Ph stands for “power of hydrogen” or “potential of hydrogen.” It is the chemical variable that denotes a solution’s acidity or alkalinity. The pH value of a solution indicates its concentration of hydrogen ions. As the concentration of hydrogen ions in a solution decreases, it becomes more alkaline (caustic), that is, its pH value increases. Conversely, as the level of hydrogen ions increases, the solution becomes more acid (corrosive), that is, its pH value decreases.

Numerically, pH is measured on a logarithmic scale from 0 (the most acidic) to 14 (the most alkaline), with 7 being neutral. Distilled water (pure H2O) is the standard for pH neutrality. All acids, therefore, have a pH value of 0 to 7; all bases have a pH value of 7 to 14. An extremely acidic solution with a pH value of 1, for instance, has a 10 times greater concentration of hydrogen ions than a solution with a pH value of 2; it is 100 times more acidic than a solution with a pH value of 3, and so on. On the alkaline side, a base with a pH value of 10 is 10 times more caustic than a solution with a pH value of 9 and 100 times more caustic than one with a pH value of 8. The most common ways of measuring pH values are disposable indicator strips made of litmus or phenolphthalein paper, or pH meters, most of which rely on hydrogen-sensitive electrodes to give LED read-outs. The pH scale was first developed in 1909, by Søren Peder Lauritz Sørensen, the head chemist of the Carlsberg Brewery in Copenhagen, Denmark.

Among the key sources of hydrogen ions in brewing water are mineral salts and the compounds formed by them. More specifically, calcium and to a lesser extent magnesium react with phosphate and other materials to release hydrogen ions and thereby lower the pH. Conversely, waters rich in bicarbonate are alkaline. Next to brewing water, a beer’s malt bill is also a significant pH factor, because very dark, heavily kilned, or roasted malts decrease the pH values of the mash, wort, and beer, while pale, gently kilned malts raise them.

Brewers manage pH values mostly to enhance enzyme performance in the mash and yeast performance in the fermenter, as well as the quality of the finished beer. Enzymes perform at their peak only within narrow pH ranges (optima): Gum-converting endo-beta-glucanases have a pH optimum of 4.7 to 5; protein-converting endopeptidase and carboxypeptidase are at 5 to 5.2; and starch-converting beta- and alpha-amylase are at 5.4 to 5.6 and 5.6 to 5.8, respectively. In mashes that are too acidic or too alkaline, enzyme activity is impaired and can even stop. Most mashes, therefore, are kept at a compromise pH range close to 5.4 to 5.6, which is also where proper amounts of zinc, an important yeast nutrient, are leached from the malt into the wort. Some brewers acidify their mashes, for example, to a pH of 5.2, which among other things promotes limited dextrinase activity.

At the start of the boil, the wort pH drops by about 0.2 or 0.3, mostly because of the precipitation of calcium compounds into the trub. This brings the wort pH close to 5, which is ideal for most yeast strains at the start of vigorous fermentation. The yeast’s metabolism, in turn, causes another pH drop in the fermenter, usually by about 0.5 to 0.7. At the end of fermentation, the typical pH value of a barley-based beer is usually a pleasant (to humans) 4.1 to 4.5 and that of a wheat-based beer is slightly lower. Some beers, such as the lambics and other sour beer types, have much lower pHs because of acids produced by bacterial strains.

See also acid, lambic, and mashing.