Your morning cup of coffee is more than 98% water, so we shouldn’t be surprised that the water you use to brew your coffee has an enormous impact on flavor. The same coffee brewed with different waters can taste fruity and bright or dull and bitter. And this largely has to do with the dissolved minerals in your water.
In the right amounts, these minerals will aid in extraction and help balance out the acids in your coffee. When there’s too many minerals it can give your coffee a dull taste with a chalky mouth feel.
In the very least, water used to make coffee should be clean, odor free, and not contain any heavy metals or chemicals such as chlorine. But to get the best results from your morning pour-over, we need to know the total hardness, alkalinity, and pH.
We all learned in school that water is comprised of two hydrogen atoms bonded with one oxygen atom-- i.e. H₂O. But water is an extremely effective solvent and tap water contains dissolved minerals, which is expressed as total hardness.
Specifically, total hardness measures the dissolved calcium, magnesium, and sodium bicarbonate in your water. The Specialty Coffee Association recommends a maximum 150 ppm total hardness. (Sometimes bottled water will list the total hardness as mg/L. This is the same as ppm.)
When it comes to brewing coffee, minerals like calcium and magnesium help aid in extraction and impart a positive flavor to the finish brew. There can be, however, too much of a good thing. Calcium in particular is prone to calcify inside boilers and clog valves, which can damage and even destroy equipment over time.
Alkalinity is also referred to as carbonate hardness. It reflects the water’s ability to buffer acids. A coffee brewed with water that has too much carbonate hardness will likely taste dull and flat. You won’t be able to taste the citric and malic acid in your cup as easily. SCA standards are 40-75 ppm alkalinity. Below 40 ppm alkalinity and your coffee will likely taste sour— there isn’t enough bicarbonate to buffer the acids.
For pH, the standards are basically the same as drinking water: 6.5-8. So if your water is safe to drink, the pH is probably fine.
Can you Make Coffee with Tap Water?
Most North Americans and Europeans use tap water to make coffee. If you live in San Francisco or Oslo, that’s perfectly fine! Your tap water meets SCA guidelines. But if you live in Phoenix, Indianapolis, or countless other cities, your water is likely too hard. Excessive amounts of bicarbonate will make your coffee bitter and flat.
If your tap water has too much alkalinity, the only option to correct your water is a reverse osmosis filtration system. A carbon or ceramic filter will not affect the alkalinity.
As the name implies, reverse osmosis filters use a semi-permeable membrane and pressure to reverse the osmosis process, essentially stripping the minerals from the water. The filter will then remineralize the water with mineral cartridges or by adding some "bypass"-- i.e. unfiltered water -- back in. Unfortunately, reverse osmosis filtration systems are expensive and take up a lot of space. For most home baristas it makes more sense to buy filtered water or make your own water for coffee brewing.
How to Make Your Own Water
The easiest way to make sure your water is suitable for making coffee is to simply make you own. Our friends at Third Wave Water sell mineral kits that can be added to distilled water to make an ideal cup. By starting with pure H₂O, you can control which minerals get added to the water in precise amounts. Simply add a mineral packet to water and wait for it to dissolve.
What Temperature Water Should You Use?
According to the SCA, the ideal temperature water for brewing coffee is 90-96° C/ 195-205° F. Personally, I find it depends on the roast degree of the coffee you're brewing.
When brewing light roast, I think you should use the hottest water possible. Light roast coffees are less soluble, so using a higher temperature will help ensure a good extraction.
For medium and dark roast, it’s typically better to use a slightly cooler water temperature to help avoid dissolving more ashy/bitter flavor compounds.
You’ve invested in a good coffee brewer like the Etkin 8-cup Dripper and you’re buying fresh roasted specialty coffee from a local roaster. Don’t mess up the final step by making coffee with less than ideal water.
If you want to learn more about the impact water has on your coffee, we recommend the books Water Quality Handbook from the SCA and Water for Coffee by Chris Hendon and Maxwell Colonna-Dashwood.