The Physics Factbook
Edited by Glenn Elert -- Written by his students
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|Espresso Machines. Consumer Search, 2002.||"The best machines meet industry guidelines for espresso preparation: 1.5 oz. of water heated to 195 degrees Fahrenheit, forced through 7 grams of finely ground espresso at no less than 9 atmospheres of pressure (130 psi) within 20-25 seconds. This results in a shot topped with approximately one-quarter inch of crema, the golden foam that's the hallmark of good espresso."||90 °C|
|Espresso machine. Wikipedia, May 2004.||"An espresso machine forces water at 90 °C (195 °F) and 9 bar of pressure through a packed cake of finely ground coffee."||90 °C|
|Espresso Tips. 800-espresso.com.||"Espresso machines must be able to heat water up to 90'C (194'F) and to exert pressure equal to 9 atmospheres or more."||90 °C|
|Fundamentals for reaching Espresso Nirvana. 1st-Cup Coffee Equipment, 2001.||"The Optimal Brewing Temperature, As Measured At The Point Where The Water Contacts The Coffee Grind, Is 190-204 Degrees Fahrenheit. The Temperature of The Extracted Espresso Will Vary From 150 Degrees Fahrenheit to 180 Degrees Fahrenheit. The Temperature Loss Is Attributable to the Ambient Temperature, The Brew Group Temperature, and The Temperature of The Cup."||88–96 °C|
|From Origin to Cup. Boyd Coffee Company.||"The fresh water is heated to 194 °F when in contact with the coffee, and forced through the coffee with pressure in excess of 100 pounds per square inch. Some brewers have an external gauge which registers the pressure in the boiler. When the gauge shows the pressure between .6 and .8 bar, there is adequate pressure to produce 194 °F water. The pressurized water makes almost instant contact with every grain of ground coffee. The process is completed in approximately 15 to 20 seconds and produces 1.5 ounces of 185 °F espresso."||90 °C|
There are two espresso machines that make two different qualities of espresso. There's the modern pump powered one and the steam generated machine.
Espresso is a kind of Italian coffee. There are several factors that affect the quality of the espresso- the beans,and the machine, for example. Like coffee because it requires the use of a specialized machine. It differs from from coffee because its specific temperature and pressure requirements.
The traditional method of producing espresso in the 1920s would be with the use of the steam-powered machine. Water was boiled at 100 °C [212 °F] or above to create steam. The steam in the chamber, because of he increased pressure created by the phase change, forces the steam through the grinded coffee beans. The pressure, 1.5–3.5 atm, forces the steam through the finely ground coffee beans. At this temperature the grounds would be scorched. Lowering the temperature would produce less pressure and under extraction of essential flavor. High temperatures produce over extraction.
The modern espresso machine is pump powered. It requires the user to put water into a pressure tight chamber reservoir. The beans sit above it. The water is raised to a temperature of 90–96 °C [195–202 °F], not yet boiling. Steam is forced through the grounds by the machine pump under 8 to 9 atmospheres of pressure. When the steam passes though the beans, it condenses back into water extracting all of the coffee and oils. The steam chamber is usually made of steel, aluminum, or brass. These are common because they retain more heat and so do not allow heat to be lost as easily. This maintains the high temperature.
The process of making espresso by steam power is not recommended but is still used. It results in bitter coffee needing lots of sugar. Espresso making is an art that is not mastered upon first trial.
Stacey Johnson -- 2004
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