Espresso is a traditional Italian coffee produced by espresso machine. It is a concentrated coffee brewed by forcing steam under high pressure through coffee. Espresso was produced solely with steam pressure until other methods to produce espresso was invented in 1940s. The earliest form of espresso machine is steam driven; it operates by forcing water through the coffee by using steam or steam pressure. The key element of the espresso machine where the coffee is brewed is the filter holder. Water at temperature of 200 °F and at a pressure of 9 bars forced the 6-7 grams dose of ground coffee for a period of 25 to 30 seconds. The average steam in a espresso machine is 86 °C to 95 °C. Other types of espresso machine include machines that is piston driven. It uses lever, pumped by the operator to pressurize hot water and send it through the coffee grinds. Another type of espresso machine is pump driven; it uses a motor driven pump to provide the force necessary for espresso brewing. Espresso contains three times the caffeine content of brewed coffee. A 30 ml espresso contains half the caffeine as a 180 ml American Style coffee.

Mei Xue Zhu -- 2008