Sip on Science: Coffee

5 min read

Many people claim they can’t survive without their daily dose of coffee. You may even be reading this with a cup in hand. Have you ever wondered how the humble bean turned into such an addictive beverage? We’ve got the journey condensed into an espresso-sized sip below.

Where does coffee come from?

Coffee plants thrive in places with all of the following: mild to warm temperatures (within the 20s), rich soil, clear dry and wet seasons with ample rainfall in the latter, shade from harsh sun, and no frost. On Earth, such conditions are found almost exclusively within the tropics. 

Coffee growing regions around the world with common origin countries or states pointed out. Source: Press Coffee.

Coffee can be divided into two broad varieties—arabica and robusta. Arabica plant varieties generally prefer to grow at higher altitudes than robusta varieties. Robusta coffee plants produce rounder beans that pack almost twice as much caffeine and acidity as arabica beans.

What is a coffee bean?

The coffee “bean” is not actually a bean, but the seed of the coffee fruit, which looks like tiny cherries. The seed takes up most of the volume of the cherry.

Coffee cherries in Kona, Hawai’i. September 2022.

Most coffee cherries contain two seeds. However, they can also sometimes have three seeds or just one. A cherry with just one round seed is called a peaberry. Peaberry seeds make up about 5% of a harvest and produce a smoother, stronger flavour as they have taken up all the caffeine and substance in the fruit!

Three sets of seeds or “beans” from three different cherries: a triplet, a pair, and a peaberry. Look at the size and shape differences of seeds from each situation! The peaberry took up the whole cherry until what would have been the second seed is just a bit of husk and the mucilage. Taken in Kona, Hawai’i.

Between the skin and the seed are the pulp, mucilage (a slimy, sugary coat), parchment, and silverlining. The last two are sometimes referred to together as the husk of the seed. An unroasted seed without its husk is called a “green bean”, not to be confused with the green bean veggie you might add to your caifan/economy rice. (Now that is an actual bean, not a seed.) In any case, the green bean won’t stay green for long if it’s going into someone’s cuppa.

Coffee seeds stripped down to different layers. September 2022 in Kona, Hawai’i.

Processing and Roasting

Once the cherries are harvested, seeds are removed, dried, and milled until we get the green bean. Then, the beans are sorted by size, density, and colour. Sorting filters out beans that may be over- or under-fermented, or damaged by pests. It also allows processors and roasteries to produce coffee of different grades or quality to sell at different price points or different consumer bases. Just like peaberries, beans that are larger and more dense pack more substance, which produces a better flavour.

Finally, the beans are ready to be roasted. How long the beans are roasted determines the strength of the flavour, aroma, and caffeine of the brew. It also changes the bean colour to different shades of brown, hence you dark, medium, and light roast. A longer roast turns the bean darker and releases more oils from the seeds, but risks burning off some of the caffeine and other acids. Hence, a dark roast may have a bolder aroma and bittersweet taste, but actually has less caffeine.

Some brands may offer a ‘black and tan’ blend – this mixes dark-roasted and light-roasted seeds so you get the flavour of a dark roast but the caffeine of a light roast. ‘Brown and tan’ does the same but with medium and light-roasted seeds.

Climate changing coffee

If temperatures are too high, coffee cherries may ripen too fast and produce seeds of poorer quality. Coffee will need to be grown at higher elevations to escape the warming temperatures. This reduces the viable areas for growing coffee worldwide. More extreme weather such as heavier rains and longer draughts are already hampering the productivity of other areas.

Warming temperatures also cater to insect and fungal pests that ravage coffee crops.

When coffee plants become harder to grow, it can cause a trickle down effect that leads to your next cup getting more expensive. As if we needed more inflation.

If your coffee tasted good this morning, be wary that it might taste worse in fifty years and still cost you more. That is unless we are able to decelerate climate change.

For more food science-related topics, check out Science Café happening this Sunday, November 19. You can register here.

Text and photos by Ellen Ng
Featured image by Zhang Yingjie


Mountain Thunder Coffee Plantation



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