Why is thermal stability essential for handbook espresso brewing?


Water temperature performs an essential function in handbook brewing. In accordance with the Specialty Coffee Association, to correctly brew high-quality espresso, your water must be between 90°C and 96°C (195°F to 205°F).

Nevertheless, alongside this, your brewing water also needs to stay at a secure temperature all through the whole course of. It is a idea often known as thermal stability – a vital a part of handbook brewing.

Whereas temperature-controlled kettles can actually assist to enhance thermal stability, your brewer additionally has an impact on it.

To be taught extra about methods to keep a secure brew temperature, I spoke with two espresso professionals. Learn on to be taught extra on why thermal stability is so essential. 

You may additionally like our article on how your dripper material affects your pour over coffee.

steadfast manual coffee brewer and gooseneck kettle

What’s thermal stability?

There are various variables to maintain observe of when brewing filter espresso. These embrace espresso to water ratio, complete brew time, grind measurement, agitation, and extra. Temperature, nonetheless, is without doubt one of the most essential.

When espresso is brewed, various soluble unstable compounds that contribute to flavour – akin to oils and acids – are extracted. Nevertheless, as completely different compounds are extracted at completely different temperatures, making certain that you just preserve your temperature secure all through the brew is vital. 

Hyunhwa is the Head Barista at Nothin Coffee in Seoul, South Korea.

“Thermal stability in handbook brewing means extra consistency in your extraction,” he tells me. “Consistency helps to mirror the espresso’s inherent flavours and allows you to put together the best-tasting espresso.”

Any important temperature fluctuations (3°C in both course is important sufficient) can lead to both too many or too few compounds being extracted. Both manner, this could have a notable affect on flavour.

If brewing temperatures are too excessive, the water extracts extra unstable compounds, in addition to rising the speed of extraction – thereby decreasing the overall brew time. 

This makes it tougher for the brewer to regulate the extraction charge, and can lead to overextracted espresso, which tastes bitter and hole.

Decrease brewing temperatures, in the meantime, extend the extraction course of because the water molecules have much less kinetic power, that means that they transfer round much less. In case your water temperature is simply too low, you threat underextraction, which implies your espresso will style bitter and astringent.

Furthermore, if the espresso mattress is extracted with brewing water which isn’t at a secure temperature (for instance, if the temperature of water decreases over the course of the brew time), among the espresso grounds could also be extracted greater than others. This results in espresso which is each underextracted and overextracted on the identical time.

electronic gooseneck kettle maintaining temperature stability

Explaining thermal stability

Whereas sustaining thermal stability throughout handbook brewing might sound straightforward sufficient with assistance from tools like a temperature-controlled kettle, it might really be tougher than you may suppose. 

That is typically due to the form and materials of the brewer.

Hyunjun Kim is the co-founder of Nothin Coffee and primary developer of the Steadfast filter espresso brewer.

“The extensive opening on the prime of a typical filter espresso brewer permits extra warmth to flee into the encircling air, slightly than retaining it within the slurry,” he explains. “Due to this fact, handbook brewing is extra liable to warmth loss throughout extraction. 

“This implies the fabric of your brewer drastically impacts thermal stability,” he provides.

Completely different supplies used to fabricate filter brewers have completely different thermal masses. Thermal mass is a measurement of how a lot warmth a cloth wants to soak up earlier than its temperature will increase. 

Supplies like concrete and stone, as an example, have a excessive thermal mass, as a result of they should take up lots of warmth earlier than their temperature rises. 

“A filter brewer with the next thermal mass requires extra power from the brewing water, which it absorbs by way of warmth,” Hyunjun says. “This implies it takes longer to heat up.

“Alternatively, supplies with a decrease thermal mass require much less power to heat up, they usually take up warmth out of the slurry way more rapidly.”

Hyunhwa, in the meantime, emphasises the significance of utilizing a dripper which is constituted of supplies that may retain warmth for longer. It’s because brewers which lose warmth rapidly can disrupt the temperature of the water, and subsequently have an effect on extraction.

“Sometimes, metal conducts warmth a lot quicker, whereas plastics take up and emit warmth at a slower charge,” Hyunhwa tells me. “Which means that, typically talking, plastic filter drippers are extra insulated than steel ones.

“Nevertheless, specially-insulated steel brewers like Steadfast conduct and retain warmth extra successfully for the absolute best thermal stability.”

steadfast coffee brewer with person drinking coffee

Evaluating completely different brewer supplies

We all know completely different supplies have completely different thermal plenty, and subsequently we all know that they conduct warmth at completely different charges.

