March 1, 2019

How Ground Coffee Temperature Causes Uneven Espresso Extraction


Are you concerned about the 3ºC variation in the water temperature inside your espresso machine’s group head? Yes? Well, what if I told you that the temperature of your ground coffee could vary a lot more than that?

The relationship between ground coffee temperature and espresso quality is often overlooked. However, it exists – and the industry is starting to pay attention to it.

To find out more, I spoke to two experts: Lauro Fioretti, Product Manager at Simonelli Group, the maker of the Nuova Simonelli and Victoria Arduino grinders and espresso machines; and Colin Harmon, four-time Irish Barista Champion and owner of 3FE Coffee in Ireland. Lauro and Colin both worked on the development of the Mythos One and Mythos II, grinders with milling chamber temperature control. Who better to tell me all about this issue?

You might also like Understanding Coffee Extraction For Better Coffee

pulling a shot of espresso

Pulling a double espresso shot at The Fix, Madrid. Credit: Fernando Pocasangre

Is Your Coffee Grinder Heating Up Your Ground Coffee?

There are many different types of grinders available. However, whether conical or flat, ceramic or metal, specialty coffee grinders typically use burrs rather than blades.

What this means is that, within the grinder, you’ll find two burrs. The distance between them determines the grind size. When the grinder is turned on, at least one of them will rotate, grinding the beans into smaller pieces.

However, this process involves friction, which in turn generates heat. This heat then spreads to the rest of the grinder and to the coffee itself. So, unless you’re working with specialized equipment, the exact temperature of the grinding chamber will vary according to how long the grinder has been operating for.

Of course, professional grinders normally have a cooling system, comprised of fans, heat exchangers, and heat sinks. However, while these systems cool the machine down, they rarely do so with precision. Nor do most of them enable baristas to monitor the temperature of the ground coffee.

Lauro Fioretti has experimented with several grinders from different manufacturers. He tells me, “Depending on how fast or slow, or how busy you’ve been or how quiet you’ve been, the grind temperature of your coffee can go anywhere from 20ºC/68ºF to, I bet, 80ºC/176ºF. So, there is a 60ºC/176ºF swing. 

A temperature change like this has an impact on your coffee’s extraction, leading to issues with both quality and consistency.

disassembled burr grinder

The disassembled flat burr set of a Mythos One grinder. Credit: Victoria Arduino

The Impact of Inconsistent Ground Coffee Temperature

Lauro tells me, “When the temperature [inside your grind chamber] is increasing, you see that your shot times get shorter and shorter and shorter. So your coffee is extracting faster, so they have to tune up the grinder to make it finer, finer, and finer to compensate for this effect.”

And then, when the peak time ends and the baristas need to use the grinder less? The temperature starts to fall. Once more, the baristas have to amend the grind setting. While this may not sound like a big deal, the more often baristas have to adjust the settings, the greater the risk of errors, inconsistency, and poor extraction.

What’s more, Lauro tells me that as the grinder heats up, it also starts to grind the coffee faster. This has a knock-on effect. “The temperature of the grinder affects the dose, because most of the grinders on demand that are actually available on the market, they dose by time. So, you do a prediction of what you will get in your portafilter according to a timer, but the performance of the grinder changes according to the temperature.”

I ask him why temperature has this impact on coffee extraction and grind time, but he tells me that they’re still trying to work that out. “The first idea we had was a change in the granulometry,” he says, “so we have ground the same coffee at different temperatures and we have analysed it with our Master Sizer.

“Also, André Eirmann [Marketing Director at UCC Switzerland and Communications at Swiss SCA] has done the same test on another grinder brand. The results show a small difference in the fines. [It’s] not statistically significant, according to André, but I believe that small difference in fines can make a big difference in the exposed surface of particles (but that needs to be proved) and/or it may be that we are not using the right analyser for it.”

Yet regardless of the reasons why, one thing is clear to Lauro: the changing temperature has an effect on the coffee.

knob for adjusting grind size

A rotary knob for adjusting grind size. Credit: Simonelli Group

Why Is It So Hard to Keep Grinders Cool?

Cooling down a grinder chamber isn’t impossible, although it’s a lot easier if you can avoid it getting hot in the first place. Take the Mythos II: the grinding chamber is external and made of aluminium to keep it as cool as possible.

However, as Colin says, “to cool a room is far more expensive than to heat a room; to refrigerate something is very expensive and very difficult, and it uses a lot of energy.”

In other words, you can cool your grinding chamber – at a cost. But for café owners looking for increased efficiency and energy savings, this might not be the ideal option.

And, in fact, how much do we actually need to cool down the grinding chamber? Colin tells me that, in 2010, “the widespread belief was that hot ground coffee is bad and cold ground coffee is good.”

But some people are now questioning this assumption.

shot of espresso and a glass of water

Drinking espresso at VESTA Coffee Roasters, Las Vegas. Credit: Neil Soque

Stabilising Ground Coffee Temperatures

“Maybe the issue was not that the ground coffee was hot or cold, but, in fact, the issue was that it was changing,” Colin says.

So, the team put the theory to the test: they ground several different coffees at different temperatures and then analysed the extraction. The result?

“I believe there is a different solubility of the compounds at different temperatures and this will affect the extraction,” Lauro stresses. But the difference in quality wasn’t notable in this experiment.

What’s more, regardless of the exact number of degrees inside the chamber, as long as it was steady, the team could dial in their grinder and achieve the same extraction time and flavour profile for every espresso – no matter how quickly they were dosing those shots.

In other words, their research indicated that the biggest issue with grinding chamber temperature was inconsistency, not the temperature itself.

The team still emphasised the importance of cooling the chamber to avoid extremely hot conditions. However, when building the Mythos II, they also built in a preheating function. This allowed them to reduce how many degrees much they needed to cool down the chamber, and so save energy, without risking inconsistency.

mythos 2 yellow coffee grinder

Mythos 2 grinders on display. Credit: Victoria Arduino

Ground coffee temperature: it’s not a variable we’re used to discussing, but it can impact both quality and consistency. For busy coffee shops, it can cause particular problems as the grinding chamber heats up during peak hours.

But like most things in coffee, consistency is key. It’s not so important what temperature the beans are ground at, providing it’s the same temperature every single time.

You might also like Understanding Coffee Extraction for Better Coffee

Please note: This article has been sponsored by Simonelli Group.

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