Grinding coffee beans just before making coffee is the way to ensure the freshest taste. Grinding increases the surface area of the coffee exposed to the water for brewing, but of course the same process increases the surface area exposed to the air during storage if the beans are stored in the ground state. Ideally the beans should have been roasted about three weeks before they are ground, but keeping them whole means that the aroma does not escape into the air too easily, so the coffee keeps its quality longer. Burr grinder is one of the best coffee makers!
How Fine should Coffee Beans be Ground?
The shorter the time the coffee is in contact with the water, the finer the grind needs to be. This is why espresso coffee tends to be ground more finely than the relatively coarse grind that is right for a cafetiere. In an espresso machine the water/steam is forced through the coffee so it is in contact for only a short time, compared to the leisurely pace at which the coffee is exposed to the water in a cafetiere before the plunger is pushed down.
If the water stays in contact with the coffee for too long then it starts to become more bitter as well as stronger, which is not usually what the coffee drinker wants. So much for the theory, however, not all home coffee grinders are made equal.
Coffee Mill, Burr Grinder or Blender-style whirling Blades?
Traditional coffee mills are gravity fed into the grinder, where a moving grooved mandrel breaks up the coffee beans against a stationary grooved structure. These are often hand-cranked, and the size of the coffee grounds particles is determined by the clearance between the moving parts, which is adjustable. The modern equivalent of this is the burr grinder, in which the moving part is driven by a geared down motor. There is usually a control to set the grade of coffee, and these produce a nice evenly ground result.
A cheaper method is to use a similar mechanism to a food blender, where a high-speed whirling blade slices up the coffee beans randomly. Here the fine-ness of the grind is roughly determined by the amount of time the coffee spends in the machine. However, there is a lot of variation in the grind quality since there is no mechanical control of the particle size. The result ends up a mixture of grain size right for espresso, medium size, and a coarse grain size better for a cafetiere.
Use a burr grinder or coffee mill for consistent coffee quality, set to the right size setting for the particular brewing method used. A blade grinder is an approximate match to a drip filter, which needs a medium-fine grind, but the burr grinder gives a more controllable result for that perfect cup of coffee each time whatever brewing method is preferred.