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If you have been reading about coffee extraction lately, you might be familiar with the term astringencywhich is often used to describe poorly extracted coffee. This is a descriptive for which the meaning is generally not well known, and it’s worth pondering why we dislike it so much in the specialty coffee world, contrary to other communities like wine and beer.
First, let me try to describe what astringency feels like. Astringency creates a dryness sensation in your mouth, and generally mutes a lot of other flavors, especially when it’s strong. It can often be found in fruits or vegetables, especially unripe ones − in fact, the astringency that is present in wine comes from the grape’s seeds and skin (Mattivi et al. 2009). If you ever ate an unripe banana and could not resist from grinning as the dryness sensation invaded your whole face, then you might know what I’m talking about. The one thing I recall being the most astringent I have experienced was the spongy white pulp separating seeds inside of a pomegranate. Another good example of astringency is over-extracted green tea, but it also contains other strong flavors so it’s not the most precise example.
Astringency is not seen as a necessarily bad thing by wine experts, and I suspect that this is due at least in part to red wine being much more concentrated than filter coffee; typically wine has total dissolved solids concentrations between 1.7% and 3.0% compared to 1.3% to 1.5% for filter coffee (e.g. see Schopfer & Lipka, 1973 and this French post), so astringency won’t easily mask everything else, and can instead be in balance with the global taste profile. In tea and coffee, we try to get rid of astringency as much as we can because it can easily become the dominant sensation, and can even come close to being the only perceptible one.
The astringency sensation is caused by soluble organic compounds that belong to a class called polyphenols, which includes tannins but also other complex molecules. They are often produced by plants as a defense mechanism against insects and other predators. These complex and large molecules can attach to proteins like Lego blocks, and form large clusters that can precipitate out of solution. This is what happens when we feel astringency; the polyphenols bind to proteins in our saliva and precipitate, forming clumps that inhibit our taste buds’ ability to taste the coffee properly, and cause this rugged and dry sensation.
Fortunately for us, polyphenols are mostly very large molecules, which makes them heavier and harder to extract from coffee particles, compared to smaller molecules like caffeine and the various acids we enjoy. This makes it possible to extract the good molecules, while avoiding the polyphenols that cause this astringent sensation. If you prepare coffee by immersion, you likely won’t often encounter astringency, because the immersion method extracts coffee solubles more gently, especially if you grind coarse and don’t agitate too much. Furthermore, the extraction happens in a relatively uniform way, in other words you won’t have large amounts of fresh water extracting a small amount of coffee particles. Remember that fresh water is a much more potent solvent compared to concentrated water. However, immersion brews also never achieve a very good filtration of coffee fines and other insoluble compounds.
If you happen to prefer the taste of coffee without these insoluble particles as I do, you might prefer coffee prepared by the percolation method, where fresh water is poured on a bed of coffee, filtering out any undissolved solids (see this previous post for a more detailed discussion of the differences between percolation and immersion). Percolation brews are however much more prone to causing astringency, because channels can form where a larger fraction of water passes through preferential paths, which can over-extract some small regions of the coffee bed (we sometimes call this local over-extraction). This allows heavier polyphenols to be extracted, and makes the resulting brew astringent.
One thing I do not know is whether polyphenols can be filtered out by the coffee bed itself. When preparing percolation brews, the coffee bed filters out a lot of undissolved solids, and prevents them from getting in your brew. This is why a coffee bed full of fines will only clog the paper filter if you agitate it a lot; if you don’t agitate it, then the coffee bed acts as a filter and retains these fines, preventing your paper filter from clogging. This is also why a V60 has much less undissolved solids than a typical Aeropress brew; the depth of the coffee bed in the latter is typically much smaller, so it lets more fines in your cup.
Based on experience, I know that a typical V60 coffee bed can filter out a lot more compounds than just a paper filter, even in the extreme case of Whatman Grade 5 paper filters that have average pore sizes of 2.5 micron; I’ll talk more about this in a future post. However, I’m not sure if coffee beds are such good filters that they would remove polyphenols that were extracted and dissolved in the slurry. I think it is unlikely, because although polyphenols can be much larger than other coffee compounds, they are still much smaller than a micron. For example, some wine tannins have sizes in the range 50−70 Å (McRae et al., 2014). If you are not familiar with these units, 1 micron is equivalent to 10,000 Angstrom (Å), so a coffee bed would need to be a much better filter filter than a Whatman Grade 5 paper filter to remove polyphenols.
