My new machine harnesses the properties of steam to provide very accurate and quick heating of the brew-water and group-head.
To produce decent coffee, we need to push water through the beans at a steady temperature. There is some disagreement as to which temperature is best, but lets for the sake of argument settle on 94C. 100C is too hot, and will make a terrible espresso.
Now, every body knows that water boiles at 100C at sealevel, and that a kettle boiled on top of Everest doesn’t boil at 100C but will in fact boil at around 71C. As it happens, if you boil a kettle at around 6,000 feet altitude, it will boil at precisely 94C: perfect for espresso! The summit of Cerro La Campana in Chile would be ideal.
It takes an enormous amount of energy to turn water into steam. Immagine boiling a kettle on top of Cerro La Campana: it will quickly reach 94C and start boiling vigorously. If you keep the kettle on the stove, it will stay at exactly 94C until all of the water has evaporated, which will take ages and ages and a lot of gas. Now, the steam coming out of the spout will be at exactly 94C, but will be carrying away all of the heat the stove’s flame is putting into the water!
The same thing happens in reverse. When that steam hits a cold surface, it will condense, and turn back into water. When it condenses, it releases all of that stored up heat, yet doesn’t change temperature whilst doing so. Afterwards, when it is water again, it might cool down more, but at the instant of turning back into water, it is at a constant 94C.
My new machine works by creating inside itself the same conditions found on top of that mountain: reduced pressure. The machine takes steam from the high pressure, milk-frothing boiler and reduces its pressure and temperature. That low pressure steam then heats up the brew water and warms up the group-head.
Because the steam has loads of energy, yet releases it all at the same temperature, the brew water or group-head never overheat, and the small volumes of water needed to make espresso can be heated up “on the fly”. There is no need for an actively heated group-head, or a electrically heated large seperate brew-boiler. The machine warms up and stabilises really quickly – my working model is ready to go for steam and espresso in just under fifteen minutes. There are a number of changes to make, and I invisage it going from cold to ready in under ten minutes.
When compared to a traditional heat-exchange unit, as the brew-water is heated by the steam at 94C (or similar), the brew-water cannot overheat so no “cooling flush” is ever required.
Another advantage is that the steam can be transported quite long distances through relatively small hoses, so the group-head can be far away from the boiler and pump yet still heat up effectively. This allows the machine to be built as a number of seperate, over the counter dispensing units connected an out of the way boiler and pump unit.