What causes bitterness in coffee? Is it the roast? Or the brewing? Or maybe even the green coffee itself? The answer is actually all three. Let's break down the causes at each stage and how we can fix it.
Bitterness from brewing
First cause: grind size. The first and probably primary cause of bitterness comes from the over-extraction of coffee. Specifically, from the "fines" in ground coffee. The finer you grind a coffee, the faster water is able to dissolve the solubles from the coffee. Extract too much, and you get increasing bitterness and astringency in your coffee.
Solution? Grind coarser.
But what if the coffee still tastes bitter with a coarse grind? Well then we have to look at the second factor: time. Even when we grind at coarser settings, there will still be the presence of "fines" that occur as a side effect of how the coffee is ground. The impact of the grinder burrs hitting the coffee will chip off small parts of the coffee. These chips are smaller than our desired grind size. Of course the coarser we go, the more we minimize this. But what if the coffee still tastes bitter?
Even though we extract at a slower rate with a coarser grind size, if there is still a prolonged period of contact time, the water will continue to dissolve the solubles in the coffee and eventually over-extract, causing bitterness.
Solution? Reduce the brew time.
A caveat with going coarser; even though you slow down extraction of the bitterness, you are also able to extract less of the delicious sweetness from the coffee. This means less of the sugars that help to balance the coffee and make it taste good.
So while going coarser can help, it also comes at a cost (the sweetness and flavour of the coffee).
Solution? Updose to increase the good stuff extracted into the cup.
Now what if all these dont work? What if after all these solutions, the coffee still tastes bitter, or worse, ashy, or burnt? That brings us to the next stage, the roast.
Bitterness from roasting
First cause: burnt coffee. As you roast coffee, sugars present in the green coffee continue to caramelise. This caramelised sugar is actually not sweet. Thus the darker we roast a coffee (more caramelisation), the less sweet a coffee actually becomes. In addition, the darker we roast, the higher the chance of over-caramelisation or burning the sugars. This will dramatically increase the bitterness in the coffee.
But in reality, darker roasts actually tend to taste sweeter than a light roast because it usually has a lower acidity. This makes it easier to balance and highlight the sweetness of the coffee. It is also easier to extract compared to lighter roasts because darker roasted coffee tends to be more porous and allow easier access to water. However, this easier extraction can also mean a higher chance for bitterness from over-extraction.
Solution? Roast light enough to retain sugars in the coffee but dark enough for balanced acidity and easy extraction.
Finding this balance can be extremely tricky. If you want to learn more about roasting and how to modulate coffee flavours, check out our Ultimate Roasting Course.
Second Cause: grindability vs solubility. During roasting, coffee expands in volume and the cell walls weaken and break down. This means the brittleness of the coffee changes based on how the coffee was roasted. A very brittle coffee could have more "fines" chipped off during the grinding process, even at a coarse grind setting. This increased amount of "fines" then experiences over-extraction and imparts bitterness even with the coarse grind.
Generally, a less developed coffee (lighter roast) is less brittle compared to a more developed coffee (darker roast). This means lesser fines created, but the coffee would also be less porous as the bean has not expanded in volume as much. As a result, you require a finer grind to extract the coffee sufficiently and balance the coffee. But what happens when you grind finer? That's right, more fines and more bitterness and astringency.
In contrast, a more developed coffee is more brittle, and hence results in more fines even with a coarser grind. A darker roast would also mean increased risk of some part of the coffee being burnt or over-caramelised. Combine this with increased porosity from the expansion of the bean and you get a bitter brew even at a coarser grind size.
Solution? Balance grindability with the taste aspect (acidity/sweetness) of the coffee.
A coffee with lower sugar levels like washed coffees will generally need a finer grind to help water access more of the coffee and dissolve sugars to balance the acidity. This means lesser development or a lighter roast to reduce the brittleness of the coffee. Likewise a sweeter coffee such as natural processed coffees, can afford to be roasted darker or go through more development. The sweetness is easier to extract through a coarse grind.
This is quite counter-intuitive because most roasters would argue that going lighter would also simultaneously increase the acidity of the coffee and make it harder to balance. But there are actually ways to roast lighter and soften the acidity of a coffee at the same time. We teach all of these in our Ultimate Roasting Course.
Bitterness in green coffee
Finally, if everything you've tried in the roast and brew doesn't work, might the cause be the green coffee? What if low quality coffee doesn't carry enough sweetness to help balance and create an enjoyable drink?
I would say this is rarely the case with specialty coffee or even high grade commodity coffee. It may be possible with robusta or really low grade, defective coffee where bitterness tends to be higher. Lucky for you and me, we both don't drink low quality coffee right? That's why we always look for specialty coffee from traceable and sustainable sources. If not, you can always support other roasters from Singapore which do so by heading to beans.sg to buy their delicious coffees!
Hi, I'm home roaster so my ability to control roast conditions is fairly limited, however, I'm always striving to control bitterness so this article gave me some ideas which look really promising – many thanks. Cheers, – Mike