Given the high pressure used in espresso machines, the CO2 that releases upon contact between roasted coffee and hot water reacts a bit differently than in a french press. The pressure creates lots of tiny bubbles that result in a foamy layer of crema. While some CO2 is good for espresso and full-bodied crema, too much can actually slow down extraction. An overly-gassy espresso shot may look nice, but it will taste sour and sharp.