However how important is the distinction? And what else do you have to contemplate for thermal stability?


Hyunjun tells me that the majority plastic brewers are constituted of “a variety of polymers, generally methyl methacrylate, which can be generally known as acrylic”.

Plastics have a excessive thermal mass (round thrice that of metal), which implies it takes longer for the fabric to soak up warmth from the brewing water.

In reality, plastics emit warmth round 20 occasions slower than different supplies, which helps to take care of a secure brew temperature for longer durations. 

Ceramic and porcelain 

Hyunhwa says ceramic and porcelain are well-liked supplies for handbook brewers due to their “excessive insulating qualities and visible attraction”.

Ceramic has a barely decrease thermal mass than plastic, which additionally makes it a very good materials for making certain thermal stability.

Nevertheless, since ceramic is a dense materials, it absorbs extra warmth than plastic, even on the identical temperature ranges.

“Ceramic brewers take up round 4 occasions extra warmth out of your brewing water than plastic ones,” Hyunhwa explains. 


Glass is one other well-liked materials for handbook brewers. 

Alongside its visible attraction, it additionally has a considerably decrease particular thermal mass than plastic and ceramic brewers, so it wants much less power to achieve the proper temperatures for brewing. 

“Glass filter brewers additionally are likely to weigh round half as a lot as most ceramic or porcelain ones,” Hyunhwa says. “This implies they’re extra insulated in terms of the water temperature.”


Metal has a really low thermal mass in comparison with different supplies, so it might warmth up way more rapidly. This implies it requires a lot much less time and power to achieve the optimum brewing temperature. 

“Nevertheless, due to the thinner supplies, metal brewers can take up extra warmth from the brew and launch it into the environment,” Hyunjun says. 

Because of this, he recommends utilizing metal drippers for recipes which have a complete brew time of three minutes or much less. 

Nevertheless, Hyunjun additionally factors out that utilizing an insulated metal filter brewer, such because the one made by Steadfast, means taking much less power and time to achieve the perfect temperature, in addition to higher warmth retention over time.

“The Steadfast espresso brewer is constituted of high-quality stainless-steel (SUS304) and comes with an outer insulated leather-based jacket, which helps to stabilise the brewing temperature,” Hyunjun tells me.

“This implies it might attain the goal temperature extra rapidly due to the metal’s decrease thermal mass, whereas the insulated jacket improves thermal stability all through the brew,” he provides.

Hyunhwa explains that Steadfast just lately carried out a examine to check plastic and ceramic brewers to their dripper.

“Among the many three brewers, the Steadfast reached the goal temperature the quickest, and maintained the temperature for longer,” Hyunhwa says.

He provides that alongside the insulated leather-based jacket, Steadfast can be particularly designed to take care of thermal stability throughout handbook brewing.

The dripper has a trapezoid wedge form, with partitions which slope at a 31° angle (much like a conical brewer just like the V60). This helps to not solely keep brew temperature, but in addition improves circulation charge throughout extraction. 

Steadfast brewing device

Are you able to enhance thermal stability?

Hyunhwa tells me that when filter espresso is brewed, warmth loss typically happens throughout the floor of the slurry, due to the extensive opening on the prime of the brewer.

“Due to this, we suggest preheating your brewer to keep away from shedding an excessive amount of warmth out of your brew water,” he provides.

Denser supplies like ceramic and plastic must be extra completely preheated than metal and glass, as they take longer to warmth up. This prevents them from absorbing an excessive amount of warmth through the preliminary phases of extraction.

Finally, utilizing high-quality brewers constituted of sturdy supplies lets you obtain a constant, secure temperature when making ready filter espresso.

“With the Steadfast, we centered on comfort, sturdiness, and ensuring individuals loved brewing espresso with the dripper,” Hyunjun says. “To attain all of this, we used SUS304 – a food-grade stainless-steel recognized for its sturdiness – and a leather-based jacket constituted of plant-based pure tannins.”

Steadfast devices on countertop in coffee shop

Your dripper materials undoubtedly has an affect on thermal stability and brew temperature, which in turns impacts extraction and the flavours you style in your cup.

When selecting a brewer, contemplate the supplies you employ and the way they’ll have an effect on thermal stability. By deciding on a brewer with a decrease thermal mass, or one that’s insulated, you usually tend to keep the required temperature of your brewing water.

This fashion, you may give attention to the numerous different extraction variables which are essential for making ready high-quality espresso.

Loved this? Then learn our article on how temperature can impact your experience of coffee.

Picture credit: Steadfast

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