I thought it might be time to move on to the most fêted piece of equipment in the coffee world – machines designed to produce espresso. Now you may have determined that espresso machines are not my favourite piece of kit. I believe that the coffee grinder is the most important single factor in a great coffee set up. Nevertheless, a good coffee machine makes the process of producing great coffee seamless and enjoyable.
What does a good coffee machine need to be able to do? In order to produce espresso, the machine needs to heat water to a relatively precise temperature that is not quite at boiling point (anywhere between 91 and 95 degrees Celsius seems to do the trick quite nicely) and then apply pressure to the ground coffee puck (6 to 9 bar of pressure works well). Bonus points are awarded if a lower pressure first stage of coffee infusion can be performed… and star awards come with the ability to continuously alter pressure across the entire extraction (manually or automated). The other main function of the machine is to heat water to beyond boiling point in order to produce steam which can be used to heat and texture milk for milk-based espresso drinks. Typically, the steam boiler allows for high pressure to be created so that milk texturing can be done more quickly and with the ability to more easily incorporate air bubbles into the milk resulting in luxuriously silky microfoam.
Both functions involve heating water, but the rub is that two functions require water at different temperatures. Coffee extracted with boiling water is over extracted and bitter, and it’s just not physically possible to produce steam at a temperature which is appropriate for espresso extraction. This is a nasty catch 22 situation. There are three main design architectures which are used to solve this issue (generally correlated with an increase in cost as we move down the list):
- Single boiler;
- Heat exchanger; and
- Dual boiler.
Single boiler switch between the two temperatures. You set the machine up to extract espresso at less than boiling point temperature, extract your espresso, switch to steam production, wait for a decently long time for the water to reach boiling point and also to build some pressure in the boiler, and then steam your milk. If you want to make espresso again you’ll have to wait for the boiler to drop back down to espresso temperature. This process works, but it is not seamless, doesn’t at all suit those of us with a lack of patience, and also tends to be the least expensive style of machine in any manufacturer’s line up. Components are therefore likely to be cheap and unlikely to be very precise.
Heat exchangers (often referred to as HX machines) also have a single boiler but this boiler is heated up to steam temperature and the steam is held at relatively high pressure. There is a heat exchanger, basically a long tube designed to release temperature from the water that is being drawn up through it, which is calibrated such that by the time the water reaches the brew head it will be at the correct temperature for good extraction. Heat exchangers allow for coffee extraction and simultaneous or immediate milk texturing. It’s a very clever solution to the problem, and well-designed heat exchangers work extremely well. However, for those of you who are very finicky, there remains the problem that the temperature of the water used for extraction cannot be directly controlled, rather it is a function of the temperature of the steam boiler and the parameters of the heat exchange tube itself.
The dual boiler houses two entirely different circuits to allow for complete control of water used for coffee extraction and that for milk texturing. Generally proportional integral derivative (PID) controllers are used to very precisely control both sets of water temperatures. This is the type of machine you want if you’re a total control freak.
Can you guess which style of machine we have on our bench?
In future newsletters we’ll talk about different ways that espresso machines develop pressure for coffee extraction as well as the more interesting manufacturers with plenty of associated eye candy images. We’ll finish the espresso machine series with a highly aspirational post describing halo machines!