Ifthe coffee bed is able to filter out polyphenols however, the presence of a large channel could provide an additional reason why they let polyphenols in our beverage, because they would create localized regions of bad filtration, where polyphenols and undissolved solids pass through. That is an interesting question to me, because it would mean that over-extraction could potentially be fixed by a good non-channeling coffee bed, even if polyphenols are getting extracted in the slurry for other reasons.
This whole idea of channels causing localized poor filtration made me want to directly measure the amount of fines and other undissolved solids in my coffee brews in an objective way. This can be done with the help of turbidity meters, but until about a month ago, I though those were extremely expensive equipment only built for labs. When I saw Ray Murakawa using what seemed like a portable turbidity meter on Instagram, I got very excited and he told me they are actually portable and affordable ! Thanks to the help of my Patreon supporters, I promptly ordered one and started measuring some of my brews. Turbidity meters work through a different principle than refractometers, but they give us information that is a bit similar. Refractometers inform us about the concentration of dissolved solids, and turbidity meters inform us about the concentration of undissolved solids.
When taking turbidity measurement, I realized something really interesting; the cloudiness goes up pretty fast as a brew cools down and stales. This was not too surprising at first because cooler water is less good at dissolving things, so we can expect the total amount of undissolved solids to increase as the brew cools. What I found really surprising is that even if you quickly cool down filter coffee to room temperature, its cloudiness keeps going up quite quickly for about half an hour, and then very slowly for more than 12 hours (I haven’t tried measuring older coffee). I’m not sure about this, but I suspect this increasing cloudiness at room temperature might be caused by polyphenols binding to some proteins that were also extracted from the coffee. Undoubtedly there are many other things that affect the absolute cloudiness of a coffee beverage, especially in an immersion, but it’s possible that the rate of increase at room temperature correlates with astringency. Furthermore, given that most of this turbidity increase happens within half an hour or so, I now wonder whether it’s related at all with coffee tasting bad after a brew stales.
Reading about polyphenols also made me realize there may be a reason beyond channeling why filter brew methods with very finely ground coffee (i.e., finer than espresso) often come out tasting astringent. For example, the high-extraction siphon method I posted a while ago only worked well with some specific roasts (some of which I listed in the post), and others came out very astringent regardless of whether channeling seemed to occur or not. Other examples include a few finely ground Aeropress and Buchner siphon brews I made that came out very astringent. I think some coffees may simply naturally contain a much lower amount of polyphenols, whether it is because of their varietal, terroir, processing or roasting. Grinding so fine ends up breaking most coffee cells, and therefore the chemical compounds are washed out by water (a process sometimes called erosion) rather than having to diffuse through the small pores in the cellulose walls of coffee cells. In that situation, polyphenols may very easily end up in the slurry, and if the coffee bed can’t filter them out, they may end up in the final brew even in the absence of channeling.
You may think that the same should happen with espresso and Turkish coffee, and you’re probably right. Assuming I’m not entirely mistaken about polyphenols extracting easily from broken coffee cells even in the absence of channels, I think either the high concentration or other things present in these types of brews (oils, suspended solids, etc.), may balance out the presence of polyphenols and make them less overwhelmingly astringent. That might also explain why it’s hard to take an evenly, highly extracted shot of espresso and dilute it into an amazing filter brew.
I think that one promising avenue may be to precipitate the polyphenols post-brewing by adding proteins in the finished coffee beverage, much like is routinely done in wine or beer making. Some things that are used for precipitation include egg whites, gum arabic, silica gels and a product called Polyclar (e.g., see this article about beer filtration) − these things are all rich in proteins. One potential major problem is that those compounds are typically left in the beer or wine for several hours to allow the precipitation to happen; if we wait this long with brewed coffee, it will probably taste very bad even if we remove all polyphenols from it.
I know this post probably opens up more questions than it answers, but I’m hoping it will help us think more clearly about what makes a brew taste astringent and how to avoid it !
I’d like to thank Scott Rao and Sylvain Mussigmann for useful comments.