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The purpose of grinding coffee is to increase the surface area that contacts the water. This makes extraction of the coffee more efficient. As you can see in the graphic below, each brewing method requires a different level of grind.
Coarse grinds are like the consistency of kosher salt. The finest grinds are powder like.
There is a relationship of degree of grind with time of brew. The finer the grind, the less brew time that is needed. Brewing a fine grind too long will lead to overextracted coffee, which can taste bitter. You can probably guess then that since cold brew requires hours of contact of water with the coffee, that a coarser grind is recommended.
Two types of grinders—blade and burr
Blade grinders are inexpensive are also called spice grinders (they can be used for both purposes but I recommend having one dedicated for coffee and the other for spices). They have a spinning blade (like a simple food processor) that chops the beans into fine bits. The longer you run it, the more finely the coffee is ground.
There are two potential problems with the blade grinder. The most important is that grinds can have uneven particle sizes. This can lead to subpar flavor as some the coarse bits may get underextracted while the fine bits get overextracted. When dealing with the long extraction times in cold brews, this flaw may be more apparent. The second problem with the blade grinder is that when attempting a fine grind, the coffee may heat up, causing some of the flavorful coffee oils to evaporate.
Burr grinders work by crushing the beans between two surfaces. This creates a more even particle size than blade grinders.
There are disc and conical shaped burr grinders. Disc grinders have a flat grinding surface, which could be better for finer grinds. Conical burr grinders have cone shaped burrs. Both have one stationary plate and one rotating plate. The disc grinders tend to rotate a little faster, which can generate some frictional heat. With small batches, this may be insignificant. In both types, size of the grind is controlled by adjusting the distance between the two plates.
Conical burr grinders are considered the best choice as they rotate slower and a less likely to clog than disc grinders. Conical burr grinders are also the most expensive type of grinder. For the home user grinding single use amounts of coffee, a disc grinder may be perfectly acceptable and save you some money. If you are an exclusive cold brew drinker however, consider that to make a concentrate you need a lot of coffee so you will not be grinding in small batches.
To summarize then, burr grinders are the weapon of choice for cold brew aficionados. Their design allows them to produce a more even, coarse grind when compared to blade grinders. For cold brew, we want a coarse grind to compensate for the extended brewing time (up to 24 hours). This will prevent bitter flavors as a result of overextraction.
To see my top picks for grinders, click here.