So, we know where the bean comes from, but have you ever been curious on how it gets from that green bean in the wild to the delicious brown bean that is used for your morning coffee?

This happens by coffee roasting, which is different to the trend going round at the moment where you roast your friends. Coffee roasting is where the beans are subjected to extreme heat and heated until they turn that perfect shade of brown (which can change depending on what flavour you want to go for).


Not the type of roasting that we are talking about…

The process begins as an endothermic reaction. This just means that the reaction absorbs heat. At this stage the beans are dried slowly until they begin to smell – this will normally smell like popcorn or toast. People from Dunedin will be very familiar with this smell, as the Greggs factory wafts its burnt toast smell across town – especially bad on my side of uni (apparently there have even been concerned people in the department who think a fire has started).


Coffee Roasting

After this smell begins to occur, the next step known as ‘the first crack’ happens at about 205°C. During this stage, the bean become a lighter brown colour, double in size (due to carbon dioxide loss) but loses about 5% of its weight.

The beans lose their green colour through this process due to the breakdown in their chlorophyll and well as the Maillard reaction. This is a chemical reaction between amino acids and sugars which end up producing flavour compounds and melanoidins (which also contribute to that lovely brown colour).

Anyway, back to the roasting. The temperature is then increased to about 220°C. The bean loses more of its weight – about 13% and the colour changes to a darker colour.

This process is called pyrolysis. This is the decomposition of organic material and is basically what happens to all your food when you cook it and is also responsible for the colour of the roasted bean.


Different coffee roasts – Taken by Jessica Spengler

The next step is exothermic (the heat is released) and then the second crack occurs. The bean gets darker still and the temperature gets cranked up to bout 225-230°C. This is when the beans get their oily outer layer.

Depending on how long the roasters leave the bean in during this process will alter the type of roast they will get in the end. The degree of the roast will change the flavours that come through from the bean.

There you go, another week. #SCIENCED

Make sure you are back on Sunday for more coffee science!!

Photos taken from:


Cool Beans!

Hey everybody, hope you are all looking forward to your weekend ahead!

This week I thought we could talk about the science behind the actual coffee beans – it’s easy to forget these days with your instant coffee and self-foaming drinks that once that your coffee was a little green bean thousands of miles away.


The coffee plant (a tropical evergreen shrub, genus Coffea) started off  being grown in Africa and is now mainly grown in The Tropics of Cancer and Capricorn. If like me, you have no idea where that is then this picture should help.

There are two main types of coffee beans that are used in commercial drinks and these are the Arabica and Robusta beans.

The Arabica plant has dark green leaves and is quite large. It takes around 7-9 months to be able to pick the cherries (where the beans live), which contain two seeds on average.


Arabica Plant

In contrast the Robusta plant is a small tree which grows to about 10 metres. It takes the beans around 10-11 months to mature in this species. The beans are also more oval and smaller than Arabica.


Robusta Plant

The overall trend for these beans is that the robusta is far more outgoing than the Arabica. Robusta can handle hotter temperatures (24-30°C vs 15-24°C) and are easier to grow and maintain. One reason for this is the level of caffeine you find inside.

The Robusta beans have about double the amount of caffeine. This is thought to be a safety mechanism as the bitter flavour deters insects from biting into their delicious flesh. This is why Arabica does better at higher altitudes as there are less pesky bugs hanging out there.

There is also an effect on the flavour as well.

Here are some quick-fire differences between the beans.



  • Stronger, harsher taste.
  • Twice as much caffeine but are a lower quality.
  • If good quality though they are good in espressos for their crema and flavour.
  • Easier to grow – less likely to get attacked by pests or the weather.
  • Produce more beans.

Although Arabica is the preferred bean to have in your coffee, a lot of places will put Robusta beans into their blends to reduce the cost, as these beans are way cheaper to produce.

Hope you enjoyed learning about the different types of beans in your coffee – now you can check what type you are having when you buy.

Have a good weekend and see you on Sunday!



Photos sourced from: