Water is water, is water, right? Wrong. Pure water is H2O but its pure form not what we drink and brew with. The water we use for brewing has had a long journey before it gets to our mash tun. Its found its way through streams and lakes, through cracks in rocks and into wells. The landscape that the water has travelled along before we get it has imparted many minerals into the water. All of these minerals have some impact on the beer and the brewing process. Some have enough of an impact that we have to account for them in our mash, if not elsewhere.
If you’re an extract brewer and don’t care to learn more, you might just want to skip the next bit—we’re heading to the weeds. When it comes to extract brewing, most water is fine as long as theres not too much chlorine, otherwise some filtered or store bought “spring” water is a good alternative. For all-grain brewing, you might have good results without taking care with your brew water, but it’s a real crap shoot.
Ignoring whatever else is there in the water (lets assume that the water is potable and not laced with bacteria or toxins), there are a few mineral/compounds to be concerned with.
Chlorine. As with extract brewing, you want as little chlorine in the water as possible. Boiling the water or leaving it out overnight will get rid of most chlorine, but may not do the job it theres too much. Filtered or reverse osmosis (RO) water is your best replacement. Helps enzyme activity in the mash, and it helps the finished beer in stability and clarity. Typical brewing ranges are 50 to 150ppm.
Calcium (Ca) is the primary determining factor in water hardness. Its important as, in the right quantities, promotes enzyme activity in the mash, and it helps the finished beer in stability and clarity. Typical brewing ranges are 50 to 150ppm.
Bicarbonate (HCO3) helps to buffer the pH of the mash. The darker the malt, the higher the acidity. Bicarbonate is alkaline and will help to raise the pH of the wort. Water that is high in bicarbonate is generally better for brewing darker beers. For low bicarbonate water, bicarbonate can be added to increase the buffering potential for a darker beer, but for the opposite (high bicarbonate water in a light coloured beer), the water will need to be diluted or the bicarbonate has to be removed by aerating and boiling. Typical amounts are 0-50ppm for pale beers, 50-150ppm for amber beers, and 150-250ppm for dark beers.
Sulfate (SO4) helps to bring out the hops, making them seem crisp and more bitter. Also, adds to water hardness. Typically 50-159ppm, or 150-350ppm in very bitter beer.
Sodium (Na) adds a roundness to the malt flavour, but can add a saltiness in too high of amounts. When higher amounts of sodium are combined with higher levels of sulfate it can make a harsh bitterness best to keep the sodium low. Typically, 0-150ppm.
Magnesium (Mg) acts a lot like calcium, but used in far smaller quantities. Typical brewing amounts of 10-30ppm, with amounts more than 50ppm giving a sour/bitter taste. Magnesium is also an important nutrient for yeast.
Chloride (Cl) can help with fullness and flavour in the beer, but in too high of concentrations can add a mediciney flavour.Typically 0-250ppm.
For more on water, John Palmer is the man. He literally wrote the book on water. His website, HowToBrew.com, is a great resource for info on water (and all things home brew).