Why does 92 degree espresso taste different?

Water temperature affects both the flavor of espresso and the speed at which it flows. Higher temperatures extract more from the grounds and can push a shot toward bitterness, while lower temperatures slow extraction and can leave the cup thin or sour. Machines commonly ship set somewhere between 85 and 95 degrees Celsius, with many leaning to 92 to 96.

Older machines fixed the boiler temperature at the factory, so baristas treated heat as a given and adjusted grind, dose and ratio instead. Modern machines let users set the temperature, which turns it into an adjustable variable. Anyone used to how to use a drip coffee machine will find the same logic applies to espresso.

Many coffee enthusiasts, when discussing espresso extraction, tend to focus on grind size, dose, ratio, and pressure, while overlooking an equally critical variable—water temperature. In reality, water temperature not only affects the flavor profile of the coffee but also alters the extraction flow rate, and there is even a temperature difference between the boiler temperature and the water temperature that actually contacts the coffee grounds. This article, from the perspective of Front Street Coffee, systematically examines the mechanisms by which water temperature affects espresso extraction, analyzes the pros and cons of high and low temperatures, and, drawing on the hypothetical experiments in Rao's "The Extraction Science of Espresso," explains how to choose appropriate water temperature settings under different dose ratios.

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In the past, when Front Street discussed espresso extraction, the focus was usually on parameters such as grind size, dose, ratio, extraction time, and pressure, and considerable space was also devoted to analyzing operational details like receiving grounds, distributing grounds, and tamping. In comparison, the topic of extraction water temperature was rarely singled out for detailed discussion.

This does not mean that water temperature is unimportant for espresso extraction, but rather that in earlier times, the boiler temperature of many machines was fixed at the factory, making adjustments inconvenient. Therefore, in practice, water temperature was often treated as a given condition, and other variables were used to adapt to it.

Today's new-style machines, as well as high-end models from the past decade, basically allow users to set the water temperature themselves, which has turned water temperature from a fixed value into an important parameter that can be flexibly adjusted during extraction.

What water temperature setting is generally appropriate for a coffee machine?

Generally, a coffee machine water temperature between 85 and 95 degrees Celsius is considered a reasonable range, and some machines have initial settings leaning toward the higher range of 92 to 96 degrees Celsius. Anyone who has studied pour-over knows that the higher the water temperature, the stronger the extraction efficiency and the more coffee substances can be dissolved; conversely, when the water temperature is lower, extraction efficiency decreases, and naturally fewer substances are dissolved.

The logic of water temperature in espresso is consistent with that of pour-over. If the temperature is set too high, burnt bitterness and woody flavors are likely to appear, but high temperatures can also extract thicker crema. In the early years, espresso tended to be bitter partly because the coffee beans were roasted very dark, and partly because the machine boiler temperature was set high, which also happened to meet the need for producing abundant crema. The extraction rhythm also differs, with time usually needing to be controlled between 20 and 25 seconds, because if it drags on too long, burnt and sawdust-like flavors emerge.

When the water temperature is too low, the extracted espresso will be very sour, and the crema will appear thin. Precisely for this reason, in the past the water temperature settings of mainstream espresso machines were rarely below 92 degrees Celsius.

Water temperature affects the flow rate of coffee extraction

When the water temperature is higher, the coffee liquid flows out more slowly; when the water temperature is lower, the flow rate is somewhat faster. High temperatures cause the puck to expand more quickly and simultaneously release gases. These gases block the water flow until they are compressed into crema and carried out, at which point the coffee flow rate gradually increases. The higher the temperature, the more bubbles are produced, and the greater the obstruction to water flow.

When the temperature is low, the coffee flow rate is relatively fast. A rather extreme example is this: when the machine is first turned on and the coffee boiler has just begun heating, the brew head can actually extract at this point, but the water temperature is very low, only about 60 degrees Celsius. If the same parameters are used for extraction, the time will be greatly shortened, and the coffee will flow very quickly.

Similarly, if a small-boiler coffee machine is used for high-intensity continuous shots, you will also find that espresso extraction gradually becomes unstable and the flow rate gets faster and faster. Part of the reason lies in the fact that after the boiler refills with water, the temperature cannot recover in time.

Boiler temperature is not equal to extraction temperature

What we can set is the water temperature inside the boiler, but as water flows from the boiler to the point where it extracts the coffee, it still has to pass through the dispersion screen and the portafilter (basket). If more than 10 minutes have passed since the last extraction, the temperature of the brew head (dispersion screen) will be relatively high. At this time, if the portafilter is locked on directly for extraction, the water in the boiler will absorb heat as it passes through the dispersion screen, causing the initial water temperature to be higher than the boiler setting.

To avoid this situation, before locking on the portafilter, the brew switch is first turned on to release water and lower the temperature, allowing the temperature to fall back to close to the set level.

The temperature of the portafilter itself also affects the initial water temperature of extraction. If the portafilter is at room temperature, at the beginning of extraction the portafilter will absorb heat from the water, so the actual water temperature will be lower than preset. Therefore, before receiving grounds, hot water needs to be released to preheat the portafilter, which is the same principle as preheating the dripper in pour-over coffee. Of course, some people choose to keep the portafilter locked on the brew head to maintain its temperature when not extracting, but Front Street does not recommend this practice; for the specific reasons, click here to view.

And during the extraction process, it is also impossible for the extraction temperature to remain consistent with the boiler water temperature from beginning to end.

What temperature setting is appropriate for daily espresso extraction?

Here, let me cite a hypothetical experiment from Rao's "The Extraction Science of Espresso": Italian baristas habitually extract espresso at 85 to 91 degrees Celsius, while American baristas prefer 92 to 96 degrees Celsius. The reason for the difference lies in the different ratios used in the two places—the Italian standard is to extract 1 ounce of espresso from 7 grams of coffee grounds, while the American standard is to extract 1 ounce of espresso from 18 to 21 grams of coffee grounds.

If 30 grams of water at 88 degrees Celsius is mixed with 7 grams of coffee grounds at 27 degrees Celsius, and then 38 grams of water at 95.3 degrees Celsius is mixed with 21 grams of coffee grounds at 26.7 degrees Celsius, the final mixing temperature in both cases is theoretically 82.8 degrees Celsius. Of course, this is only a hypothetical experiment, but it illustrates a fact: coffee grounds absorb heat, and the larger the dose, the more heat they absorb. To reach a certain extraction temperature, the preset water temperature must be increased.

Book cover of "The Extraction Science of Espresso"

The ratio we use today is similar to the American baristas' approach, both extracting a single espresso with a large dose, so in terms of temperature selection, it needs to be adjusted to 92 to 96 degrees Celsius.

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The above content is compiled by CoffeeHunters, a coffee news website.