My answer to How does a coffee machine work?
Answer by Peter Baskerville:
If you are talking about espresso coffee machine, then this image below explains best
As seen the the diagram:
- Mains water flows into the espresso coffee machine, usually via a water softener and carbon filter to remove the impurities and hard corrosive minerals.
- When the desired button is pushed on the control panel button by the barista, the volumetric pump springs into action and pumps the cold mains water through a series of pipes, known as the heat exchanger, which is located inside the boiler.
- The water that emerges from the heat exchanger and boiler is at an optimum temperature to extract the very best of the coffee oils.
- The heated pressurized water is then directed to the top of the E61 group head where it meets a resistible force in the form of a recently inserted portafilter, filled with compacted ground coffee.
- The balance between the pressure applied by the volumetric pump and the barista’s skill in creating the perfect resistance in coffee granularity, coffee volume and tamper pressure ensures that the extraction of only the best of the coffee oils occurs, leaving the naturally occurring tanic acid still inside the ground coffee and so not leaching it into the brew.
- What results is about 20ml of pure coffee essence, like this.