Traditional coffee conventions dictate that the best water temperature for coffee brewing falls within a narrow range of temperatures, between 195 and 205 degrees Fahrenheit. According to this precept, brewing coffee with colder water will result in under-extraction, while hotter water will produce over-extraction.
But, if you're familiar with the most recent trends in coffee brewing, you'll notice that some experiments suggest that brewing coffee using a French press can have better results with boiling water.
Something similar has been happening with popular Aeropress recipes with boiling water that seem to produce great results. So, what's the truth behind these contradictory findings and trends?
Let's find out exploring the basics of coffee extraction to learn how and when it's most appropriate to play with water temperature to improve coffee brewing.
Water and coffee extraction
Coffee beans are made of carbohydrates, lipids, proteins, cellulose, sugars, acids, and caffeine. The most important thing to know about coffee is that its flavor comes from the soluble substances it contains. Soluble substances are what water dissolves during the brewing process.
The coffee compounds that dissolve in water vary depending on temperature. The first to come out of coffee grounds is the more soluble substances, so they are the first ones to appear in your brewed coffee. They are primarily acids, so sour flavors are prevalent when the water temperature is lower than optimal.
Some people claim that coffee grounds burn if the water temperature is too high. However, it's unlikely—coffee beans roast at over 200 °C, more than double than boiling water. What happens is that the less soluble compounds in coffee grounds dissolve at higher temperatures, and some of them might get into the brew, increasing bitterness and other unpleasant flavors.
Other critical factors in coffee extraction
Although water temperature tends to be quite a thing in coffee brewing recipes, other factors are more important and tend to play a huge role in taste and aroma.
An espresso machine produces a radically different cup of coffee than a dripper or a siphon. Devices alter how water and coffee mix together and depending on their construction, they can retain heat in different ways.
Plastic and glass have been showing better heat retention, while metals and ceramic are the worse in this department.
Heat retention, pressure, and turbulence change depending on the brewing device, which changes coffee extraction.
Although water makes more than 90% of a cup of coffee, it's largely underestimated. However, it's well known that using tap water can be horrific for coffee taste and aroma.
It all depends on water hardness, which is primarily caused by calcium and magnesium salts. These minerals make the water hard because they dissolve in it.
Hard water can be positive for coffee brewing since these compounds bind with fatty acids from coffee to form insoluble substances that get deposited on the water's surface along with other sediments. For this reason, zero hardness as in distilled water isn't ideal.
Excessive water hardness hampers coffee extraction. As these minerals dissolve in water, they reduce water's ability to extract coffee soluble compounds too.
Coffee grind size is so important for coffee taste and aroma that most espresso experts play a lot more with grind size than with any other variable when dialing an espresso.
More importantly, obtaining an even grind size distribution is critical to get a more pronounced and fully differentiated aromatic profile. Many flavors and scents get messed up when the grinder produces too many fines or boulders. Why? Because an uneven grind makes an equally uneven extraction.
Brewing coffee with uneven grounds is like frying potato chips with different sizes and thicknesses. Once the chips in different sizes start cooking in hot oil, some will burn, and others will be undercooked if we take them out at the same time.
Something similar happens when brewing coffee because it's impossible to let larger coffee particles for a longer time than smaller ones, so the sour taste from large particles gets into the cup simultaneously as the bitter taste from fines.
Most coffee brewing recipes play little with turbulence. Turkish coffee is an exception, as tradition demands to stir the coffee in boiling water during the brewing process.
Some World Aeropress Champions use turbulence to get different extractions, and one of the most notorious that I can recall now is Carolina Garay's recipe. Carolina stirred the coffee and the hot water with wooden chopsticks before pressing the plunger.
Manual brewing gets different results than electric drip coffee makers because of turbulence, along with the uniformity of water flow.
Automatic coffee makers create little turbulence depending on their showerheads. It's the opposite of manual brewing, particularly when using a gooseneck kettle that agitates the coffee bed, affecting coffee extraction.
Extraction changes radically depending on how long coffee grounds are in contact with water.
Normally, to get a good brew, experts recommend using shorter times as the temperature increases. For this reason, cold brew usually takes hours, while a pour-over with boiling water takes a few minutes.
The bottom line is that water temperature is important for brewing coffee, but can't make a great cup of coffee alone without considering the grind size, brewing time, and even turbulence.
Fortunately, a barista can control these variables through manual methods, brewing device selection, and grinder calibration. In the end, all this information serves to improve one's understanding of how to get the best cup of coffee possible.