30ml espresso
First invented in 19th-century Italy, an espresso shot is created by forcing a small amount of nearly boiling water (86-95º C) under pressure through finely ground coffee. The naturally occurring crema (foam) on top is the measure of the perfect espresso. The Centro Studi Assaggiatori defines the perfect espresso in their ISO norm as follows:
Weight of ground coffee: 7 g ± 0,5
Temperature of water after heating: 88°C ± 2°C
Temperature in the cup: 67°C ± 3°C
Water pressure on ground coffee: 9 bar ± 1
Percolation time: 25 seconds ± 5 seconds
Viscosity at 45°C: > 1,5 mPa s
Total fat: > 2 mg/ml
Caffeine: < 100 mg/cup
Milliliters in the cup: 25 ml ± 2,5
Source: http://www.espressoitaliano.org/files/File/istituzionale_inei_hq_en.pdf