Pressurized Vs. Unpressurized - What's the Difference?
A classic question for new espresso machine customers is what's the difference between an unpressurized and pressurized portafilter basket. The answer is actually pretty simple, but to understand the whole story we need to start with the basics. First of all, the portafilter is what holds the coffee when brewing espresso. It's what you grind, distribute, and tamp in. Water is then pushed through the "puck" of coffee contained in the portafilter to brew the espresso. With all of this in mind, we know that we need pressure to make this happen. That pressure is created when the pumped water meets the finely ground coffee. All of this is why espresso requires very fine coffee, so that there is the perfect amount of resistance as water is pressed through.
The basket is the insert that holds the coffee in the portafilter. Removed from the portafilter, it looks like a metal cup with holes in the bottom. These holes are what ultimately lets the brewed coffee through into your cup. So where does the pressurized vs. unpressurized terminology come in?
Unpressurized baskets are what you find in higher end machines, and always how professional espresso is brewed. These are baskets that function entirely as mentioned above. In an unpressurized basket, nearly all of the resistance is created by the coffee, not by the basket. When everything is dialed in, this creates the best result, because you're making the coffee and the pump do all the work.
Pressurized baskets are a little bit different. These baskets use one of a few different methods to artificially add resistance. In some cases, a small chamber with offset holes acts as that resistance. Others designs use a smaller set of holes centered in the middle of the basket to slow extraction. In any case, the result is artificial resistance that slows down the brewing process, giving more time for the coffee to extract.
So why would you want one over the other?
As noted above, unpressurized portafilters naturally provide the best extraction, creating the best espresso. With that said, this also takes a very fine grind that is carefully dialed in. This means that you have to spend time on getting everything just right, and have an espresso specific grinder. Pressurized baskets create less perfect espresso, but are less finicky and can handle a coarser grind. It's up to you to determine where the sweet spot is for you!