title_body: How does pressure affect the flavor of espresso?
upvoted_answer: 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.