decaffeinated coffee benas

How coffee beans are processed

Long before that steaming cup of (Turkish?) coffee stands in front of you, or even before you buy that bag of coffee beans, your coffee beans have been through quite a bit. From plant to farmer to processor and finally that cup or bag of coffee in front of you. Interestingly, apart from its colour, the coffee bean doesn’t change that much along the way. But within it, a lot happens to develop that special coffee flavour!

It starts with coffee beans

And even before those beans, it really starts out with coffee fruits, berries more specifically. These coffee berries grow on plants that only thrive around the equator (approx. 1000 miles north and south of it). Once the berries are ripe, they are harvested from the plant. Each of those berries contains just two coffee beans.

Those fresh berries don’t keep long, nor would the beans, so they need to be preserved. There are several ways that this can be done. The two basic processes are either dry or wet (washed) processing.

Wet/washed processing of coffee beans

In this method, the processing of the beans starts by removing a large amount of the fruit (coffee cherries).The beans with the remainder of the fruit is then placed in water to soak. While soaking the beans, the remainder of the fruits soften, thanks to enzymes present naturally. During this process the beans ferment to some extent, developing all sorts of flavours.

Soaking in the water will not take longer than a few days after which the remainder of he fruit is removed. The coffee beans then need to dry to ensure that they can be kept and transported. If the beans are too wet (high water activity), they might spoil because of growth of moulds and yeasts. The moisture content of dried beans should again be <11%.

Dry processing of beans

In this process the beans + cherries are left to dry together (often in the sun). It can take weeks for the beans to dry enough. However, for areas where water is scarce, this may be a far more sustainable method of processing.

The fruits contain a considerable amount of sugar. While drying the fruits and the beans, these sugars allow (good) micro organisms to thrive and ferment the beans. This will change the flavours of the beans. Since this process is so much longer than the washed process, the flavour changes are more significant.

Again, the aim is to dry the beans enough so they won’t spoil.

coffee beans in sacks
Coffee beans are transported in jute sacks

Dehulling

Whichever method is used, whether it’s washed or dry, or a combination of the two, all fruit remainders need to be gone at the end before they’re sorted by size and packed for export.

Once the beans are dry and packaged in sacks, they can be kept safely for months. This is great, because once the beans are roasted, their shelf life will be limited (hence packaging is so important!).

Decaffeination

At this point coffee beans are green, not the brown colour that you might expect. Instead, they only get this brown colour after they’ve been roasted.

This is also the point that the beans may be decaffeinated. Coffee beans naturally contain a considerable amount of caffeine. Since not all consumers appreciate this caffeine, a decent amount of coffee is sold decaffeinated. Caffeine is just one of the many chemicals inside the coffee bean though. Therefore taking it out requires a well controlled process.

Caffeine is extracted from coffee beans, chemically it is pretty similar to vanilla extraction! Caffeine is soluble in water, so you can extract caffeine from coffee beans by soaking it in water. However, a lot of other molecules in the beans will also dissolve in the water, which you do not want. This is why processes are a little more advanced, either using water that is already full of sugars and flavours so they won’t soak from the coffee bean (the Swiss meth)d or by using solvents or supercritical carbon dioxide (CO2).

All decaffeinated coffee beans do change the appearance of those green beans. It makes them a little brownish in colour instead.

Roasting coffee beans

Before you can actually use those green coffee beans for a delicious cup of coffee, they need to be turned brown. In other words: roasted. Roasting coffee beans is nothing more than heating them up for several minutes under well controlled conditions. During this process, the colour, but especially the flavour of coffee drastically changes. The Maillard reaction and many others take place creating a wide variety of molecules with each their own flavour.

Controlling all these reactions and having them occur to the right level, is the job of a coffee roaster. By controlling roasting temperature & time, cooling time, humidity, mass of your beans, etc. Coffee beans are a natural product and as such, every batch will be slightly different.

coffee roasting set up
An example of a coffee roaster, suitable for roasting about 50kg of beans at a time.

Large scale manufacturers have developed well controlled processes and controls to ensure their coffee gets roasted consistently. Nevertheless, they will have to consider these same factors.

A simple roasting process consists of three main steps. First of, the green unroasted coffee beans needs to be dosed into the roaster. The right amount needs to be added.

The roaster

Next, the beans are roasted in the roaster. A roaster can be a rotating heated drum. By continuously moving the beans, you ensure that they are all roasted evenly. By keeping track of the smell and colour of the beans, a roaster can check whether his roast is still on track.

The roaster can control temperature during the process. They might heat the beans very quickly, or take a longer time to heat them for instance. This will all result in a different flavour profile. Roasting coffee beans for too long, at too high temperatures will burn the coffee beans. They will turn bitter and most consumers won’t appreciate the coffee made from these. Roasting too quickly at too high temperature may result in a sour coffee bean.

Coffee beans that have only been roasted lightly have less roast flavour of course. However, that does result in them still having their characteristic natural flavours. If you have a very special bean from a special farm, it would be a waste to roast it very darkly. If you did, you would mostly taste roast flavours and not the specific from that special bean!

Since decaffeinated coffee beans have already turned slightly brown during the decaffeination process, roasting them is more challenging than regular ones. They are more prone to browning too much. for a roaster it’s trickier to keep them in control.

outlet of coffee roaster
The outlet of a coffee roaster

Cooling & packing

Once the beans are finished, they need to cool down. It is important that this is done consistently as well. If you just leave all the beans stacked together, they won’t cool well and will continue roasting and developing flavour. In the worst case, they might burn. By spreading them out and moving them the coffee beans can again be cooled evenly.

Roasted beans have a lot of flavour. But as soon as they are done and cooled down, they start losing this flavour already. That’s why roasted coffee has a shorter shelf life than fresh green coffee. It’s also why it is packed so carefully, in vacuum or other specially designed packaging.

Coffee chemistry

The coffee is now ready to be ground down and transformed into your warm (morning) drink, whether it’s Turkish coffee or an espresso, a creme brulee or ice cream. When you drink or eat your coffee treat, just have a quick thought of the complex chemistry you’re drinking. It’s not something a flavour factory would have been able to create. All those thousands of molecules in your coffee create the amazing flavour profile.

To mention just a few of those molecules: methanethiol, dimethyl sulfides, 2-methyl butanal, 2,3-pentanedione and many many more. Which are present exactly and at which quantity, depends on the roasting & cooling process, the age of the beans, but definitely also on the type of bean that was used.

Sources

Crema, Coffee processing explained, link

Folmer, B., The craft and science of coffee, 2016, Academic Press, link ; great book if you want to learn more about coffee!

Kington L., How to make coffee: the science behind the bean, 2015, Abrams, link

Metropolis Coffee, Variables involved in roasting, Feb-7, 2019, link ; We also got to visit their manufacturing site in Chicago, however, we did not get paid for writing about coffee

Mole, B. The science behind a good cup of coffee, Jan-31, 2016, link ; With some tips for roasting your beans in home style equipment

Science meets food, The flavor of coffee, link

Scientific American, How is caffeine produced to produce decaffeinated coffee?, 1999, link

Wrigley, B., Coffee: washed vs. natural process, Aug-1, 2017, link

Zanato, E., The decaffeination of coffee beans, Coffee extraction, Sep-3, 2018, link

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