Why grinds become coarser when the grinder heats up and how it impacts extraction

Why grinds become coarser when the grinder heats up and its impact on extraction speed

Dr. Monika Fekete discovers why coffee grinds become coarser when the grinder heats up and how it changes the speed of espresso extraction. 

Dr Monika Fekete is the Founder of Coffee Science Lab.

Ice, fresh, cold milk, and a perfectly balanced espresso shot are the key ingredients to a delicious iced latte over the summer months. The warm, balmy nights might give you a head-start waking up your machine and grinder from their slumber. But as the day goes on, your perfectly dialled-in shots will speed up and you will need to adjust your grinder a bit finer. 

The reason we need to adjust our grinders throughout the day to meet with the climatic demands of summer or winter is a surprisingly complex question without a solid explanation.

Rising temperatures as the grinder and machine heat up are clearly central to this problem. As we discussed in the August 2019 issue of BeanScene, temperature affects practically all aspects of the brewing process. 

Why grinds become coarser when the grinder heats up and how it impacts extraction
Figure 1. Experimental design: Two coffees tested on two grinders, both running cold and after warming up.

In the mystery of increasingly faster shots, we can hypothesise that two factors in particular will have an impact:

  1. Increased extraction temperature as a result of warmer grinds in the espresso basket.
  2. Potential changes in size of the grind particles as the grinder heats up.

When scientists try to understand a complex system, they like to break it down into simpler chunks. In other words, study just one feature at a time and hold everything else constant.

In the June 2019 issue of BeanScene, we focused purely on the temperature of the grinds, eliminating any potential variation in grind particle size. In that study, we found that hotter grinds and consequently hotter extraction temperatures reduced the viscosity of shots. This makes sense, as liquids with lower viscosity can flow faster. However, this “shot viscosity” effect only accounted for a few seconds decrease in shot time, while in reality, the shift can be much more dramatic. 

This leads us to the second chapter of the story: grind particle size distribution. To counteract the shift towards faster shots, baristas need to adjust the grinder finer during the day. Does this intuitive solution address the real issue of grinds becoming coarser at higher temperatures? Let’s find out.

In the following set of experiments, I tested two grinders (a conical burr grinder and a flat burr grinder) and two single origin espresso roast coffees, aged over seven days from roast (Colombian washed and Ethiopian natural process beans, thanks to Vacation Coffee Roasters). 

Each coffee was ground on each grinder “cold” and again after a
warm-up.

For the “cold” samples we ground each coffee in the morning, just after the grinder had been switched on. The grinder temperature was measured in each case, and was typically about 13°C. Then, keeping the grind setting constant, two kilograms of coffee were ground through on both grinders to simulate intense grinder use in a café. Immediately afterwards, the participating coffees were also ground without changing settings – these were our “hot” samples. 

Why grinds become coarser when the grinder heats up and how it impacts extraction
Figure 2: Particle size distribution of cold and hot grinds.
Top: flat burr grinder.
Bottom: Conical burr grinder.
No clear shifts can be observed between cold and hot samples.

As I wanted to separate the effect of grind particle size from grind temperature and outgassing, both cold and hot grinds were allowed to rest for an hour after grinding before a set of 10 repeat shots were made on a LaMarzocco Linea PB. The brew recipe was kept constant for dose (20.0 grams) and beverage weight (40 ± 0.5 grams). Extraction times were close to 30 seconds for the cold samples but ran up to 12 seconds faster for the hot samples.

Grind samples of each condition were taken to La Trobe University in Victoria and their particle size measured on a laser particle sizer in triplicate runs. 

Figure 1. shows changes in particle size distribution for both the flat burr and the conical burr grinders between “cold” and “hot” grinds. While we might expect to see a characteristic shift between the particle size distribution of the samples going from cold to hot, this doesn’t seem to be the case. It’s clear that some of the peaks have shifted, but it’s hard to tell if the grinds have become coarser or finer overall. And which particles should we pay attention to anyway, the smaller ones or the bigger ones? 

A little dive into some theory will help.

When we think about how water flowing through the puck wets the grinds, it’s important to consider the surface area as well as their volume. For applications where the active surface area of powders is especially important, chemical engineers tend to focus on the Sauter mean diameter (SMD or d(3,2)). SMD is an average particle size that takes into account both particle volume and surface area. A typical SMD for espresso grinds is around 40 micrometres. That’s about the diameter of a human hair. 

Condensing the information from the graphs shown in Figure 2 to just one number for each sample gives a new insight into how particle sizes shift after the grinder heats up. This time we can observe a clear trend: the SMDs of all four “hot” samples increased by 8 to 27 per cent compared to the “cold” grinds, an indication that the grinds have indeed become somewhat coarser.

Why grinds become coarser when the grinder heats up and how it impacts extraction
Figure 3: Increase in the mean diameter d(3,2) shows a clear correlation with reduction in extraction times for all four samples. Dots: flat burr grinder. Squares: Conical burr grinder.

What’s more, as shown in Figure 3, there looks to be a strong correlation between the increase in SMD and the drop-in shot times. This holds true for both bean origins and both grinders. In all cases, coarser grinds produce quicker shots. 

Why exactly the grinds become coarser when the grinder heats up is still a question to be studied in more detail. One explanation could be that beans fracture differently depending on their temperature. As Christopher Hendon and co-workers showed in their 2016 paper in Scientific Reports, grinding beans cold results in somewhat finer average particle sizes. It would make sense that the opposite happens when the beans are ground at higher temperatures. Measuring fracture force needed to break coffee beans under different conditions is one of the next projects I would like to explore.

What we can take away from this experiment is that coffee grinds become a little coarser when the grinder heats up, and this small shift has a large effect which can account for a reduction in extraction times as much as 10 to 15 seconds. Combined with the smaller effect of increased extraction temperatures resulting from hotter grinds (up to two seconds), I believe now we have a better understanding of what happens to espresso flow when grinder, machine, and baristas work hard on a busy day in the café. 

I would like to thank Dr. David Hoxley and the Webb Lab at LaTrobe University for their kind assistance with the grind particle size measurements. 

This article appears in the February 2020 edition of BeanScene. Subscribe HERE

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