Espresso is coffee brewed by forcing a small amount of nearly boiling water under pressure through finely ground coffee beans.
Espresso is generally thicker than coffee brewed by other methods, has a higher concentration of suspended and dissolved solids, and has crema on top (a foam with a creamy consistency).
Tamping down the coffee promotes the water's even penetration of the grounds. This process produces an almost syrupy beverage by extracting both solid and dissolved components. The "crema" is produced by emulsifying the oils in the ground coffee into a colloid, which does not occur in other brewing methods.
Espresso is both a coffee beverage and a brewing method. It is not a specific bean, bean blend, or roast level. Any bean or roasting level can be used to produce authentic espresso. For example, in southern Italy, a darker roast is generally preferred. Farther north, the trend moves toward slightly lighter roasts, while outside Italy, a wide range is popular.
As a result of the pressurized brewing process, the flavors and chemicals in a typical cup of espresso are very concentrated. Espresso has more caffeine per unit volume than most beverages, but the usual serving size is smaller—a typical 60 mL (2 US fluid ounces) of espresso has 80 to 150 mg of caffeine, a little less than the 95 to 200 mg of a standard 240 mL (8 US fluid ounces) cup of drip-brewed coffee.
Snippets from Espresso from Wikipedia, the free encyclopedia.