How Does Coffee Machine Water Temperature Affect Extraction? How is Crema and Oils Formed? Do Oils Affect Flow Rate?

The Importance of Water Temperature in Espresso Extraction

When FrontStreet Coffee discusses espresso extraction, we typically emphasize parameters such as grind size, ratio, time, and pressure, rarely mentioning the coffee machine's water temperature! However, this doesn't mean that extraction water temperature is unimportant. It's simply because many coffee machines' boiler temperatures cannot be easily adjusted, so we treat water temperature as a fixed value and adjust other extraction parameters to work with it, thereby achieving proper extraction.

Water Temperature's Impact on Extraction Efficiency

As the solvent for coffee, water temperature determines extraction efficiency! The higher the temperature, the higher the extraction efficiency; the lower the temperature, the lower the extraction efficiency! This is well-known. But did you know? For "aggressive" pressurized extraction, water temperature affects not only extraction efficiency but also other aspects!

Another Impact of Water Temperature on Pressurized Extraction - Flow Rate

A couple of days ago, FrontStreet Coffee shared reasons for espresso crema hardening, mentioning that when extraction parameters are too efficient, it causes rapid release of carbon dioxide, making the crema richer! Water temperature is one of these factors!

The Relationship Between Temperature, Carbon Dioxide, and Flow Rate

What we call extraction efficiency can be simply understood as the speed at which substances are extracted from coffee. When extraction efficiency is higher, all substances, including carbon dioxide, are released more quickly. In pressurized extraction, released carbon dioxide gets wrapped by proteins and oils, forming numerous bubbles. These bubbles, when pressed through the coffee puck, become crema! Before being pressed out, these bubbles create obstacles during extraction, reducing the gaps through which hot water can flow, thereby slowing the flow rate.

When water temperature is not high enough, extraction efficiency decreases, so carbon dioxide doesn't release rapidly in a short time. Without the obstruction of bubbles, the flow rate naturally becomes faster. We can observe this at the beginning of each day when the machine is first turned on. Although the temperature doesn't quickly rise to the target temperature, the group head can still extract. If you use the same parameters to extract espresso at this time, you'll find that the coffee flows faster, and the time to reach the target liquid weight is significantly reduced. Of course, the taste obviously won't be very good.

Commercial vs. Home Machine Differences

Additionally, most single-head home coffee machines have relatively small boilers, so they can store less hot water! After continuous extraction, we can observe that espresso extraction flow rates become unstable. This occurs because the boiler cannot heat up quickly enough after replenishing water, causing differences in extraction flow rate and coffee taste! This is also the biggest difference between commercial and home machines. Since flow rate is also a controlling factor for extraction efficiency, water temperature, which plays two important roles, is very important!

What Temperature Should Coffee Machine Water Be Set To?

The vast majority of coffee machines are factory-set to 92°C, which is a very reasonable temperature. Without special requirements, we generally don't need to adjust the water temperature. Unless you're extracting lighter roasted coffee or your grind isn't fine enough, you can raise the water temperature to increase extraction efficiency, but this should be determined based on the specific situation. However, try not to let the water temperature fall below 92°C (unless there are special requirements), because the temperature we see is not the extraction temperature!

Why Isn't the Displayed Temperature the Extraction Temperature?

Although we can see the water temperature through the coffee machine's built-in thermometer, in reality, this thermometer only displays the water temperature inside the boiler, not the temperature of the water used for coffee extraction.

The Temperature Difference Journey

Hot water travels through pipes from the boiler to the shower screen. These pipes, including the shower screen, absorb heat from the hot water, causing a deviation between the final extraction water temperature and the displayed boiler water temperature. For example, the Lelit coffee machine used by FrontStreet Coffee is set to 94°C, but without preheating, the hot water it releases is less than 80°C. After releasing hot water for preheating, the measured result is around 82°C! From this, we can learn that the thermometer's water temperature is not the extraction water temperature, and without preheating, the temperature drops significantly, affecting coffee extraction.

Therefore, to avoid this situation, we always preheat by flushing water before extracting coffee to make extraction more stable! This also reminds us to be more cautious, as the temperature display doesn't show the actual extraction water temperature.

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