Grinders and Extraction
Extraction is the process of water removing the soluble material from the coffee grounds to give you your finished beverage. If you don’t extract enough soluble material, your coffee will be weak, watery, sour, vegetal, and/or underwhelming. This kind of brew is commonly referred to as “underextracted.” If you extract “too much” (this is in parenthesis for a reason - keep reading) soluble material, your coffee will be harsh, bitter, and/or astringent (mouth-drying). This kind of brew is commonly referred to as “overextracted.” If you extract the right amount of soluble material, your brew will be balanced, sweet, smooth, and full of origin character (perhaps fruity, perhaps floral, perhaps savory, etc).
The main tool at your disposal to adjust extraction is grind size. How much water you use in comparison to how many coffee grounds you use (your brew ratio), your brew water temperature, the amount of agitation you introduce during the extraction, and the brew time are also variables that will adjust extraction though. If you grind finer (up until a certain point), or use more water per unit coffee, hotter water, more agitation, or a longer brew time, you will get more extraction. If you grind coarser, or use less water per unit coffee, cooler water, less agitation, or a shorter brew time, you will get less extraction.
Extraction Yield versus Strength
We’ve actually been a little careless. Coffee that is weak and watery is technically coffee that has a low level of total dissolved solids (TDS). The more precise way to describe coffee like this is to say that is has low strength. High strength = high TDS. Low strength = low TDS. The complication is that strength does not tell you whether a coffee tastes sour or bitter. You can have coffee that is very low strength and very bitter, or very low strength and very sour, or very low strength and sweet. What determines the sour/bitter/sweet flavor is extraction yield. Extraction yield tells us what percentage of the coffee ground we have extracted. Only about 30% of it can possibly be extracted under normal circumstances. Typically extraction yields will be around 20% when a coffee tastes sweet and balanced. Below that, they will generally taste quite sour, and above that, you may taste some harshness or dryness.
To really drive home this concept, consider a huge batch brew brewed with several gallons of water but only a small handful of coffee beans that were ground pretty fine. Since there is so much water, and a lot of surface area of grounds due to the fine grind, it is quite easy for the water to remove essentially all of the soluble material. The extraction yield might be in the high 20s as a percentage. This coffee will likely taste bitter/harsh/astringent. However, because there is so much water, and not many grounds, the finished brew is very weak. This is a weak and overextracted brew. It might have a TDS of less than one percent. Typically filter coffee has a TDS of about 1.35% and espresso has a TDS of about 10%. That means that 1.35% (or 10%, if espresso) of the finished beverage, by mass, is material that was in the coffee grounds, and the rest is water.
To fix the bitter/harsh/astringent flavor, we could grind coarser. Extraction yield might drop to 20% but TDS will go even lower, down to let’s say 0.5%. This coffee is now balanced and sweet but incredibly weak. It tastes good, but we want it to be stronger. We can decrease the water amount and/or increase the amount of coffee grounds to accomplish that. This does technically change the extraction yield a little bit (downwards) but the primary effect is to just make the brew stronger. We might end up at 19.5% extraction and 1.4% TDS. Now we have a coffee that tastes balanced and sweet and is not too strong and not too weak. Success! If we decreased water amount and/or increased coffee dose even more we might end up at 16% extraction and 1.9% TDS. This will taste too strong and also be sour/vegetal. This is a strong and underextracted brew.
Percolation versus Immersion Brewing and the concept of Overextraction
Percolation
Percolation is any kind of coffee brewing where water is flowing through the grounds. This includes automatic drip machines, pourovers, commercial batch brews, and espresso.
As is the case with just about everything, the deeper you dig into something, the more complicated it becomes. There are two different types of coffee brewing: percolation and immersion. Percolation is any kind of coffee brewing where water is flowing through the grounds. This includes automatic drip machines, pourovers, commercial batch brews, and espresso. Immersion is any kind of brewing where the brew water just sits with the grounds and does not flow through them. The two best examples of immersion are french press and cupping. Brewers like the AeroPress and Clever Dripper/Hario Switch are a hybrid as they start with an immersion phase and then finish with a percolation phase. Distinguishing between these types of brewers is important because “overextraction” is a different concept in immersion than it is in percolation brewing.
Immersion
Immersion is any kind of brewing where the brew water just sits with the grounds and does not flow through them. The two best examples of immersion are french press and cupping.
