I just wanted to know what “Crema” was. So I searched seventeen articles of CREMA including world famous coffee personalities video blogs and websites and none of them answered the question. Each article defined Crema as the stuff that sits on top of an espresso – WRONG! That is where crema is found, not what crema IS!
Crema is a colloidal suspension of vegetable oils and water, a temporary state caused by the extraction of fresh coffee using the solvent of hot water. The fresher the coffee the more gasses and the more crema.
“Colloidal suspension” – A mixture in which small particles of a substance are dispersed throughout a gas or liquid. If a suspension is left undisturbed, the particles are likely to settle to the bottom. The particles in a suspension are larger than those in either a colloid or a solution. Muddy water is an example of a suspension. (Dictionary.com)
Other examples of a colloidal suspension is: milk foam. It often arises when oil and water are forced to coexist and will eventually separate.
The crema is a foam of air suspended in an emulsion composed of vegetable oils and water. (Royal Soiety of Chemistry)
More helpful information from Barista Hustle
This is the most pressing reason not to use shot glasses for espresso.
When coffee is fresh, the crema is much thicker. This is because there is still a lot of carbon dioxide in the beans that hasn’t escaped.
As a coffee ages after roasting, that carbon dioxide dissipates and the espresso has much less crema.
Crema is mostly air and its density is far lower than the espresso liquid below, which makes volume measurements close to meaningless.