There are a lot of things that go into making espresso, but the general idea is to brew a small amount of coffee under pressure for about twenty five seconds. The pressure at which that coffee is brewed matters a great deal, and getting it right can mean the difference between a nice smooth espresso shot and one that bitter.
You may see when looking at an espresso machine for home use that it is rated for 15 bars of pressure, unforunately that is too much. All that 15 bar of pressure means is the max outage of the pump, not what should actually be used. The reason is that if the water gets to the espresso at too high of pressure it can cause a crack in the coffee puck, which will cause for that bitter coffee I was talking about.
However, at a lower pressure at around 9 bars of pressure the water doesn't cause nearly as much force the the coffee and tends to keep together better. So the water is able to flow through all of the coffee and you should be able to get a more even extraction.
Now the only way you can get this in a lot of machines, including my own, is a device that makes sure the pressure never reaches the coffee at a high pressure. Luckily there is a little device that is in some espresso machines called an Over Pressure Valve (OPV) this allows the excess pressure from the pump to be released. This allows for the coffee to be extracted at the right pressure.
Unfortunately most machines if they even have an OPV have to adjusted when you first get them. This is because they are calibrated i nthe factory to use espresso pods. I guess the idea is that you NEED to have the higher pressure to get an espresso pod to work, but the high pressure for normal ground coffee in your portafilter will work, but not ideal. So I have some experience with using a machine with less than ideal pressure.
When I first bought my Gaggia Classic, all I could get was bitter espresso when drank straight. Sure it was good for a milk drink, but left something to be desired in an americano or shot. I really almost returned the machine to be honest and eat the restocking fee. However, before I did that I adjusted the pressure, and the difference was dramatic. Where once all I could get was a bitter shot, they immediately became more mellow.
Because of this experience I can honestly say that for almost any espresso machine that you buy you should look for one with this over pressure valve that you can adjust! Look at the over pressure valve below.
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