Grinding coffee beans is an important step in preparing a cup of coffee, because it releases flavor and aroma compounds that are normally trapped inside the bean. There are different sizes of grind for different types of coffee. Coarser grinds are used for Americanos, which use a drip system; finer grinds are required to prepare espresso, which is made by forcing hot water through finely ground coffee powder with a pump.
Some types of grinders also produce a fine powder that is used to make Turkish coffee and other unique brews.
There are many different ways to grind coffee, but most use one or more rotating cylinders with rough edges called burrs mounted in a frame. They can be made from metal, ceramic, plastic or diamond-tipped glass. The burrs are driven by a revolving wheel, and periodically the grinder will be stopped to clean the ground coffee out of the burrs.
A hand-turned crank machine has a hopper which holds the beans, and two wooden or plastic rollers between which the beans are crushed (Hario recommends using their mills only with arabica beans, as the oil tends to make its way into other bean varieties and causes gumming). The crank machine is turned by hand, and usually only one or two cups of coffee are grind at a time.
Electric burr grinders come in many shapes and sizes. Some may offer additional features such as adjusting the fineness of the grind, time delay before grinding begins and grind-and-brew. Most of these machines work with a two-step process in which the beans are ground into small pieces (typically called “fines”) by rotating cutting blades called burrs, then the grounds feed through a hole into a chamber where they are held while waiting to be used. The grinder’s motor spins the burrs, and a spring pushes the grounds into a narrow tube leading to the chamber. The user presses a start button to activate the grinder and release the ground coffee into the chamber; he or she then checks that all of the grounds have made it through. Some machines allow users to stop grinding before all of the coffee has been released from the grinder.
Some very expensive high-end electric burr grinders have built-in scales to measure out the precise amount of beans for each batch.
The term “blade grinder” is often used as a synonym for burr mill, which is misleading: coffee grinders with blades actually cut the beans into fragments rather than chop them into smaller pieces. A blade grinder can be any device equipped with one or more electronic scales to measure out the exact amount of coffee grounds based on how much coffee is desired. These machines may also allow the user to adjust the fineness of their grind and automatically start a brew cycle or even program it for that morning. Blade grinders are typically used to grind coffee for commercial use, but can also be used at home.
A blade grinder is sometimes called a whirly blade or propeller grinder because the blades spin around quickly in a circle-like motion while they chop up the beans. The more advanced models employ thick metallic disks, which chop the beans up into smaller pieces than their thin counterparts.
Blade grinders use two or three blades to crush coffee beans. A blade grinder produces a coarse grind comparable in size to breadcrumbs whereas a burr grinder, using two rough-edged disks called Burr, produces a fine powder ground bean similar to powdered sugar. Burr grinders are more expensive than blade grinders, but the coffee produced is preferred by some people.
A few simple modifications can be made to a blade grinder (usually a household blender) to produce an effective and safe hand-powered burr grinder for grinding coffee without electricity. This is good because damage from power surges can destroy expensive electronic coffee grinders. A simple blade grinder using a blender can be built for less than $10 USD, as the motor of a household blender is easily substituted for the original motor and blades.
The open source electronics company Arduino released an open source Coffee Grinder project which offers an alternative way to implement an automated blade grinder, using a stepper motor that drives the spinning blades.
The French press, also known as “plunger pot”, is by far the most traditional method for coffee preparation. However, because of its low-tech nature, the “press pot” is not usually considered a coffee grinder, though it does the same job. The French press consists of a beaker and lid made from heat proof glass or plastic, where coarsely ground coffee is placed, and then hot water (usually about 50% more than required to fill the beaker). Because of the coarse grind required, this method also extracts more caffeine and oils vs. many other brewing methods.