Of the Five Important Factors in Brewing Great Coffee at Home, grind size is crucial. It allows for much greater or lesser extraction, and can mean the difference between great cup of coffee and a terrible one.
Select the right grind.
It can make all the difference…
A quick refresher on extraction
Extraction is the process by which water pulls soluble materials out of coffee particles. When too much material is pulled from the coffee, it’s referred to as “over-extraction.” Similarly, pulling too little material from coffee is referred to as “under-extraction.”
Remember that finer grinding exposes surface area, which makes it faster and easier for water to extract soluble materials from coffee. It also slows the flow of water through the brew bed, resulting in longer contact time than you’d otherwise experience. This is why espresso requires a fine grind – it restricts flow of water to enable pressurized extraction, and exposes a ton of surface area for quick extractions.
This is why French press is so coarse – coffee is steeping in water (no need to worry about flow) for a long time, so less surface area needs to be exposed.
SIGNS YOUR COFFEE IS UNDER-EXTRACTED
Sour, even salty flavors dominate. This isn’t just the acidity of your coffee; it’s an unpleasant, almost metallic flavor. The coffee may also taste quite weak. This is due to that quick contact time and low surface area. Water just sprinted past ground coffee, only picking up a little soluble material on the way.
If those flavors are very obvious – even over-powering – you’ll want to move your grind finer. We recommend incremental adjustments, especially if you’re still new to dialing in. Basically, don’t over-compensate. Ten grind steps is probably too many if your coffee is just a little sour.
If the brew tastes okay but leans a little sour or weak, then consider either a small grind change (if your machine has micro-adjustability) or a small decrease of your dose. If you use less coffee, what’s still there will extract a little more. We’re taking a gram or so here, since you don’t want to speed the brew up too much by removing a lot of coffee.
SIGNS YOUR COFFEE IS OVER-EXTRACTED
Bitter, dry, ashy, astringent, or strong/intense flavors suggest over-extraction. These flavors occur when water draws too much material out of coffee – either because too much surface area was exposed, or because water was in contact with coffee for too long (or both, in which case I’m so sorry). It’s typically very gross, and a good sign that you need to increase your grind size.
Similar to down-dosing an under-extracted brew, you can up-dose when over-extracting. In both cases, this should only be for very slight changes, as your dose has a bigger effect on concentration than on extraction.
SIGNS YOUR COFFEE IS JUST RIGHT
There’s no “correct” extraction. Rather, we want a balance of over and under extraction flavors. Put these together and you’ll get something more than the sum of its parts: great-tasting coffee. Clear flavor notes, pleasant aftertaste, a nice mouthfeel – these are all signs that your extraction is right on target. The biggest sign of success, however, is if you think it tastes good!
Thanks to the great people at Baratza grinders for the info.