For a given coffee and roasting method, what factors of the espresso making process affect the crema and in what way(s) do they affect it? By my own empirical evidence, I would guess that time since grinding, coarseness of the grind, and tamping pressure are the biggest factors.
The "crema" is produced by emulsifying the oils in the ground coffee into a colloid, which does not occur in other brewing methods.
Using the right material from the start
Personally, I think this definition could use some updating. Crema exists because of the freshness of the coffee, in relation to it's roast. It contains the gasses we find from roasting, such as carbon dioxide and nitrogen gasses. You will notice that if you dig up coffee that is months old and brew it as espresso, there will be little crema. Bottoml ine, if you want crema, you need fresh coffee.
The other factor in producing crema, is correct extraction. If you have a poorly extracted shot of espresso, it will be primarily water, with some seriously over-extracted coffee. An even extraction will produce the highest yield of crema.
In their famous study on coffee crema, Illy and Navarini states:
The key to interpret the several factors affecting the crema, seems to be the carbon dioxide content of roasted coffee in addition to CO2 possibly present as bicarbonate ions in the water ingredient. Most of the data reported in the present review may suggest that espresso brewing can be described as “a quick way to transfer carbon dioxide from roasted and ground coffee to a small cup by means of hot water under pressure”. This then leads to the facts that for espresso coffee, carbon dioxide has to be:
- generated by roasting
- maintained in the bean by proper packaging
- maintained in the ground coffee
- solubilized in water
- released into the beverage.
In this framework statements such as “any error in grinding or in percolation, in temperature or extraction level, has an immediate effect on denounced by the color, the texture and the persistence of the foam” or “the foam is the signature of a well-prepared espresso” can be well justified. In fact, foam volume, persistence, and consistence are the consequences of the carbon dioxide content originally present in the coffee. In addition to the importance of carbon dioxide in espresso coffee foam, we believe that carbon dioxide can play a role even from a taste point of view. This aspect has not yet been the subject of investigation.