Water temperature plays an important role in manual brewing. According to the Specialty Coffee Association, to properly brew high-quality coffee, your water should be between 90°C and 96°C (195°F to 205°F).

However, alongside this, your brewing water should also remain at a stable temperature throughout the entire process. This is a concept known as thermal stability – an essential part of manual brewing.

While temperature-controlled kettles can certainly help to improve thermal stability, your brewer also has an effect on it.

To learn more about how to maintain a stable brew temperature, I spoke with two coffee professionals. Read on to learn more on why thermal stability is so important. 

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

What is thermal stability?

There are many variables to keep track of when brewing filter coffee. These include coffee to water ratio, total brew time, grind size, agitation, and more. Temperature, however, is one of the most important.

When coffee is brewed, a number of soluble volatile compounds that contribute to flavour – such as oils and acids – are extracted. However, as different compounds are extracted at different temperatures, ensuring that you keep your temperature stable throughout the brew is key. 

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

“Thermal stability in manual brewing means more consistency in your extraction,” he tells me. “Consistency helps to reflect the coffee’s inherent flavours and lets you prepare the best-tasting coffee.”

Any significant temperature fluctuations (3°C in either direction is significant enough) can result in either too many or too few compounds being extracted. Either way, this can have a notable impact on flavour.

If brewing temperatures are too high, the water extracts more volatile compounds, as well as increasing the rate of extraction – thereby reducing the total brew time. 

This makes it harder for the brewer to control the extraction rate, and can result in overextracted coffee, which tastes bitter and hollow.

Lower brewing temperatures, meanwhile, prolong the extraction process as the water molecules have less kinetic energy, meaning that they move around less. If your water temperature is too low, you risk underextraction, which means your coffee will taste sour and astringent.

Moreover, if the coffee bed is extracted with brewing water which is not at a stable temperature (for example, if the temperature of water decreases over the course of the brew time), some of the coffee grounds may be extracted more than others. This leads to coffee which is both underextracted and overextracted at the same time.

Explaining thermal stability

While maintaining thermal stability during manual brewing might seem easy enough with the aid of equipment like a temperature-controlled kettle, it can actually be more difficult than you might think. 

This is often because of the shape and material of the brewer.

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

“The wide opening at the top of a standard filter coffee brewer allows more heat to escape into the surrounding air, rather than retaining it in the slurry,” he explains. “Therefore, manual brewing is more prone to heat loss during extraction. 

“This means the material of your brewer greatly affects thermal stability,” he adds.

Different materials used to manufacture filter brewers have different thermal masses. Thermal mass is a measurement of how much heat a material needs to absorb before its temperature increases. 

Materials like concrete and stone, for instance, have a high thermal mass, because they need to absorb a lot of heat before their temperature rises. 

“A filter brewer with a higher thermal mass requires more energy from the brewing water, which it absorbs through heat,” Hyunjun says. “This means it takes longer to warm up.

“On the other hand, materials with a lower thermal mass require less energy to warm up, and they absorb heat out of the slurry much more quickly.”

Hyunhwa, meanwhile, emphasises the importance of using a dripper which is made from materials that will retain heat for longer. This is because brewers which lose heat quickly can disrupt the temperature of the water, and therefore affect extraction.

“Typically, steel conducts heat much faster, while plastics absorb and emit heat at a slower rate,” Hyunhwa tells me. “This means that, generally speaking, plastic filter drippers are more insulated than metal ones.

“However, specially-insulated metal brewers like Steadfast conduct and retain heat more effectively for the best possible thermal stability.”

Comparing different brewer materials

We know different materials have different thermal masses, and therefore we know that they conduct heat at different rates.

But how significant is the difference? And what else should you consider for thermal stability?

Plastics 

Hyunjun tells me that most plastic brewers are made from “a range of polymers, commonly methyl methacrylate, which is also commonly referred to as acrylic”.

Plastics have a high thermal mass (around three times that of steel), which means it takes longer for the material to absorb heat from the brewing water.

In fact, plastics emit heat around 20 times slower than other materials, which helps to maintain a stable brew temperature for longer periods. 

Ceramic and porcelain 

Hyunhwa says ceramic and porcelain are popular materials for manual brewers because of their “high insulating qualities and visual appeal”.

Ceramic has a slightly lower thermal mass than plastic, which also makes it a good material for ensuring thermal stability.

However, since ceramic is a dense material, it absorbs more heat than plastic, even at the same temperature ranges.

“Ceramic brewers absorb around four times more heat from your brewing water than plastic ones,” Hyunhwa explains. 

Glass

Glass is another popular material for manual brewers. 

Alongside its visual appeal, it also has a significantly lower specific thermal mass than plastic and ceramic brewers, so it needs less energy to reach the right temperatures for brewing. 

“Glass filter brewers also tend to weigh around half as much as most ceramic or porcelain ones,” Hyunhwa says. “This means they are more insulated when it comes to the water temperature.”

Steel

Steel has a very low thermal mass compared to other materials, so it can heat up much more quickly. This means it requires much less time and energy to reach the optimal brewing temperature. 

“However, because of the thinner materials, steel brewers can absorb more heat from the brew and release it into the atmosphere,” Hyunjun says. 

As a result of this, he recommends using steel drippers for recipes which have a total brew time of three minutes or less. 

However, Hyunjun also points out that using an insulated steel filter brewer, such as the one made by Steadfast, means taking less energy and time to reach the ideal temperature, as well as better heat retention over time.

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

“This means it can reach the target temperature more quickly because of the steel’s lower thermal mass, while the insulated jacket improves thermal stability throughout the brew,” he adds.

Hyunhwa explains that Steadfast recently carried out a study to compare plastic and ceramic brewers to their dripper.

“Among the three brewers, the Steadfast reached the target temperature the fastest, and maintained the temperature for longer,” Hyunhwa says.

He adds that alongside the insulated leather jacket, Steadfast is also specifically designed to maintain thermal stability during manual brewing.

The dripper has a trapezoid wedge shape, with walls which slope at a 31° angle (similar to a conical brewer like the V60). This helps to not only maintain brew temperature, but also improves flow rate during extraction. 

Can you improve thermal stability?

Hyunhwa tells me that when filter coffee is brewed, heat loss generally occurs across the surface of the slurry, because of the wide opening at the top of the brewer.

“Because of this, we recommend preheating your brewer to avoid losing too much heat from your brew water,” he adds.

Denser materials like ceramic and plastic should be more thoroughly preheated than steel and glass, as they take longer to heat up. This prevents them from absorbing too much heat during the initial phases of extraction.

Ultimately, using high-quality brewers made from durable materials helps you to achieve a consistent, stable temperature when preparing filter coffee.

“With the Steadfast, we focused on convenience, durability, and making sure people enjoyed brewing coffee with the dripper,” Hyunjun says. “To achieve all of this, we used SUS304 – a food-grade stainless steel known for its durability – and a leather jacket made from plant-based natural tannins.”

Your dripper material undoubtedly has an impact on thermal stability and brew temperature, which in turns affects extraction and the flavours you taste in your cup.

When choosing a brewer, consider the materials you use and how they can affect thermal stability. By selecting a brewer with a lower thermal mass, or one that is insulated, you are more likely to maintain the required temperature of your brewing water.

This way, you can focus on the many other extraction variables that are important for preparing high-quality coffee.

Enjoyed this? Then read our article on how temperature can impact your experience of coffee.

Photo credits: Steadfast

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