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I have a grinding machine with a lot of different settings. Even between the categories of Fine and Medium there are sub levels of grinding. I'm not sure how they relate to the flavor of the coffee. Is there any research that shows how the grind affects the taste of the different types of roasts?
The main reason for different grinds is the different brewing processes. If the water is exposed to the grounds for a very short amount of time (e.g., 10-15 seconds for espresso), you need a finer grind to provide enough surface area to extract the coffee from the beans. If the brewing time is long (e.g., the ~4 minutes of french press), you need a coarse grind to avoid over-extracting the grounds and getting something too bitter to drink.
That's really the heart of it. If the grind is too coarse for your brewing method, your coffee will end up under-extracted, weak, and acidic. If it is too fine a grind, your coffee ends up over-extracted and bitter. Your grinder is a good brand; if you still have the manual, it includes a guide to grind vs. brewing method. If not, the capresso.com site includes PDF versions.
In your title, you mention grind consistency. This is different from but related to grind size - an inconsistent grind (such as a whirling-blade type "grinder") will have sizes all over the map, from dust to half a bean. The dust will be over-extracted, and the half-a-bean won't contribute much flavor at all.
A consistent grind (one with particles in a small range of sizes, neither excessively large nor excessively small) makes it more feasible to get the contribution of all the coffee grounds to the beverage without over-extracting any of the grounds - thus making it possible to get a more consistently predicable cup of coffee, without off flavors from some part of the grounds and no flavor from another part.
Having fine control over the grind size in a consistent grinder makes it possible to adjust the grind to produce the best cup (to your taste) in the equipment you have for brewing.
It's already been stated that grind size will change depending on your brewing method.
Here is a quick break down of which grind size aligns with what brew type(the most popular ones).
Keep in mind everybody's taste is different, You may have to make small adjustments to find yours.
Coarse - French press , Cold Brew
Medium to Medium/fine - Manual drip, Automatic drip
Fine - Espresso, Turkish
You might find this resource helpful.
Happy Coffee!
Basically the finer you grind your beans the more extraction you will get with more of the soluble coffee being in contact with your filtered water. Finer= stronger. Espresso is very fine as the extraction process takes about 20 seconds with water under a lot of pressure being forced through the bed of firmly tamped coffee grounds. While using a French press/ Cafetiere the grounds are quite course so as to not let too much insoluble coffee through the filter gauze & the coffee is steeped for minutes( time is up to you) & not seconds. Longer you let it steep the stronger & bitter your coffee will be much like a cuppa tea!
