Water, obviously, plays a vital role in making a great cup of coffee. It is said that 90% of an espresso is water, while 98.5% of a filtered coffee is water. In fact, water is the component that acts as a solvent; it is the agent that extracts the flavors from the coffee beans. This reality, then, makes it necessary to use quality water and the right temperature when brewing coffee.
There are specifically two things to consider when dealing with water quality: hardness and mineral content. Hardness has to do with the quantity of limescale, or calcium carbonate, dissolved in water. This does not affect everyone the same way, because the quantity of limescale dissolved in water has to do with the local area’s bedrock. The hardness of the water actually affects the brewing process at a chemical level, affecting the rate of solution. Truly, some calcium hardness is necessary, but not very much (the SCAA suggests 4 grains or 68mg/L). Therefore, for those in soft water areas, it may be just fine to use tap water. But for those in hard water areas, it may be necessary to either filter the water or to buy bottled water. Honestly, an active-carbon filter, like those made by Brita and Pur, can help significantly. I use this faucet filter, and it helps tremendously.
Mineral content, or total dissolved solids (TDS), also affects the brewing process. The EPA requirement for safe drinking water is anything less than 500mg/L, but much lower than this is preferable when making coffee (the SCAA recommends 150mg/L). A higher TDS often suggests a harder water. Again, this is dependent upon where you live, and an active-carbon filter can especially help with this issue.
Experimentation is the coffee lovers game. It truly is easy to recognize the difference between coffee brewed with quality water and coffee brewed with not-so-quality water. If you have not tried this, I encourage you to do so. You do not have to be a coffee expert to recognize the difference between coffee brewed with tap water versus coffee brewed with mineral water. All that you need to do is to brew two cups of water in the same way but one with tap water and the other with mineral water. Try it; it’s fun.
When it comes to water, proper temperature is a very important factor to consider. This is because water temperature affects extraction. If the water is too hot, then your coffee will be over-extracted, resulting in a bitter tasting coffee. But if the water is too cold, then your coffee will be under-extracted, resulting in a weak coffee that could even taste sour. That said though, there are certain ways of brewing coffee that require cooler water, but those will be addressed in the future; this post is only dealing with hot coffee.
The perfect water temperature for brewing coffee is between 195-205F (91-96C). Generally speaking, it is said that the lighter the roast, the hotter that your water should be. But if you find yourself within the 195-205 range, you will have a good cup of coffee. Another thing that is often considered when dealing with water temperature is the length of the extraction period: if the extraction period is longer, then a lower water temperature is necessary.
In the end, when it comes to water, you want to be aware of three things: hardness, mineral content, and temperature. A little hardness is good, but too much will negatively affect the flavor. A lower mineral content is preferable. And, a temperature of 195-205F is best for making hot coffee.
P.S. Correct water temperature is one of the most well-known flaws of a significant amount of automatic home coffee makers. This is one of the major downsides of automatic coffee makers: you are unable to adjust any of the variables other than grind size.