The Physics
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Pressure in an Espresso Machine

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Fischer. Nestec S.A. Espresso Brewing Device. US Patent 6,820,535. 2004. "During this process the upper surface of the single-serving capsule 16 is pierced by the capsule piercer 6 so that subsequently the brewing water is able to flow into the capsule at a pumping pressure of 10 to 15 bar and a temperature of approximately 96° C." 1,000–1,500 kPa
Davids, Kenneth. Espresso,Ultimate Coffee 2nd Edition. New York: St. Martin's Griffin, 1993: 24. "Although these machines forced water through the coffee at a paltry 1½ atmospheres, compared to the 9 or more atmospheres considered ideal today, they introduced several innovations that carried the day for espresso: a richer coffee, brewed under pressure; a fresher coffee, custom brewed by the cup; and milk heated by having steam run through it, eliminating the flat taste acquired by milk heated in conventional manner." 900+ kPa
Espresso Machines. Shopping Made Easy. 12 June 2005. "In 1905, Desidero Pavoni bought the patent from Bezzera and espresso culture was born. After researching the variables, Pavoni discovered that the coffee was best brewed at a pressure of 8-9 BAR and at a temperature of 195° F" 800–900 kPa
Lewis, Walter H. III. Is a bar a bar a bar? May 23, 2005. "For espresso extraction you need much higher pressure- 8-9 bars, and this is achieved by either the use of a pump, or in older machines the use of a compressed lever." 800–900 kPa

Espresso is a type of coffee drink that is much richer and thicker in taste. Not only is there a great amount of flavor, but espresso is made differently by forcing hot water and/or steam through finely roasted coffee beans.

In Italy espressos are considered as their average coffee yet in America our average coffee is watered down with hot water and milk. So a lot of the flavor is lost in the brewing process. Italians consider the making of espressos a fine art while Americans just want their coffee and this might explaining the misspelling of the word espresso as "expresso".

The invention of the espresso machine began with the help of Luigi Bezzera in 1901, who created a machine that produced coffee at a faster rate. Then in 1905, Desidero Pavoni bought Bezzera's design and researched how to make a better quality coffee also known as the espresso. He finally realized that the ideal pressure is from 8 to 9 BAR or 800-900 kilopascals and the ideal temperature was 90.6° C or 195° F. His machine was called La Pavoi and most espresso machine today have a similar function.

The machine works by first filling the portafilter with about 6.5 grams of finely grounded coffee beans. Then the beans are tamped or compressed with about 30 pounds of force resulting in a "puck". When the machine is turned on a pressurized stream of hot water and/or steam is forced through the portafilter and espresso is made in the glass.

Kenny Ngo -- 2005