In percolation brewing, when the brew water is reasonably slowly and evenly flowing through the grounds, it does not have a whole lot of energy. This kind of brew water will not extract tannins and tannin-like compounds that give overextracted brews their harsh/bitter/astringent flavors as these compounds are bonded too strongly to other compounds in the grounds. This is true even if we grind finer and finer, as long as the water keeps flowing evenly (this is essentially the holy grail of coffee brewing: fine grind but no channeling = super sweet coffee with very obvious unique origin character).
Hybrid
Brewers like the AeroPress and Clever Dripper/Hario Switch are a hybrid as they start with an immersion phase and then finish with a percolation phase.
However, what does happen as we grind finer and finer is that it becomes more and more difficult for the water to flow evenly through all the grounds. Channeling starts to take place. This is just fast, turbulent water flow through a pathway in the grounds that has slightly less resistance. The turbulent flow has a lot more energy, which means that water is able to extract the tannins and tannin-like compounds that give us those negative overextracted flavors.
In immersion brewing, there is no water flow, apart from the very beginning when the water is being introduced, so there is no channeling, and therefore no overextraction due to channeling. The only reason to not grind as fine as possible for immersion brewing is that the very fine grounds tend to clump together and effectively reduce your coffee dose (since the water doesn’t get inside the clumps) so you end up with a very weak brew. The only way to get negative, overextracted kinds of flavors from an immersion brew is to use a coffee that is roasted darker than is your preference. If you do that, then it will taste too bitter if you have a nice even, high extraction (achieved with a relatively fine grind). With a well-developed light roast, you simply cannot achieve overextraction in an immersion brew.
It is of course also possible to achieve overextraction in a percolation brew without any channeling if you are using a roast than is darker than is your preference. It will simply taste too bitter. But it likely won’t be astringent. The astringency, or mouth-drying sensation, is really the easiest way to tell if your brew is channeling or not.
One final note on channeling: if your grind is too coarse in a percolation brew, you can run into a situation where it’s almost as if the entire slurry is a channel as all the water will be flowing very quickly through the grounds. Counterintuitively, you need to grind finer to fix the channeling in this case, rather than the usual solution, which is grinding coarser. Generally you’ll know you are having this “whole slurry channeling from grinding too coarse” issue because the brew will taste underextracted (sour/vegetal) and also be weak AND also have a lot of noticeable astringency. If you are having channeling from grinding too fine, the brew should mostly taste good but just leave your mouth dry in the aftertaste.
Grinders
Hopefully by now you are starting to realize why grinders are so important. Grind size is a crucial factor in getting your coffee to taste the way you want it to. Unfortunately, coffee grinders don’t really do exactly what we want them to do. We want them to turn the whole beans into grounds that are all basically the same size. If they are all the same size, water will extract flavor at the same rate from all of them, giving us more control over our extraction. Unfortunately, coffee beans tend to kind of explode like glass when they hit the burrs. We end up with some pieces that are the size that we want them to be, some a little bigger, some a little smaller, and a lot that are very tiny (these are called fines). This means that we extract more flavor from some grounds than we do from others, because the surface area of the grounds is one of the primary things that dictates extraction rate. If the grounds are very uneven, we end up with a wide spectrum of extractions, so the finished brew will be kind of sour/vegetal and kind of bitter/astringent, which can end up tasting somewhat balanced but lacks any real sweetness and origin character - it tends to make all coffees taste kind of the same.
The even bigger problem with grinders producing this spectrum of sizes of coffee grounds is that the fines tend to fill in the spaces between the “normal”-sized coffee grounds, which massively increases the likelihood of channeling in a percolation brew as they are essentially clogging up pathways where water could flow evenly and forcing it to flow fast and turbulently through other pathways. If you have a lot of fines, then you have to grind coarser in order to allow the coffee to flow more evenly through the grounds. The coarser grind limits your extraction - with a relatively bad grinder, you may only be able to extract 18% before the astringency becomes too off-putting. At 18% extraction, the brew will probably be a bit sour (depending on roast level - the darker it is, the lower the extraction yield that is required for it to taste balanced) and lacking in sweetness and origin character. This is why you want a grinder that produces relatively few fines and a relatively narrow distribution of grind sizes for percolation brewing.
Sadly, it is expensive to make a grinder that produces relatively few fines. It requires precision manufacturing with very tight tolerances, using enough material (not to mention the right material) so that nothing deforms, and careful assembly. What do we recommend? If you are primarily doing immersion brewing, the Baratza Encore is a great choice. If you like doing pourovers or other percolation-style brews, we can’t recommend the Baratza Vario-W+ highly enough.
Got questions? Email us at coffeeisafruit@vibrantcoffeeroasters.com
