In 1996, David Schomer released his book Espresso Coffee: Professional Techniques. In it he promulgated the importance of espresso machine temperature stability for delicious espresso. In the time since, espresso machine manufacturers have been hell bent on achieving temperature stability.
When extracting tasty chemicals out of coffee grinds within a ~30 second window, we rely on temperature to do some heavy lifting. Without it we’d only get a small fraction of the available coffee. The role of temperature in speeding up chemical reactions like this is described by the Arrhenius equation. Proposed by Svante Arrhenius in 1889, the Arrhenius equation states that the more reaction energy you have (temperature) the more quickly things will happen (diffusing coffee into water).
The simplest interpretation of Arrhenius is that for every increase of 10 degrees Celsius, our speed of extraction will double. Or, if you go down by 10C, the time needed to extract the same amount will double. In this frame, cold brew makes perfect sense. If you were using the same grind size, a 90C espresso that took 30 seconds would extract similarly to a 20C cold brew for 1 hour.
If we dig deeper on this concept then it becomes apparent that a 30 second window isn’t a very long time. Even small changes in temperature during that brief window could have massive effects on our extraction. We absolutely need an accurate and repeatable water temperature to ensure that we’re not getting wildly variable extractions. This is where modern espresso machines come in.
Long gone are the days where Baristas need to “surf” the temperature of their heat exchange lever machines by purging excess water before pulling a shot. Almost every machine manufacturer is fully subscribed to multi-boiler configurations. It’s a massive triumph for the industry, and we should all be grateful for many an engineer's sleepless nights to get us there!