Coffea arabica: the bean with a buzz

Coffea arabica flowers. Image by B.navez (CC BY-SA 3.0)

Coffea arabica flowers. Image by B.navez (CC BY-SA 3.0)

When I started my PhD studies 4 years ago I did not like coffee. Now, I actually rather like the aroma of freshly brewed coffee and sometimes I even seriously consider drinking it. You might think that this is due to the long working hours associated with working in science but I think it is because there is a coffee machine in my office right behind my desk. Throughout the day there is a steady stream of coffee making around me so for the last four years I have spent my working week  surrounded by a lovely coffee aroma. Although I’m still hoping to avoid becoming a coffee drinker I realised that the sum total of my knowledge of where coffee comes from is “roasted coffee beans”. So I did some research…

Coffee is made from the beans of members of the Coffea genus. Coffea plants are evergreen shrubs/small trees that grow to about 5 m tall, although when cultivated they are usually much shorter due to pruning. The beans are contained within the fruit, known as “cherries”. After harvesting the soft flesh of the fruit is removed and the beans fermented to remove the mucilage (a slimy substance) leaving the bean. The beans are then dried and roasted before they are ready for coffee making.

Immature fruit on Coffea arabica.  Image by Marcelo Corrêa (CC BY-SA 3.0)

Immature fruit on Coffea arabica. Image by Marcelo Corrêa (CC BY-SA 3.0)

Coffea plants originate from Ethiopa. According to legend, in the 9th Century a goat herder called Kaldi stumbled across a group of monks while he was searching for his flock of goats (1). The monks were eating the cherries from a Coffea plant. Kaldi danced with delight when he tasted the cherries and shared them with a passerby. The passerby took them to his monastic friends who cultivated the tree and brewed a drink that enabled them to stay wide awake during prayers (1). Whether or not this story has any element of truth in it, there is evidence that coffee was cultivated in the Sufi monasteries around Mocha in the Yemen as far back as the 15th Century (2).

Today the coffee industry is huge with coffee produced in equatorial regions across the world (2). In 2005, it was the seventh-largest legal agricultural export worldwide (2). Of this, Coffea arabica accounts for 70% of coffee production but C. canephora (also known as robusta coffee) also widely grown.

Coffee contains the stimulant caffeine. The caffeine content of coffee beans varies but in C. arabica is typically around 1.2% of its dry weight.  C. canephora has a higher caffeine content and tastes more bitter so varieties of C. arabica are usually the bean of choice for high quality coffees. However, C. canephora is less susceptible to disease and can be grown in lower altitudes and warmer climates than C. arabica (2) so it is often used in cheaper coffees.

Roasted C. arabica beans

Roasted C. arabica beans. Image by Ragesoss (CC BY-SA 3.0)

There is increasing demand for decaffeinated coffee. To reduce the caffeine content unroasted (green) beans are steamed and then rinsed with a solvent to extract the caffeine (2). This process is repeated 8-12 times until the caffeine content has been sufficiently reduced for the beans to be classed as “decaffeinated” (in EU 99.9% caffeine free by mass) (2). Scientists have found some mutant C. arabica plants that have naturally low caffeine content (3). Caffeine is made from its precursor theobromine through the activity of the caffeine synthase enzyme. The mutants lack a functional caffeine synthase so accumulate theobromine instead (3). Cultivating naturally low caffeine plants would remove or reduce the need for the decaffeination process so naturally there is a lot of interest in this research.


1) Laws, B. (2010) Fifty plants that changed the course of history. Pub: David and Charles.

2)Wikipedia: Coffee (retrieved 12/01/14)

3) Silverolla et al (2004) Plant biochemistry:  A naturally decaffeinated arabica coffee. Nature.


8 thoughts on “Coffea arabica: the bean with a buzz

  1. Thank you for an interesting post, Sarah. The pharmacological properties of caffeine are quite interesting. Its stimulant effects are believed to be due to its antagonism of adenosine receptors, which normally have inhibitory roles in the central nervous system. Additionally, it acts peripherally as a non-selective phosphodiesterase inhibitor. Do you know whether the ability to produce caffeine evolved independently in different plant species? I was quite surprised by the diversity of plants which produce it.

    • Thank you. In addition to its effects on animals, caffeine can induce calcium oscillations in plant cells! But we dont know how or whether it is biologically relevant.

      I dont know much about the evolution of caffeine production. Plants produce lots of secondary metabolites though so if there are only a few reaction steps between caffeine and a precursor that is commonly found in plants then it would be relatively easy for it to evolve independently in different plant species. Also caffeine is toxic to many insects and animals so its pretty advantageous for plant defence!

      I shall look into it and get back to you 🙂

  2. Like most crop plants, the severe monoculture of arabica makes it highly susceptible to disease. Coffee rust (fungus), Hemileia vastatrix, caused great losses in the 1800s and is still a big problem today. Spores were spread locally by wind and rain, but the large scale spread of the rust across the world was down to human movement. Coffee then became quite expensive (as yield was low) and was partly responsible for the masses in the UK switching to tea as a cheaper caffeinated beverage.

  3. Pingback: Camellia sinensis: the plant behind a comforting cup of tea | Plant Scientist

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  6. Sarah,

    Lots of great info about caffeine. I noticed you referenced a Wikipedia page about coffee and mentioned coffee roasting. Here is a fantastic link to another Wikipedia page that describes different types of roasts and what categorizes each of the different types. I, like you, am surrounded by coffee drinkers and have so far avoided taking the plunge (with the exception of an occasional “froofy” carmel machiato when I need a caffeine kick for a long drive). Anyway, here is the “coffee roasting” link. I think you’ll find it interesting:


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