Bar pressure
Good bar pressure is essential for making a good ‘crema’. It means that the steam meets the coffee granules at the correct speed. Too slow and it could result in a bitter taste. 15 – 19 bar is the optimum, though some suppliers claim 9-11 bar is enough if the beans have been ground correctly.
Thermoblock
Rather like a shower, the Thermoblock is a type of boiler that heats up the water to about 90°C, via a pump. To achieve an excellent flavour it’s important that you don’t scald the coffee – unlike tea. Some models have a Thermoblock system for frothing the milk, which needs to be at about 120°C.
Filter holder
This holds the coffee granules and should be kept warm. Basic models have aluminium ones but more expensive models have brass holders that retain the heat for longer.
Valve
Most machines have a mechanical valve to control the water flow. Top-of-the-range machines have solenoid valves that increase the water pressure at the point of delivery and also shut off as soon as you finish the extraction
Wattage
Like kettles, the higher the wattage the faster the water is boiled. High wattage machines are good if you are making several espresso