Mash pH is important to enzymes to properly convert the sugars, but also to the overall flavour of the beer. If the pH is too low the beer will be tart, almost sour. Tannins with leach from the husks of the grains when the pH is out of range and this will also have an effect on the beer, and not in a good way. Some beers can due with a slightly lowered pH, but this is pretty rare and only slight (unless we’re talking sour ales, but that’s a whole other discussion for a later time and has nothing to do with the mash pH).
The mash pH will try to stabilize itself, but it needs some help. The makeup of the brewing water and the grains used in the mash have a direct impact on the mash pH. Pure water is pH neutral, 7 (on a 0-14 scale, below 7 is acidic and above 7 is alkaline), but most waster in North America since it comes from underground sources is slightly alkaline, or higher than pH 7—usually in the 7.1 to 7.6 range, but can be higher (some sources might be neutral or very slightly acidic). The pH of the mater is a direct result of the minerals that it has picked up from the
ground its traveled across and through. Surface water sources are likely near pH neutral where underground sources that have travelled through limestone rich ground will be slightly alkaline.
Grains are acidic by nature. The darker the roast, the more acidic the result. Its easy to understand that the darker the beer being brewed, the more alkaline the brewing water should be, and vice versa for lighter beers. This is part of the reason why some beers were developed in specific areas of the world (dark stout in Dublin and pale helles in Munich, for example)—this was a result of their natural water sources and the kind (or colour) of beer that brewed well.
Now, the understanding of mash pH is starting to sound like a university chemistry class, but it is really quite simple to control. First, you have to understand your brewing water. Most municipal water authorities will provide you with the water report (I actually got mine from the town website, and it is updated every year). This should give you all the information you need to know. What do you need to know? Calcium, Magnesium, Sulphate, Bicarbonate Alkalinity, and pH—you can get all those from your water report.
Beyond your water report, you will need to know the colour of your beer and the mash volume (amount of water being used in the mash). These should be no brainers and already sorted out if you’re getting ready to sort out you mash pH.
There are some other options to adjust your water. To acidify you wort you can add a measured amount of food grade acid or you can use a buffer agent that forces the pH into the proper range. Also there is acid malt—a malt that adds lactic acid to the mash.
No mater which way you treat your water, a little understanding and precise measurement will go a long way.