Temperature Equilibrium In Espresso

November 01, 2015

The advent of PID control on espresso machines brought with it a feverish obsession with temperature control. “We want more precise temperatures” the professionals cried; and almost all manufacturers have been delivering with aplomb. Reflecting on these advances, one might ask: how important is precise machine temperature control on a busy bar?

We’re going to need some basic science.

Everyone who took chemistry or similar classes knows about thermal equilibrium. Mix two things at different temperatures together and they end up with a temperature somewhere in between. Espresso is no different; it’s hot water and cool/warm coffee.

To calculate thermal equilibrium we need to know the weight, starting temperature and specific heat (how much energy is needed to change the temperature of one gram of it by one degree celsius) of each material.

For this little thought experiment we’re going to assume the basket, group head, showerscreen etc. don’t exist. Approximating their effect is far beyond my abilities and not necessary to illustrate my point.

Let’s get the numbers.

If we use 20g of coffee grounds to make a 40g espresso we’ll be mixing ~65g water and 20g coffee grinds (assuming a liquid retained ratio of 1.2).

The machine is probably set to a standard 93C (199.5F) and the coffee is likely at room temperature of 20C (68F)

The specific heat of water is 4.18j/g/K (joules per gram per kelvin). Coffee is mostly made of plant material similar to wood, and isn’t entirely dry, so I’ll assume a specific heat of 1.4j/g/K.

The beginning of an espresso shot is cooler, and the end is hotter so please remember we’re going to get the average temperature of this extraction - the equilibrium - not the absolute temperature.

So what’s the result?

86.2C (187F). Probably cooler than you were expecting.

So what’s the big deal? What does this mean for consistency a busy espresso bar?

It’s all in the temperature of the coffee grinds.

Our machines might be stupendously consistent, but our coffee grinds are not. Grinding creates a lot of friction heat. This heat is absorbed by the coffee grinds, which changes the above equation.

Grinders get pretty hot. During busy service they can easily be spitting out coffee grinds at 50C (122F). If we change the equation above to include this, we get a result of 89C (192F). That's 3.8C hotter than before.

Here we are applauding machines for 0.1C accuracy when a typical scenario can see swings of nearly 4C based on the coffee grinds temperature.

Of course, accurate machines are important because they reduce the total swing. But we also need to think about coffee grinds temperature to get the full picture. If you can, measure your coffee grinds with an infrared thermometer before and during service. The difference could be startling!

Though keep in mind, what's most important for quality is consistency, so if your grinds are maintaining a relatively consistent temperature (i.e. you have a relatively consistent volume of service), you can still be achieving relatively high overall temperature consistency.

PS If you hear a Barista say that they use Fahrenheit because it provides better temperature resolution you can throw an espresso at them for me.

PPS Yes, I know the Mythos reduces the temperature delta but a) it still heats the coffee higher than its set point when busy and b) hot grinding isn’t optimal for the resulting particle distribution.

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