, a higher amount of energy at the beginning of a roast is really beneficial to developing any coffee and especially a lighter roast.
As the roast gets a bit darker, the acidity will become smooth, and you’ll get plenty of sweetness, and more body. If you are not sure of what you want, going for a medium roast is the simple solution.
The acidity will be slightly lower than a light roast. Generally, the sweet spot for a medium roast is larger than it is for a light roast. By that I mean there are going to be more than one way to make a coffee delicious in the medium roast style, but also when they are being brewed it’s usually easier to make them taste good.
Also called City + by some groups, the medium roast lives between the end of first crack and the beginning of second crack.
Dark roasts tend to have very unique flavours, with a subtle and simple acidity; think flavours reminiscent of smoke, ash or molasses.
Dark roasts have very reduced acidity and unique flavours. The body is very high and the beans, being broken down more, are very soluble. These roasts are often used in coffee shops in order to bring a velvety feel to espresso shots, making them great for milky coffees.
Dark roasts have gone past second crack where the structure of the coffee begins to crack at a cellular level. Oils are released and migrate to the outside of the bean, either immediately or as the coffee rests over a week or two.
This isn’t to say that all dark roasts are bad – a good dark roast of the right green coffee is enjoyable. It can be dense and sweet with enough acidity to cut through and balance the coffee. However, like a light roast, the balance is fine and a dark roast is usually easier to get wrong than right.
The stages of roasting coffee beans
Either at home or in a commercial roastery, the stages of a roast are the same.
At home, you might be tempted to look at your beans while they transform; be very careful as the temperature of the air goes high and can cause damage.
We designed our roaster in order to allow you to safely check the progress of your roast with minimal heat and smoke coming into your face.
The key stages are:
- The first crack
- Roast development
- The second crack
1: The Drying Stage
The green, raw coffee beans have anything between 8-12 percent moisture by weight. The coffee beans won’t begin to turn brown while there is much moisture in them, so drying them out is the first part of the process.
It takes a lot of heat to get the beans warm enough for the moisture to start evaporating, and at this stage there’s hardly any visible change. The smell stays largely the same although sometimes be described as “grassy”.
2: The Yellowing Stage
Now that the beans have dried out, they will start to turn brown, and some beans will go very yellow, or even orange, before turning brown.
The aroma of the beans will start becoming a bit like bread. As this happens, the coffee bean will expand, and the chaff (the thin, papery skin) will start to separate from the coffee. This flaky chaff can easily catch fire, so it’s blown out by the fan and collected in the glass chaff jar.
You will perhaps start to see some steam at this point.
It’s very important that the coffee is dried and turns yellow at an even rate. Uneven drying and yellowing can result in the inside of each bean being undercooked and a rather sour and grassy taste in the finished coffee. This can’t be fixed later by slowing down the roasting.
3: The First Crack
Okay, now we’re getting going!
As the coffee continues to turn brown, there’s a build-up of gases and water vapour inside the bean. When the bean just can’t handle it any more, the bean breaks open with a distinct “crack” or “pop” that you’ll get to know and love!
At this point you have roasted the coffee enough to use it to make coffee. Once you reach the first crack, you can choose to stop roasting at any point. If you were to stop roasting right at first crack, you would have a light roast, and the coffee will be fairly sour — not necessarily a bad thing!
The roasted beans will now begin to have the familiar and wonderful coffee flavours that we all love.
4: The Roast Development Stage
This is when even more magic happens. After the first crack, the beans become smoother on the surface. As the beans turn a deeper brown, the acids and sugars change, leading to more subtle acidity as the roast development gets longer. As this is happening, the level of bitterness increases as the roast gets darker in colour.
There are a number of ways to describe the roast level at this stage. However, we choose to describe the coffee by the amount of time in the roast development stage and the roast colour.
5: The Second Crack
If you continue to roast the beans further and darker, they’ll crack again, but this time it’ll be quieter and and may sound like a sizzle, rather than a loud pop.
At this point, oils in the coffee will come to the surface and you’ll get dark and shiny beans. At this point, you’ve more or less lost any unique flavours from the bean itself, and the coffee develops a generic bitter “roast” flavour.
Roasting coffee beyond the second crack can make the beans actually catch fire. The IKAWA Home roaster is equipped with safeguards to prevent a fire, when used according to the directions. However, roasting coffee that dark, may not be the tastiest option. While dark roasted specialty coffees have shown some popularity in recent years, most high-quality beans aren’t suited to roasting very dark as you lose most of their unique flavours and characteristics.
The stages of roasting coffee beans at home: a summary
You may make some mistakes along the way, or roast some coffees that you do not like, but let's use those experiences as learning opportunities.
Make lots of notes as you go along. Use your senses to watch, listen and smell as the beans get to the first crack. 30 seconds difference in roast time can make a dramatically different coffee. Make notes of everything that you like and don’t like the taste of.
With all the subtle nuances of coffee roasting and the need for finesse and consistency, the IKAWA Home roaster system, fits the bill to get the results that you are looking for.