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This morning, as with every morning, I had my coffee. However, today it was burnt, because I slightly overfilled the water. I use an Italian Percolator on a gas top.

Diagram of the Italian Percolator

Picture of the Italian Percolator

Normally, you put it on to boil, and once you hear the water bubbling, the coffee is ready, and chamber C will be full of coffee. As you can see, you fill chamber A with water, loosely fill B with coffee, and the water then rises through B, up through the red pipe, and flows over into C. However, as the picture shows, there is a small valve on A. If you fill the water above this valve, you get horrible coffee. When you hear it bubbling, very little coffee has risen into chamber C, and you have to wait a long time for it to do this, while water also escapes from between chambers B and C (which screw together).

Why does this happen? I cannot think of why there should be a valve to release what is presumably steam. Surely the machine should work regardless? Why does the steam have to be released for the mechanism to work properly - is it linked to the pressure of the system?

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I have never used one, but I read about it on the Italian Percolator (Moka Pot) Wikipedia Page.

The valve is a pressure relief just like on a pressure cooker; it is for safety and probably not involved with the burnt result. You may want to clean it with some vinegar to be sure it is in good operating condition, but it is probably fine since other pots have been turning out well.

Further down the wiki page it says:

When the lower chamber is almost empty, bubbles of steam mix with the upstreaming water, producing a characteristic gurgling noise. This "strombolian phase" allows a mixture of superheated steam and water to pass through the coffee, which leads to undesirable results, and therefore brewing should be stopped as soon as this stage is reached.

Perhaps filling the water higher changes your timing, and higher temperature steam is reaching your coffee grounds.

If I had to guess, without your anecdote, I would say that for the same amount of time, an overfilled pot would result in a more dilute, lower temperature brew, and an underfilled pot would result in dark burnt brew.

You may have to time yourself, sacrifice a few cups, and/or get the thermocouple meter out ;-) Good Luck!

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  • $\begingroup$ Without a pressure release valve the pressure in the lower chamber will increase which in turn increases the boiling point of the water and so results in the brew using hotter water than is intended. How much this happens will depend on how tightly packed and finely ground the coffee is, the harder it is for the water to pass through the coffee the greater the pressure rise will be. $\endgroup$ – Andrew Sep 5 '16 at 15:36
  • $\begingroup$ @Andrew, I thought about that, but I dont think the pressure to lift the water 8 inches and through even tightly packed grounds would be much above atmospheric pressure. I think the only way the temperature can get considerably higher is from superheated steam passing through the coffee grounds. $\endgroup$ – ericnutsch Sep 6 '16 at 2:17

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