Ever wondered what causes big bubbles to push out of the portafilter spouts when you pull a shot? I use to think it was a massive gurgle of CO2. I thought that fresher coffee gave off lots of gas, which is true. But the gurgle effect is something a little different. Steam is building up in the space between the portafilter basket and the bottom of the portafilter. The steam pushes out big bubbles once the stream of crema and espresso fills the exit chute. This may not bother you, and I didn’t give it much thought till David Lamont mentioned it. But if you like to read the signs of good extraction you may not want bubbles or a back log of espresso to get all churned up before you get to see it. If you really want or need to stabilize the flow you can ventilate your portafilter.
This video shows the effect of a ventilated portafilter as compared to a stock portafilter with both fresh gaseous espresso and a slightly more aged and stabilized espresso.