Espresso or caffè espresso (also: expresso) is a concentrated coffee beverage brewed by forcing very hot (but not boiling) water under high pressure through coffee that has been ground to a consistency between extremely fine and powder.
Espresso was developed in Milan, Italy in the early 20th century, but up until the mid-1940s it was a beverage produced solely with steam pressure. The invention of the spring piston lever machine and its subsequent commercial success changed espresso into the beverage we know today. Espresso is now produced with 0.82-1.8 MPas (8.2-18 atm; 120-265 PSI) of pressure.
The defining characteristics of espresso include a thicker consistency than drip coffee, a higher amount of dissolved solids than drip coffee per relative volume, and a serving size that is usually measured in shots, which are between 25 and 30 ml (around 1 fluid ounce) in size. Espresso is chemically complex and volatile, with many of its chemical components quickly degrading from oxidation or loss of temperature. The most distinguishing characteristic is "crema," a reddish-brown foam that floats on the surface and is composed of vegetable oils, proteins and sugars. Crema has elements of both emulsion and foam colloid.
As a result of the high-pressure brewing process, all of the flavors and chemicals in a typical cup of coffee are concentrated. For this reason, espresso lends itself to becoming the base for other drinks, such as lattes, cappuccini, macchiato and mochas.
Despite some confusion, on a per-volume basis, espresso contains approximately three times the caffeine content of regular brewed coffee (1.700 g/l (50 mg per fluid ounce) of espresso versus 0.50-0.75 g/l (14-22 mg per ounce) for brewed coffee). Compared on the basis of usual serving sizes, a 30 ml (1 fluid ounce) shot of espresso has about half the caffeine of a standard 180 ml (6 fluid ounce) cup of American-style coffee, which varies from 80 to 130 mg.
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