If you're doing it right, there's a good chance that your homebrew is over 90% water. Even so, most homebrewers hardly ever talk about water. Grain, hops and even yeast are flavor variables that are easy to understand and manipulate. But the flavor impact of water is much more subtle, and for homebrewers interested in the science behind the beer, water chemistry is a fun topic to look into.
Let me start by saying that for almost all homebrewers, the water from your tap will work just fine. Any clean drinking water will usually have a mineral and chemical content that is conducive to brewing. However, occasionally it becomes useful to make chemical adjustments to the water you use for brewing in order to produce the best homebrew possible.
The first rule of changing your water for homebrew is that a small adjustment goes a long way. A couple of drops of food grade acid or a few grams of powdered compounds known as "brewing salts" is enough to make significant changes to your brewing water chemistry. In fact, over-adjusting your brewing water is a quick way to turn an average homebrewed beer into a bad homebrewed beer. So when you start playing around with these kinds of changes, always make small additions and change one variable each time you brew. That way, you can find which water adjustments improve the quality of your homebrew without having to toss out 5 gallons at a time.
Eliminating Chlorine and Chloramine
The first, easiest, and most effective water adjustment you can make is to eliminate chlorine from your brewing water. Municipal water suppliers use chlorine compounds to keep tap water sanitary for both storage and transportation. The concentration is usually low enough where it may not be noticeable when drinking a glass of water, but the interaction with different malts and hops can create a flavor that is not dissimilar to an infection in your homebrew. If you find that different batches of homebrew have a similar plastic-like or papery flavor to them, one step you can take to fix this problem is to eliminate potential chlorine compounds from your water.
If your municipal water supplier uses typical levels of pure chlorine, then it's likely not going to impact the flavor of your homebrew. Pure chlorine is pretty volatile, and boiling the wort is usually enough to drive off flavors associated with the chemical. However, more and more frequently, a chlorine derivative called chloramine is used to sanitize public water sources. Chloramine is more stable than chlorine, making it much more useful as a sanitizer and much less desirable in homebrewing water. Boiling is mostly ineffective at driving off bad-tasting chloramine, but a simple brewing salt known as a campden tablet can quickly neutralize the negative impact of this chemical.
Campden tablets are either sodium metabisulphite or potassium metabisulphite, and both versions work the same way. They are often marketed to winemakers as a chemical that prevents the growth of wild yeast and bacteria. Less often mentioned, but more important to homebrewers, is the fact that they can effectively neutralize flavors contributed by chlorine or chloramine in water. Dissolving a quarter of a tablet is more than enough to eliminate the flavor from sanitizing chemicals found in 5 gallons of municipal water. Even better, the reaction is very quick, and your brewing water will be ready less than a minute after dissolving the campden tablet.
If you use any type of mash when you homebrew, whether brew-in-a-bag, partial mash, or all-grain, the pH of your water is an important and often overlooked variable. In chemistry, pH is the measure of hydronium ions in a solution. Brewers may not care too much about the technical definition, but they do care about the impact of pH on the brewing enzymes in the mash.
You can measure the pH of your brewing mash by dipping a beer-specific testing strip into the liquid. If the pH is outside of the range of 5.1 to 5.5, the natural enzymes may not be breaking down the starches into sugar. Even worse, if the pH is much above 5.5, you may be extracting mouth-puckering tannins from the grain that will make their way into your final beer.
Before you get too worried, the pH of the mash usually corrects itself. Any time you use a good mix of base malt and specialty malt, there's an appropriate mix of different natural compounds that moves the pH to the correct range and keeps it there. The time to check for unusual pH readings is when you're brewing a very light beer such as a pilsner, or a very dark beer like a stout.
If your pH is out of range, it's almost always going to be too high as opposed to too low. If your pH is too high, you can add 2 to 3 drops of food grade lactic acid, or a quarter of a teaspoon of gypsum to bring a 5 to 7 gallon mash back into range. These adjustments should be made no more than twice during the mash, in order to avoid unanticipated flavors derived from the water additions. Adjustments like these will not be necessary every brew day, but knowing how to test and make the changes will allow you to maximize the sugar you get from your grain and minimize the potential for off-flavor tannins.
Eliminating chlorine and correcting mash pH are just the beginning of the different water adjustments you can make in your homebrew. In the future, we'll be looking at ways to adjust water chemistry to optimize hop flavors in IPAs and to make smoother-flavored stouts.