In search of a better brew: The best water for coffee
Over the years, the culture of drinking coffee has grown to be recognized as a human necessity. Making a truly great cup of coffee requires great beans, an expert roaster, the right grind, and proper technique. But an often-overlooked element of brewing coffee at home is what constitutes 98% of the delicious drink’s weight: Water.
Water isn’t just an ‘ingredient’ added to roasted coffee seeds; it can transform the character of a coffee, and make or break its flavor. Water acts as the solvent that extracts flavor compounds from the coffee seed by forming chemical bonds and carrying them away.
While there is more to be learned about how each element in water specifically affects coffee brewing, we do have a somewhat substantial base of established knowledge about water and coffee already. For a start, here are some basic principles.
The chemistry of water in coffee
Not all waters are created equal; they vary wildly in terms of mineral content, chemicals, dissolved gases, pH, TDS, and suitability for coffee.
These elements in water affect the taste or quality of our coffee:
- minerals and other organic substances
- chemicals from water treatment like chlorine
- substances from the water supply system, such as copper and iron
- residues from pollution, microbes and germs
The Speciality Coffee Association of America (SCAA), a nonprofit group within the specialty coffee industry that works to raise the standards of coffee worldwide, published a simple list of standard criteria for brewing water: it should be clean and free of odors, colors, and off-flavors. It should be at pH 7.0 (neutral; neither acidic nor basic), and have around 150 ppm of dissolved minerals (with 75 – 250 ppm considered an acceptable range).
The TDS science bit
TDS stands for ‘total dissolved solids’, a measurement representing the total concentration of dissolved substances in the water, including minerals, salts, and other solids. It is measured in ppm (parts per million) with 50 ppm generally being considered soft water and 180 ppm hard water.
TDS readings vary greatly between different kinds of water. Most distilled water has a TDS of 0 ppm. During the distillation process steam is condensed from boiling water and the result is pure water with no dissolved solids. Spring water, on the other hand, has a relatively high TDS, which can range from 50 to 450 ppm. This is because the water picks up different minerals and salts on its journey through underground rock passages and cracks in the earth on its way to the spring. The result is water with a high level of dissolved solids. Tap water is somewhere in between these two.
The TDS of water not only can affect the initial flavor of a cup of coffee but it can also affect the extraction process. Low TDS waters tend to over-extract coffee. There are little to no solids dissolved in these waters so they have a greater ability to absorb coffee material from the ground beans. This will lead to a coffee that is bitter and dry.
On the other hand, high TDS waters often have high mineral contents and tend to under extract coffee. These waters already have a high level of solids dissolved in them and will have less capacity to absorb coffee material from the coffee grounds. This may lead to a coffee that is sour or lacking sweetness.
Thus, the target TDS of 150 ppm should lead to a properly extracted cup of coffee with balanced flavors and acidity.
Soft or hard water
Christopher H. Hendon, a chemist at MIT, discovered the importance of water in coffee after overhearing a conversation between two frustrated baristas. He teamed up with them, Lesley and Maxcell Colonna-Dashwood – who won the 2015 UK Barista Championship – and they found that different kinds of “hardness” in water bring out significantly different flavors in coffee. They went on to publish their research in the Journal of Agricultural and Food Chemistry that explains why lovers of the drink should worry about more than just beans.
Water hardness is a measurement of the amount of magnesium and calcium that is dissolved in water. Water can be “hard” (full of minerals like magnesium) or “soft” (distilled water falls into this category). These two minerals helps to bring out the best in coffee flavoring compounds. Typically, hard water has a high TDS level, whereas not all waters with high TDS values are “hard”; they could contain other minerals or contaminants while being low on magnesium and calcium levels.
Roasted coffee beans are packed with compounds that give coffee its distinct aroma, mouthfeel, and taste. Those include citric acid, lactic acid, and eugenol (a compound that adds a “woodsy” taste). The amounts vary from one roasted batch of beans to the next, giving you an enjoyably different sensory experience each time.
Calcium and magnesium are metals dissolved as charged particles in water (Ca2+ and Mg2+). The positive charge is important because most of the flavour compounds in coffee are negatively charged when dissolved in water, which means they are attracted to the positively charged metal ions, and are good at extracting coffee flavor. The more eugenol the water hangs on to, for example, the woodsier the taste of your coffee will be.
Magnesium ions in water aid the extraction of sharp, fruitier flavours, calcium emphasises heavier, creamy notes. Magnesium has a greater effect on extraction than calcium, so water that’s high in magnesium will make coffee with a stronger favor and higher levels of caffeine.
Oh the other hand, soft water results in very bad extraction power. Soft water often contains sodium that has no flavor stickiness. This means that you’ll get a much stronger flavor from the same beans if you use high-magnesium “hard water” in place of distilled or softened water.
To sum it up, it’s most ideal to have hard water as your coffee brewing ingredient. It should contain no chlorine, minimal sodium at less than 10 ppm, with calcium at 51 – 68 ppm, and magnesium (along with other trace minerals such as iron and manganese) making up the rest of the 150 ppm of dissolved minerals.
Carbonate is another important component – at or near 40 ppm, they are in charge of acid buffer capacity. This means the ability to keep the acid level steady. Carbonates do so by locking up acid when there is too much of it around, and releasing acid again when there is a shortage.
Since coffee contains weak acids by nature, and carbonates act as a “buffer” that is antagonistic towards sharper, acidic notes, the acidity of the coffee batch changes depending on the amount of carbonates in water.
In case there is high carbonate level in water, the positive acidity in taste (such as citric, fruity, sour) will be flushed away by the pH buffer. The acidity itself is still there, we just don’t find it in the taste, only the earthiness and dull, flat taste. In case of low carbonate hardness the coffee taste is vinegary and sour. So a small amount of buffer should be kept in your water.
pH is the measure of the acidity or alkalinity of your water: the lower the pH the more acidic your water and the higher the pH, the more alkaline. The scale runs from 1 to 14 with 7 being completely neutral. Though your water should be completely neutral according to the SCAA, pH between 6.5 and 7.5 is acceptable.
The ratio of coffee to water
There is a ratio of coffee to water that is used and the best ratio to use is 17.4 units of water to 1 unit of coffee. This ratio will give you the best results since the solids will be dissolved in your coffee. Strong coffee has nothing to do with bitterness, caffeine, roast profile or content, but has everything to do with the ratio of coffee to water.
The best water temperature
At a high temperature of 195° to 205° Fahrenheit (~90ºC – 96ºC), optimal extraction happens. Water first soaks the grounds to release the coffee oils and dissolve the soluble solids, then it slowly drains through to the cup or carafe carrying these components that make up the flavors and aromas we look forward to.
When cold water is used, it will result in flat and under-extracted coffee, while water that is too hot will lead to the loss of the trademark taste of coffee.
A chemically perfect cup
While the SCAA sets the criteria, these are simply guidelines. It is important to note that while two waters may have the exact same TDS values, they can have totally different aromas and flavor profiles because of the different kinds of solids dissolved in each water.
An extreme example would be if we dissolved 10 grams of salt into a liter of water and 10 grams of sugar into another liter of water. These waters might end up having the same TDS values but they would obviously taste completely different. It all comes down to the balance of minerals in your water.
Brewing great coffee is as much a science as it is an art. Once you learn the principles that underlay the brewing process, you can develop a routine that suits you perfectly – depending on how you’ve mastered the science behind the perfect cup of coffee.