How does pressure affect the flavor of espresso? I've noticed the water pressure from espresso machines varies from about 12 psi to 22 psi. What effect does this have on flavor?
Working on a Synesso (prototype), I was lucky enough to be able to play with controlled pre-infusion. They state that their pre-infusion has a pressure of 3-4 bars and " bring[s] an extra textural element to your brew in the form of a greater volume of crema. It can also lead to a more syrupy mouth-feel overall." I find this to be mostly true if the pre-infusion was used for about 5 seconds.

What I found through personal experience was that, if I kept the pressure consistent, not switching to the full 9 bars, the shot pulled at the same extraction rate, finishing at my desired time. So Keeping dose, time, and extraction the same, the only variable was pressure. I found that the most notable change was a reduction in brightness. The more pronounced acidity and citric flavors were dulled. There was however a silver-lining that gave the pre-infusion a redeeming quality: Because of the reduction in brightness, I found that when using really fresh coffee ( &lt; 4 days old), the pre-infusions reduction in brightness took away from the character metallic taste found with fresh coffee. For that reason, when I am dealing with fresh coffee, I may increase the pre-infusion time (from 5 to 9 seconds for example) and this helps make the espresso palatable.