fractionated shot

Espresso is a multiphasic beverage. What does this mean? During different parts of the brewing process, different things happen.

First 1/3 : the volatiles and the aromatics are extracted. So when you use stale coffee or coffee that has been ground for a while, this is the fraction of the shot that suffers the most. This is the most intense, syrupy part of the shot. It's the heart and soul.

Second 1/3 : the sugars and solubles are extracted. This is the body of the shot. This is where a shot becomes sweet, or if you're using crappy or over-roasted coffee, not so sweet.

Last 1/3 : here come the solids, astringents and caffeine. And most of the crema is also formed here. This is the mildest part of the shot. If you overbrew, you get more of this part, which is not necessarily desireable.