Dr Monika Fekete explores the impact of grind temperature on extraction temperature and why shots tend to speed up over time. 

Dialing in a delicious espresso shot is great way to start your day, be it at work or at home. As the day goes on, you might find that shots speed up and you need to adjust your grinder a bit finer to get the same result. It seems like your grind profile has changed. 

Does this sound like a familiar story? It might be a common observation, but to date I’m unaware of a viable explanation backed up by solid data. 

The reason is probably a result of many factors at play. Increasing temperatures over time is clearly of crucial importance. Or, as Colin Harmon of 3FE Coffee has been quoted as saying, “the daily process of ‘dialing in’ may as well be called ‘warming up’.” 

Temperature affects practically all aspects of the brewing process. We need to take into account all temperature-dependent variables involved and investigate their effects individually. This way, we might gain a better understanding of what happens when you give both grinder and machine a good workout on a busy day in the café.   

In the February issue of BeanScene, we looked at the effects of brew water temperature on extraction. I used the Rancilio RS1 espresso machine in an experiment to determine if we can control brew water temperature precisely. However, extraction temperature is determined by the combination of brew water and grind temperature. This time, I would like to focus on the latter. 

Grind temperature is generally less tightly controlled, even though it can vary significantly over time. Even a stone-cold grinder will spit out grinds about 10°C hotter than the beans due to the heat of friction. As the grinder heats up, grind temperature can reach up to 50°C. So, on a 20°C day, you’re probably going from grinds at 30°C at the start to 50°C at peak times. This significant difference will have an impact on the temperature the coffee is extracted at. 

In the experiment I’ll discuss here, I focused purely on the temperature of the grinds, rather than any potential change in grind particle distribution as a result of grinding hotter. To be able to do this, I had to separate the two effects. To eliminate variation in grind size, I ground all coffee necessary on a Mythos 1 grinder, and then I mixed the grinds thoroughly.

I let the grinds degas overnight to make sure they all retained the same levels of CO₂ and volatiles. This way, there were no significant degassing effects over the course of the experiment. Stale coffee is of course not ideal for making great-tasting espresso, but this time I didn’t need to worry about putting them in front of a tasting panel. I dosed 25 x 22 grams of the mixed grinds into sealed containers and separated them randomly into five groups. 

The five groups of containers were all stored for a minimum two hours at set temperatures ranging from -13°C in the freezer to 8°C in the fridge, 21°C room temperature, and 45°C and 55°C in a thermostat. I used such a wide range of storage temperatures to be able to demonstrate the effect of grind particle temperature more clearly. Realistically, your grinds would more likely sit in the 25°C to 50°C range. 

Once the grinds reached the desired temperatures, I removed them from storage one by one and immediately used them to brew espressos on a Rancilio RS1 operating at 93°C brew water temperature. The pump was set up to deliver 80 grams of water each time. The cups used to hold the coffees were pre-warmed to 36°C. 

I recorded extraction variables, such as shot weights, times, extraction yields, and temperature in the cup. 

As expected, shot temperature in the cup increased with increasing grind temperature (see Figure 1). When the coffee grinds were very cold, the cooling effect on the shot was, of course, quite dramatic. In the more practical range, increasing grind temperature from 21°C to 55°C led to an average 2°C increase in shot temperature. 

There wasn’t much variation in shot weight, which makes sense as coffee dose and the amount of water added were kept constant. No significant correlation between grind temperature and shot weight was measured.

In a similar way, total dissolved solids and extraction yield were also not strongly affected by grind temperature. Shot times, on the other hand, showed a dramatic difference. The shots were speeding up as the grinds were getting hotter. This effect seemed to tail off past 45°C (also shown in Figure 1).  

This aligns with the observations in the café. Increased grind temperature alone, with no change in grind particle distribution, can account for faster shots. But why does this happen?

The flow rate of liquids depends on a number of factors. (To calculate flow rate of coffee, divide shot weight by shot time). Imagine a liquid flowing through a tube. The flow depends on the following:

• the pressure drop between the ends of the tube (or the puck in our case)
• the length of the tube (or height of
  the puck)
• the diameter of tube (or the basket)
• the viscosity of the liquid (how easily a liquid can flow. The lower the viscosity, the faster the flow. Water can flow faster than honey due to its lower viscosity.)

In our experiment, most of these variables were kept constant. Pressure always drops from pump pressure to atmospheric pressure by the time the coffee drips out of the basket. The dimensions of the puck were the same, as were particle size and tamping to my best ability. This leaves us with viscosity to blame for the faster shots.

Viscosity of liquids generally decreases with temperature (imagine warm honey flowing faster than cold honey). Coffee is no exception. 

I measured the viscosity of an espresso sample over a range of temperatures with the help of a rheometer. The results show that viscosity indeed drops off as temperature increases, just enough to account for the observed increase in flow rate (see Figure 2). 

Please note that this measurement was done using coffee brewed with grinds at room temperature and doesn’t take into account the increased solubility of all flavour compounds at higher extraction temperatures. This aspect would need more investigation. 

In conclusion, we need to appreciate that a complex array of changes happen with shifts in temperature. Separating variables and studying them one by one helps to put the pieces of the puzzle together. This set of experiments showed that grind temperature affects the flow rate of espresso shots by altering their viscosity. Colder grinds lower extraction temperature and therefore increase viscosity, which results in slower shots. As grinds heat up, extraction temperature increases, while, in turn, viscosity becomes lower – at least up to a point – and shots speed up. 

I would like to thank Absolute Espresso Services and United Supplies for providing the Rancilio RS1 machine for the experiments. 

This article appears in FULL in the June 2019 edition of BeanScene. Subscribe HERE.