Too hot to handle: Is boiling water bad for coffee?
Matt Haw explores how science is attempting to calculate the perfect brewing temperature for coffee, and why for baristas, it’s all about consistency.
Young baristas entering the profession are often taught that it’s a bad idea to brew with boiling water. Some believe that the water can “scald” the coffee, while others maintain that there’s a sweet spot for water temperature.
Even the Specialty Coffee Association of America recommends using water that’s below the boiling point, asserting that water between 90-96°C is ideal for brewing, while 92–94°C is considered a good range for coffee cupping.
In recent years, prominent coffee professionals like James Hoffman, Scott Rao, and Matt Perger have countered the historical norm, actually advocating for the use of boiling water – particularly when making filter coffee.
The impetus behind this is one that many baristas can relate to: a desire for consistency. Theoretically, if the brewing water is always at boiling point, this variable will always be under control.
Many baristas and prosumers may not have access to accurate electric kettles, making anything but boiling difficult to measure. Conversely, many modern coffee machines are able to brew at precise temperatures that can be programmed by the user.
More and more research is suggesting that the idea of “scalding” coffee is inaccurate, but the science around the topic is still being understood. What is clear, however, is that very hot water can extract undesirable flavours from coffee.
In the last decade in particular, a flurry of academic papers has attempted to address the lack of scientific data to support ideal brew temperatures. One paper notes that higher-temperature water facilitates the extraction of pyridines, a flavour compound associated with bitter, astringent, roasty, and burnt notes.
In a recent video on brewing dark roasts, barista and coffee influencer James Hoffman claimed that this is especially true of more developed coffees. Because it’s easier to extract soluble compounds from darker roasts, he recommends not using water straight off the boil.
In this case, he believes that a brewing temperature of around 80-85°C will yield a “soft, gentle brew” without the over-dominant bitterness, although the same is probably not true for lighter roasts.
What role does water temperature play in extraction?
Water temperature is just one variable in coffee brewing, alongside others like roast level, grind size, and brewing method. Ultimately, coffee brewing is all about extracting soluble compounds from coffee.
Typically, a coffee bean is around 30-40% soluble in water – depending on the density of the bean – but the aim isn’t to extract all of that. Instead, it’s about creating a controlled process that extracts the desirable compounds, but not those that result in excess bitterness and other undesirable traits.
The more energy and interaction between molecules during brewing, the more extraction takes place. Heat is energy – too little leads to under extraction, and vice versa.
Scientific research into brew temperatures broadly agrees that hotter water increases extraction and the total dissolved solids (TDS) in a cup. However, there are some stark differences in the results of some investigations.
A 2009 paper published in Food Research International found that “the chemical-physical parameters of sampled espressos increased proportionally with extraction temperature”. This suggests that the effectiveness of the extraction process increases with temperature.
Surprisingly, this same study reported that samples of arabica brewed at 100°C and 110°C had “sensorial attributes typical for a fine espresso coffee”, suggesting that boiling water isn’t that bad at all.
In contrast, a 2020 paper published in Nature reported that using “colder” water (87-93°C) had “no appreciable impact” at a sensory level. The authors concluded that the “brewing temperature of the water plays a minimal role in the sensory properties of the coffee as measured by a trained descriptive analysis panel”.
Coffee professionals are acutely aware of the possible variables that might account for any inconsistencies in these results. Differences in roast level, the altitude at which the sample coffee was grown and the way in which the beans were processed all contribute to the final density of the bean.
What does the science say?
Without ensuring that such variances between research samples are controlled, it will remain difficult to establish a definitive ideal temperature for all coffee brewing. The aforementioned Nature paper, however, highlights two important pieces of previous research.
Firstly, it asserts that “50% of measured coffee components are extracted within the first quarter of the brewing volume”. Most of these are extracted early and then plateau in terms of the total amount removed from the coffee grounds.
Organic acids are among the earliest compounds to be extracted, explaining why under extracted coffee can be astringent.
This research has, to a certain extent, vindicated Hoffman’s recommended brew recipe for dark roasts. While he admits that his reasoning involves speculation, he advises blooming with water just off boiling to extract some “additional complexity” before finishing the brew with 80°C water.
Essentially, his advice is to start hot and end cool.
The research also explores lesser-known flavour compounds that are slower to extract, such as methyl cinnamate, which exhibits flavours of sweet balsam, strawberry, and cherry, as well as indole, which although small quantities, can impart a floral aroma. These flavours are often associated with high-scoring coffees.
Conversely, less desirable flavours increased towards the tail end of extraction and TDS ranges. This suggests that continued contact with boiling or close to boiling water will eventually yield flavours that negatively impact the cup profile.
Lower brewing temperatures, therefore, allow time for those more desirable slow extractors to be pulled out without risking over-extraction.
This leads to the second surprising revelation of the research: the environmental impact of brewing at higher temperatures. The authors report that energy usage in cafés is responsible for 45% of the entire coffee industry’s carbon dioxide emissions.
It’s thought that much of this energy usage stems from maintaining high temperatures in boilers inside coffee machines. Perhaps, then, the search for the ideal brew temperature is less about extraction and more about sustainability.
With the introduction of energy-saving technologies like induction heating – featured in the Heylo Coffee and Milk Modules – inefficient boilers may be on their way out. Likewise, Carimali machines with motion sensors can go into standby mode when not in use, eliminating wasteful heat generation.