Dr Monika Fekete looks at temperature-dependent variables and their consequences on grinder and espresso machine performance.

On a chilly winter morning, hot cup of coffee in hand, I’m once again thinking about the many ways temperature has influenced the liquid I’m drinking.

In the June edition of BeanScene, we investigated how changes in grind temperature affect the viscosity of espresso shots, shedding some light on why shots tend to speed up over time. This time, let’s zoom out and look at a complete map of temperature-dependent variables, and the complex ways they influence the end result (see images). Rather than discussing the details of each effect, I’m going to focus on how they are all interconnected. 

On a cold day, coffee beans will sit at a significantly lower temperature in the bag and in the hopper than they would on a summer day. This, on one level, will affect how the beans age while they wait to be used. The cold weather is perfect for keeping your beans fresher for longer: every 10°C increase in storage temperature speeds up the release of volatiles by 50 per cent, according to a research article published in 1993. This alone is a powerful reason to keep the coffee bags and hopper away from heat sources, such as direct sunlight or the espresso machine itself, as much as practically possible. 

After grinding, that aroma evaporates even faster. Grinds release volatiles three times faster for every 10°C increase. So grinds at 50°C lose aroma 27 times faster than grinds at 20°C – luckily we don’t usually let them sit around for too long.

Warm grinds affect more than just the aroma. As we discussed in the last edition, warmer grinds do increase overall extraction temperature. One effect of this was that the viscosity of the overall hotter espresso decreased, resulting in a faster shot. This effect becomes even more pronounced as the machine itself – including the basket and portafilter – heat up too. 

Another potential effect of the heating grinder is a possible shift in particle size distribution. This is a much-debated question with little solid data to back it up. Until I can report back to you with a set of my own experiments, let me refer to investigations by Maxwell Colonna-Dashwood, former UK Barista Champion and Owner of Colonna capsules. He based his findings on data provided by grinder manufacturers, which, as stated in his blogpost ‘The heat is on – more grinding puzzles’, found that cooler conditions seem to produce more fines than hotter conditions, possibly due to the increased brittleness of colder grinds. This, in addition to the change in viscosity, could help explain why shots speed up as grinder and machine warm up during the day.

One of the most important temperature dependent variables is luckily somewhat easier to control: brew water temperature. This can be quite accurately set on many espresso machines. However, as Ona Coffee Head Trainer Hugh Kelly warns in one of his blogposts, we first need to understand what the temperature reading on the machine actually refers to. 

Hugh says if the reading is taken from the boiler, the water will have changed in temperature before it hits the coffee grinds. Some machines have different heating mechanisms in the group heads to maintain brew temperature, but each is different. So 93.5°C in the boiler could mean less than 93.5°C is actually applied to coffee, and sometimes more.

Calibrating the output temperature with a Scace device and a good quality temperature probe – a job best left to machine technicians – can help get more precise readings. Keep in mind though that brew water temperature will determine extraction temperature together with the temperature of the grind and the metal parts in contact with the coffee. 

According to a heat transfer model first proposed by Matt Perger through Barista Hustle, then refined by a process engineer via TheDIYCoffeeGuy.com, a brew water temperature of 93°C can result in an effective 87°C or even 82.5°C extraction temperature (for grinds at 50°C and 20°C respectively). You are probably brewing espresso a lot cooler than you imagined.

Extraction temperature plays an all-important part in shaping the flavour profile of the final beverage. 

The solubility of all constituents of the coffee bean changes with temperature. Flavour molecules become more soluble as the heat increases, but they do so at different rates, which affects their ratio in the final brew. 

As an example, the solubility of sugars is roughly 2.5 times higher in boiling water than at room temperature. No wonder that higher brew temperatures favour the extraction of sugars, as backed by TR Lingle’s The Coffee Brewing Handbook. The handbook also points out that the concentration of acids remained similar between 74°C to 100°C. Caffeine concentration wasn’t found to change much either in the temperature range used for espresso brewing (which is also in agreement with my investigations as published in BeanScene’s February 2019 issue).

Other researchers investigated the concentration of compounds responsible for fruity flavours and found that these were extracted at higher rates when the brew water temperature was set to 92°C compared to 88°C. At 96°C and above, there was an increased concentration of bitter flavours due to the breakdown of chlorogenic acids at such high temperatures. 

When I put espressos made with brew water at 90°C, 92.5°C and 95°C to a panel of judges, the sample at 92.5°C was found to be the most balanced. Of course, the tastiest combination of extracted flavours will strongly depend on the coffee you are using. As a guideline, we can expect lower temperatures to result in more prominent acidity and less bitterness, but also less body and sweetness. Higher temperatures can help increase body and sweetness, at the cost of introducing more bitterness and slightly reducing acidity.

Higher extraction temperature also means that carbon dioxide gas in the puck will be present at higher pressure, meaning a higher resistance, which will somewhat counteract the observed “speeding up” of hotter shots. 

Lastly, the taste experience strongly depends on the temperature we taste the coffee at. A study published in the prestigious science journal Nature in 2005 found that tiny microscopic channels in our tastebuds are responsible for changing taste perception at different temperatures. These channels send stronger electrical signals to the brain when food is tasted at higher temperatures. 

When we taste coffee hotter, its sweetness and, in particular, its bitterness will become more prominent. As such, keeping your cups at a steady temperature helps deliver more consistent coffee to the customer. 

If you’re adding milk, remember that not only its temperature but also its volume will affect the final drinking temperature, along with the temperature of your dine-in or takeaway cups. 

Take another look at the map of temperature dependent variables. Whether your grinder and machine are just warming up during the day or you have decided to adjust the temperature of the brew water, the consequences are far-reaching. 

This article appears in FULL in the August edition of BeanScene Magazine. Subscribe HERE.