Jenna Gottlieb speaks with coffee scientist Adam Lees about how hot grinders can affect coffee extraction and what can be done to mitigate the problem.

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Grinders play a crucial role in preparing a coffee for extraction.

By breaking up whole beans into smaller particles, they ensure that when hot water is passed through the coffee, it successfully extracts its compounds, leading to a well-balanced cup.

To maximise freshness, most cafés grind each time they prepare a drink. In high-volume cafés, this means the grinder is often in constant use – noticeable by a constant “whirring” sound in the background.

However, while grinding is a vital step in the brewing process, it doesn’t always produce consistent results. One of the main culprits is overheating.

“Typically, the reason grinders overheat is because they have been operating for too long or grinding too much coffee in a short period of time,” explains Adam Lees, a coffee scientist at Kaffe Magnum Opus.

“But it could also be because too much coffee has been poured into the hopper or the environment in which the grinder is operating is too hot – for example, in coffee roasteries. Basically, if you can feel the coffee heating through the bag you are grinding into, it is overheating.”

Although cafés could get around this problem by purchasing ground coffee rather than whole bean, it is unlikely to have all its original, unimpaired characteristics intact. There’s also no guarantee that the roastery that grinds the coffee hasn’t allowed its grinder to overheat, too.

The impact of grind temperature on espresso shot times

During the course of a day, grind temperature can swing dramatically due to the heating and cooling of the grinder. At the start of service, it can be as low as 10ºC (68ºF), while peak times can produce grind temperatures in excess of 50ºC (122ºF).

Grind temperature matters largely due to its impact on espresso shot times.

As grind temperatures increase, shot times shorten, which accelerates the coffee’s extraction. According to recent studies, this is because higher temperatures cause a coffee’s viscosity to fall, which results in a faster flow.

Quick espressos shots tend to have a lighter body and higher acidity than slower shots, which are generally sweeter with a heavier body. If the time is too fast, the espresso will become thin and weak.

Adam explains that higher grind temperatures can also affect what happens to the coffee prior to pulling a shot.

“Raising the temperature of ground coffee will prematurely extract the coffee before brewing,” he says. “This will flatten the taste and give it a ‘long-roast’ flavour profile.” In other words, it can end up tasting “burnt”.

As a result, a cup of coffee served in the morning from the same café could end up tasting entirely different from one served later in the day.

How to prevent overheating

An overheated grinder can seriously impact a barista’s ability to consistently extract coffee. This can damage the long-term success of a café, particularly for customers who want a repeatable experience – a factor on which many chains have built a loyal customer base.

But what can coffee shop owners and baristas do to prevent their grinders from overheating? Adam suggests waiting between each grind to give the equipment time to cool.

“The best thing to do is to grind in batches,” he says. “This allows a full cool-down between each batch.”

However, this isn’t always feasible. Depending on the footfall into the coffee shop, waiting for grinders to cool down could cause queues, leaving customers feeling impatient. Busy hours such as early morning coffee runs can be especially challenging for cafés as the grinding chamber heats up during peak hours.

To combat this, cafés should ensure that the area in which the grinder is placed is well ventilated, providing proper airflow to lower external temperatures. If it is a particularly hot day, using air conditioning units can also help.

Some grinders, such as Slingshot’s volumetric series, come with inbuilt climate systems, designed to optimise the internal airflow and keep the coffee chamber at a constant temperature. It uses a powerful fan to extract hot air from the motor area, considerably reducing the risk of fluctuating grind temperatures throughout the day.

Adam also explains that proper grinder maintenance and keeping a careful watch on the quality of the ground coffee can go a long way to achieving consistent results.

“Burrs should be fully cleaned anytime coffee grinds look inconsistent,” he says. “If this fails to resolve the issue, burrs should be changed as soon as possible.”

Increasingly innovative monitoring systems can help baristas keep an eye on the quality of their ground coffee. Some even keep a countdown to ensure the grinder always runs with the ideal burr sharpness.

When you practise proper maintenance of your coffee grinder and work with a quality machine, you can easily avoid overheating, leading to a consistent cup of coffee each and every day.