Without question, the temperature of your espresso can make or break your experience. Even with perfect preparation, shots that lack temperature stability will cool too quickly, producing an espresso that leaves something to be desired. Conversely, there are circumstances, such as switching from steaming to brewing, where the water you brew with will be too hot. Fortunately, ensuring temperature stability is easier than you might think. Outlined below are several methods you can employ to ensure that your coffee is brewed at the correct temperature and stays at the correct temperature.
Prevent Heat Loss
Preheat Your Portafilter: If you're just turning your machine on, the heat from the boiler may not have had enough time to properly heat your portafilter. Before brewing, it is always a good idea to pull a blank shot to heat up the internals of the machine as well as your portafilter. A blank shot is performed simply by pulling water through the machine as if you were brewing espresso. Additionally, try to keep the portafilter locked into the group head, if you're going to be brewing multiple shots, to prevent heat loss between extractions.
Preheat Your Cup: Your espresso cup is the your coffee's final destination and the last opportunity for it to lose a significant amount of heat. By preheating your cup, you help to ensure that the temperature of the coffee does not drop when the brewing process is complete. An easy way to preheat your cups is to fill them with hot water from the steam arm or group head. Alternatively, you can use your machine's cup warmer if it has one, although, this will take longer to heat your cups.
The Cooling Flush: In certain circumstances, such as when you use the steam wand to prepare multiple specialty drinks, it is important to make sure that your water isn't too hot to be used to brew espresso. In single boiler machines, like the Gaggia Classic, which has only one boiler that heats water for both brewing and steaming, performing a cooling flush can prevent you from burning your espresso.
When you use your machine to produce steam, the water is heated to a much higher temperature than is appropriate for brewing. If you immediately go from steaming milk to brewing espresso, the water in the boiler is likely to be much hotter than it normally would be. A cooling flush involves running water through the steam arm and through the group head after steaming. Not only will you clear out any residual milk from the steam wand this way, but you will also drain the hotter water from the boiler, preventing you from brewing shots that are too hot.