Foam is an important aspect of coffee preparation, and it determines, or validate quality in some coffee drinks.
Coffee beans contain carbon dioxide immediately after roasting. The degassing starts right after roasting and is the process of seeping out the gasses from the beans. A lot of the CO2 leaves the beans in the first 24 hours after roasting, that is about 40%. Lighter roasts degass slower, whereas dark roasts degass faster. Gasses in the coffee beans is a sign of a freshly roasted coffee.
Filtration is commonly the mechanical or physical operation which is used for the separation of solids from fluids (liquids or gases) by interposing a medium through which only the fluid can pass. Oversize solids in the fluid are retained, but the separation is not complete; solids will be contaminated with some fluid and filtrate will contain fine particles (depending on the pore size and filter thickness).
Espresso crema is a brown, foamy layer on top of the brewed coffee. Crema is a mix of gas bubbles, and emulsified fats, which are the signature of a good espresso. The perfect crema is a reddish-brown foam, with tiny bubbles. If the bubbles are too big, the shot was incorrectly extracted. If the colour is too light is an under-extracted shot, and if it’s too dark, the shot is over-extracted.