I’ve frequently written about astringency on Instagram and Jonathan Gagné, author of the forthcoming book The Physics of Filter Coffee, wrote an excellent blog post about the science of astringency. Here I’d like to offer a practical guide to finding and fixing astringency in brewed coffee.

What causes astringency in coffee?

Large molecules called polyphenols, in particular chlorogenic acids (CGAs) and tannins, are the likely, primary sources of astringency in brewed coffee. Being larger molecules, polyphenols tend to extract less readily than most other coffee solubles, though CGAs seem to extract more readily than tannins do. (It has not been scientifically proven that CGAs extract more readily than tannins do, but it seems probable.) It’s likely the majority of astringent polyphenols found in brewed coffee extract via channeling in percolation brewing. Astringency is much less likely to occur in immersion brewing due to the lack of channeling.

What increases the risk of an astringent brew?

There are several potential sources of increased astringency in brewed coffee:

• beans (seeds) from underripe cherry*

• underdevelopment in roasting*

• channeling during percolation brewing

*Both underripe cherry and underdeveloped roasts tend to yield higher levels of CGAs in the cup. CGAs are both astringent polyphenols and also the most prevalent acids in coffee. Having some CGA is probably important for a delicious cup (though I’m not sure any of us have ever tasted a coffee without any CGAs), but when the CGA level is too high, the coffee becomes noticeably astringent.

How to find and fix the source of astringency

When I find a brew astringent, I go through a series of steps to find and fix the source of the astringency:

• If I have previously made a non-astringent, percolation brew of the same roast batch of the coffee in question, it is almost certain that channeling caused the astringency in the more recent brew. In that case, I would brew again with either (hopefully) better technique of a coarser grind setting.

• If I cannot rule out channeling (for example, if I don’t trust my brewing method or skill), I will taste the coffee as a cupping. If the cupping is astringent, then channeling was not the cause, or at least not the only cause, of the astringency in the percolation brew.

• If both percolation and immersion (cupping) produced astringency, the cause must be due to an underdeveloped roast or underripe cherry.

• If some roast batches of the coffee in question are astringent and others aren’t, it is likely that roast development was the cause of the astringency.

• If all roast batches of the coffee are astringent when cupped, and you are confident they are not all underdeveloped, then the green was likely from underripe cherry.