Under Pressure!

I've talked previously about the grind of the coffee, its freshness etc., however I haven't said much about cofffee machine pressure, that is the pressure in the boiler and the pressure delivered by the pumps.

I was reminded of this issue over the last couple of days following a site visit to one of my customers. I pressed the button to drive some water through the group head before making a coffee and was met with this intense spray of near boiling water. I checked the gauge on the machine to see if the boiler pressure was significantly above 1 bar pressure, it wasn't, I then checked the pump pressure and this too was correctly running at between 8 and 10 bar pressure. So what has gone wrong and why is it important?

Well if the water flows at a constant rate, the barista can set up the grinder to grind the coffee to a fineness that will allow him or her to make a perfectly extracted espresso in 20 or 30 seconds depending upon the characteristics of the coffee.

If however the water blasts out of the group head on one occasion and steadily on another there is no way that the taste can remain consistent. Water that is under undue pressure will force its way through the coffee resulting in a thin and underextracted brew; you might even end up with a burnt taste.

Now I don't experience this on all machines, but it does appear to be prevalent at sites where coffee throughput can vary dramatically throughout the day. For some reason and I hope a machine manufacturer takes note there can sometimes be an excessive build-up of pressure in the boiler which effectively means that the machine needs to be "bled" before making a cup of coffee.

I have found that this tends to happen when the machine has not been used for a while. The important thing as far as the barista is concerned is to be aware of the condition and to ensure that he /she runs the machine until a steady flow of water is established and then make that coffee.

As a customer said to me recently, great coffee isn't just about one or two things, it's about a whole series of little things any one of which can cause the whole structure to collapse. Never a truer word said.