“Espresso is a 25–35ml (.85–1.2 ounce [×2 for double]) beverage prepared from 7–9 grams (14–18 grams for a double) of coffee through which clean water of 195°–205°F (90.5°–96.1°C) has been forced at 9–10 atmospheres of pressure, and where the grind of the coffee is such that the brew time is 20–30 seconds. While brewing, the flow of espresso will appear to have the viscosity of warm honey and the resulting beverage will exhibit a thick, dark golden crema. Espresso should be prepared specifically for and immediately served to its intended consumer.” — SCA, 2018

The average shot of espresso uses a 1:2 coffee to water ratio. The average dry espresso weight (IN) starts with an 18 - 20 gram dose, has a liquid output weight of 36.5 - 45 grams (OUT), is extracted in 20 - 32 seconds, at 9 bars of pressure at a temperature of 200ºF, uses pre-infusion/pressure ramp control, through an 18 gram (double) basket.

Crema is composed primarily of CO2 and water vapor bubbles wrapped in liquid films made up of an aqueous solution of surfactants (a substance that, when added to a liquid, reduces its surface tension, thereby increasing its spreading and wetting properties).

Crema also contains suspended coffee bean cell fragments (fines) or emulsified oils containing aromatics.

Espresso is produced by the percolation of pressurized hot water through a tightly packed bed of finely ground coffee. The water erodes solids and oils from the surfaces of the coffee particles as it flows through the coffee bed and deposits the solids and oils into the vessel.

The flow rate of the water through the grounds is determined primarily by the amount of pressure applied by the machine, how much coffee is in your basket, and the fineness of the grind. Higher pressure, up to a point, increases the flow rate; beyond that pressure, flow rate decreases. A larger dose or a finer grind will produce greater flow resistance and a slower flow rate.

Water always follows the path of least resistance through the coffee bed; it is the job of the barista to create the proper amount of flow resistance, and to form the coffee bed to provide uniform and even distribution of the water. A poorly formed coffee bed is vulnerable to channeling, an area of high-velocity flow through the coffee bed resulting in uneven extraction. Channeling dilutes a shot and causes grounds along the path of the channel to overextract.

Bottomless portafilter baskets are essential in monitoring extraction and channeling. Pale, lighter yellow stripes contrasting dark brown espresso flowing through the bottom of your basket during extraction is an indication of channeling. Spouted portafilters are intended to control/hide potential channeling and are not recommended for specialty coffee.

Because of its volume, espresso should be served immediately, as it will cool quickly and espresso is intended to be served warm. Serving after 30 seconds will change the guest experience, as the temperature will have dropped significantly and the crema will have dissipated creating a less-than-ideal mouthfeel.