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Decades of trial and error have shown that most people prefer espresso extracted between 85-95°C (A Illy and R Viana, 1995). For lighter roasted, special coffee, the range typically used is rather narrower – 90-95°C. Within that small range though, there are still considerable variations in flavor to explore. Higher temperatures tend to flatter lighter roasted or more acidic coffees, by bringing out the sweetness; while lower temperatures can reduce the roasty, ashy flavors that might otherwise be found in darker roasts. In this post, we’ll investigate why this might be and why changing the temperature doesn’t always have the effect you’d expect.

 

Which Machine?

Let’s get this out of the way first: not all machines will respond to a 1-degree temperature change in the same way. Most modern, multi-boiler machines do a very good job of delivering a precise brew temperature. However, be aware that older dual boiler machines may deliver slightly different temperatures at each group, depending on how close the group is to the heating element or the temperature sensor in the boiler. Controlling the brew temperature of single boiler heat exchange machines is more complicated: changing the boiler temperature by 1 degree does not necessarily increase brewing temperature by the same amount, and the range of temperatures available to you will depend on the design of the machine.

 

How Much Difference Does 1 Degree Make?

The brewing temperature is not only determined by the water temperature at the group. Matt’s post on temperature equilibrium, and the more detailed model in the excellent follow up from DIY Coffee Guy, explain that changes in grinds temperature can have a big effect on the effective brewing temperature. During service, a grinder might easily go from room temperature to 50°C or higher – enough to change the effective brewing temperature by several degrees. It’s well worth experimenting with reducing the brew temperature during busy periods to account for this.

Shot time also affects brewing temperature. During a slower shot, the water has more time to transfer its heat to the coffee and the surroundings, resulting in a lower effective brewing temperature. These factors are perhaps harder to control, but if they’re not taken into account they could easily offset the effect of changing the group temperature by one degree.

 

Temperature and Extraction

In coffee brewing in general, increasing the temperature increases the rate of extraction. Most compounds in coffee are more soluble at higher temperatures, so they will extract more easily.

There is one important exception: gases such as CO2 actually become less soluble at higher temperatures. CO2 plays an important role in espresso extraction, interfering with extraction by creating added resistance to the flow of water. While this could conceivably make the relationship between temperature and extraction less linear in espresso than in filter, increasing temperature still increases extraction in espresso (D Albanese et al, 2009; S Anduesa et al, 2003).

Extraction yield only tells one part of the story, though – two espressos extracted to exactly 20% at different temperatures will taste quite different. To understand why, we need to look more closely at some of the individual compounds extracted in espresso.

 

The Effect of Temperature on Individual Compounds

While most compounds do become more soluble at higher temperatures, the size of that effect is quite different for different molecules. In The Coffee Brewing Handbook, Ted Lingle shows that as brewing temperature increases from 70°C to 94°C, the amount of sucrose extracted increases, while the amount of citric and malic acids remains fairly constant (TR Lingle, 1996). Similarly, the extraction rate of phenolic compounds (which bring smoky, burnt, spicy, and bitter aromas) is more sensitive to changes in temperature than the extraction rate of chlorogenic acid or caffeine.

So increasing the temperature doesn’t just increase extraction, but also changes the proportion of the different compounds extracted. This means that changing the brew temperature will alter the taste balance, even if the total extraction is kept constant.

The balance of aroma compounds changes with temperature in a similar way. Researchers found a higher concentration of ketones and aldehydes, associated with fresh and fruity flavors, in espresso brewed at 92°C than at 88°C (S Anduesa et al, 2003). At 96°C and above, there was an increased concentration of pyrazines, which are associated with roasty, earthy, and musty flavors.

At higher brew temperatures, some compounds will even start to break down. At temperatures above 96°C, the same researchers found lower concentrations of chlorogenic acid and trigonelline, because these molecules were breaking down in the espresso. This will also affect the taste balance of the shot: at high temperatures, chlorogenic acid breaks down into caffeic and quinic acids (K Izawa et al, 2010), which are more bitter than chlorogenic acid.

It’s clear that temperature has a distinct effect on the taste balance and aroma of espresso, and is a tool that can help get the most sweetness, and the most desirable aromas, out of a coffee. Changing the temperature affects different compounds in quite different ways though, which makes it hard to predict the exact effect of the change on the flavor of the espresso. Finding the ideal temperature is, as always in coffee, a matter of trial, error, and lots of tasting.

 

References

D Albanese, M Di Matteo, M Poiana, S Spagnamusso, 2009. ‘Espresso coffee (EC) by POD: Study of thermal profile during extraction process and influence of water temperature on chemical–physical and sensorial properties’. https://doi.org/10.1016/j.foodres.2009.02.027

S Andueza, L Maeztu, L Pascual, C Ibañez, MP de Peña, C Cid, 2003. ‘Influence of extraction temperature on the final quality of espresso coffee.’ https://doi.org/10.1002/jsfa.1304

A Illy and R Viana (Eds), 1995. Espresso Coffee: The Science of Quality, second edition

K Izawa, Y Amino, M Kohmura, Y Ueda, and M Kuroda, 2010. 4.16 – ‘Human–Environment Interactions – Taste.’ In: H-W Liu and L Mander (Eds). Comprehensive Natural Products II: Chemistry and Biology, pp 631-671.

TR Lingle, 1996. The Coffee Brewing Handbook, second edition, pp 29-33

 

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