Cookbook:Coffee
Coffee beans are best fresh. They must be roasted in order to be used, a process that normally takes place before the beans are sold. This roasting starts a degradation process, whereby the oils and flavor compounds break down and generally become bitter and less flavorful. Surface area is a large part of the degradation, so ground coffee degrades considerably faster than whole-bean coffee.
The flavors in coffee are extracted by hot water, and there are a number of ways by which to extract these flavors. The coffee must be ground for all of these methods -- just as the increased surface area contributes to faster degradation, it also allows the rapid extraction of flavor by water. There are two basic methods by which this is done, either by allowing the grounds to sit in hot water and steep like tea, or by forcing hot water through the grounds. Generally some combination of these two processes is done.
On the one extreme is the French press. The grounds are placed into the bottom of a vessel and hot water is poured onto them. The mixture sits for a few minutes, then a mesh strainer is pushed down onto the mass and the coffee water is poured off to drink.
The other extreme is espresso. A fine grind of strong coffee is packed into a small metal container with a hole in the bottom. Pressurized steam is forced into the grounds, where it condenses on the grounds and extracts a strong flavor, then falls into the pot below, driven by the pressurized steam still coming in.
Generally, the coffee we drink is prepared by the drip method, which is halfway between these two. The grounds are placed in a basket with a non-reactive liner of paper or sometimes gold, with a small hole in the bottom. Hot water is poured over the grounds, where it soaks the grounds and steeps for a short time while being slowly forced by gravity through the hole into the container below where it is kept warm (or at least insulated) while the brewing process finishes.
There are many factors in the final result. First is the roast and quality of the coffee beans. Freshly ground, newly roasted beans will always give a better cup of coffee than pre-ground, older beans. Clean fresh water is necessary, and the temperature of the water will have some bearing on the result. Cooler water will not extract flavors as efficiently as hot water, but boiling water can be a little too good with a very dark roast. Lastly, the coarseness of the grind and the ratio of grounds to water are also important, and vary according to the brewing method used.
A good rule of thumb for beginning brewers is to use the simple drip method. Use 2 Tablespoons of finely ground coffee for every 6-7 oz of fresh water. Bring the water just to a boil, then pour slowly over the grounds, stirring gently as you do. Stirring will result in the grounds retaining some extra water, but do not try to squeeze the water out. When it stops dripping, the coffee is ready to drink.