Pressure is the ratio of force to the area over which that force is distributed. Pressure is force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure (also spelled gage pressure) is the pressure relative to the local atmospheric or ambient pressure. While pressure may be measured in any unit of force divided by any unit of area, the SI unit of pressure is called the pascal (Pa) after the seventeenth-century philosopher and scientist Blaise Pascal.
In coffee brewing pressure is used to improve coffee extraction. By using pressure, the brewing temperature can be considerably lowered. Using pressure, undesirable compounds, that are only soluble at high temperatures, are not extracted. This improve the aroma and taste profile of the coffee.
The presence or absence of pressure in coffee brewing creates very different results in the final cup of coffee. Extracting coffee under pressure, as with espresso, yields a cup that that contains more coffee compounds such as cafestol and kahweol and is considered a much fuller cup. However, brewing coffee without pressure and through a filter creates a much cleaner cup. The absence of some of these properties extracted at high pressure is an advantage to many coffee drinkers who prefer not to consume these compounds for health reasons or because of taste preference.