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How Coffee Bean Processing Impacts Flavor
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 on quite a journey. From plant to farmer, to processor, and finally that cup or bag of coffee in front of you. Each of those steps is important to get to that final delicious cup of coffee, because that initial coffee fruit (yes, it’s a fruit) isn’t even close to being a cup of coffee yet!
So, let’s follow this been from start to finish.
Preparing the coffee beans
Long before your cup of coffee sits in front of you, it all starts with a coffee plant. You will find these plants in countries within 1500km (1000 miles) of the earth’s equator. This is where the coffee plant thrives.
These coffee plants grow coffee fruit, more specifically, coffee berries. It’s these berries that form the start of your coffee. Each of those berries contains just two coffee beans.
Once the berries are ripe, they are harvested. They don’t keep long, so they need to be preserved quickly. 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 (the coffee berries). All that remains are the beans, with some leftover fruit material. The beans are 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 a little as well, which results in the formation of all sorts of flavors.
Soaking in the water only takes a few days after which the remainder of the fruit is removed. The coffee beans are then dried to ensure they can be kept and transported. If the beans are too wet (high water activity), they might spoil because of the growth of molds and yeasts. The moisture content of dried beans should again be <11%.
Dry processing of beans
Alternatively, beans + berries 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) microorganisms to thrive and ferment the beans. This changes the flavor of the beans. Since this process is so much longer than the washed process, the flavor changes are more significant.
Again, the aim is to dry the beans enough so they won’t spoil. After drying, the remainder of the fruit is removed, so the producer is left with clean, dry beans.
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.
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 flavors 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 color instead. It can also impact the flavor, since caffeine is slightly alkaline, whereas coffee as a whole is acidic.
What happens during roasting?
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.
Controlling chemical reactions
During this process, the color, but especially the flavor of coffee drastically changes. The Maillard reaction, as well as several other chemical reactions, result in the formation of a lot of new molecules. Those hundreds of different flavors and aromas together form the coffee flavor.
To mention just a few of those molecules: methanethiol, dimethyl sulfides, 2-methyl butanal, 2,3-pentanedione, and many many more. It’s hard to say which molecules exactly will be present, and at which quantity. This depends on a lot of different factors.
Some of these can be controlled by the coffee roaster. Roasting temperature & time, cooling time, humidity, the mass of your beans, all influence the roasting process.
But the type and age of the bean have a big impact as well! Keep in mind that coffee beans are a natural product. As such, every batch will be slightly different.
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.
How coffee beans are roasted has a big impact on flavor of your final drink, such as a cold brew coffee.
The coffee roasting process
A simple roasting process consists of two main steps: roasting & cooling.
First, 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 color 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 flavor 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 beans. Roasting too quickly at too high a temperature may result in a sour coffee bean. Coffee naturally contains a lot of acids, so controlling their quantity during roasting is key.
Coffee beans that have only been roasted lightly have less roast flavor of course. However, that does result in them still having their characteristic natural flavors. 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 flavors and not the specific flavors 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.
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 flavor. 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 flavor. But as soon as they are done and cooled down, they start losing this flavor 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 a vacuum bag or other specially designed packaging.
Let’s use those beans
The coffee is now ready to be ground and transformed into your warm (morning) drink, whether it’s Turkish coffee, an espresso, or a cold-brew coffee. Or, use it to flavor your creme brulee, brownie, or ice cream?
And, while you’re savoring all those aromas and flavor. Be in awe of what such a little bean can produce. It’s something that if we humans tried to ‘design’ it from scratch, we’d hopelessly fail…
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