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Bean Insight: The science (and art) of coffee roasting

Ever wondered what makes great coffee taste so great? How it gets from bean to cup? And what the secret is to producing sumptuous flavours and aromas?

Welcome to the first post in our Bean Insight series, aimed at providing you with expert advice on our number one passion – coffee. In this post we explore the art of roasting.

Believe it or not, those little coffee beans, which first start off life as coffee cherries when first picked, go through a lot in their little lives, and it all starts with the roasting process.

Roasting is what transforms ‘green coffee’ (that’s the coffee cherries once they’ve been picked, processed and dried) into the aromatic brown beans that we all know and love so well.

What happens to the beans when we roast them?

When we roasts beans they go through a whole host of changes, some of which are detectable during the roasting. One of the most obvious is the colour change from green, to yellow, to brown and eventually to black (we don’t want to let it get to this stage!). Another obvious change during the roasting is a loud ‘pop’ known as ‘first crack’ which is where the beans release pressurised gas and water vapour and is often a sign that the roasting is nearly finished. Over the course of a roast the beans also nearly double in size and halve in weight, increase in acidity, gain and then lose sweetness and also release hundreds of aromas.

 How do roasters work?

For the most part, roasters consist of a heated, rotating drum and an exhaust flue to allow air and chaff to escape. It is a combination of the hot air in the drum and the hot surface of the drum wall that roasts the coffee beans, and the correct balance of these is what creates the perfect roast profile and ultimately the tastiest brew.

What is a roast profile?

On our roaster we have temperature probes that tell us the temperature of the beans, the roaster and the exhaust. All of this information, along with the time, is tracked on a computer program and presented as a graph called a roast profile. This allows us to see where we need to make any improvements and also to keep our batches consistent by following the roast profile closely.

How do you decide on a roast profile?

There are all sorts of factors that affect the way coffee beans will roast. Everything from where they were grown, the altitude they were grown at and even the ambient air temperature in the roastery. Using all this information we come up with a starting point and then keep tasting and tweaking until we’re satisfied that we’ve unlocked the most flavour from that coffee.

Want to know what happens after the roasting process? Stay tuned for the second part in this Bean Insight special, ‘Lifting the lid on coffee creation post-roasting (part two).’

14 March 2016 - The Barista


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