It's all in the emulsion.
Someone asked me what exactly constitutes espresso. Here is an answer from the founder of Illy:
"Italian espresso is a small cup of concentrated brew prepared on request by extraction of ground roasted coffee beans, with hot water under pressure for a defined short time.
The range of the parameters is:
Ground coffee portion 6.5 ± 1.5 g
Water temperature 90 ± 5 degrees C
Inlet water pressure 9 ± 2 bar
Percolation time 30 ± 5 second”
If it doesn’t fit these parameters, it’s not espresso! (I’m looking at you, moka pot and aeropress!) Just strong coffee. … Delicious, delicious coffee. ;)
Source: Illy, Andrea, and Rinantonio Viani. Espresso Coffee: The Science of Quality. (2nd Edition.) London: Academic Press. 2005. (It’s really expensive at Amazon. If you want a copy, try used, the first edition, or getting it out of your lcoal library.)