By Dave Miller (Brewing Techniques)
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Q: When should we check pH of beer (mash, boil, fermentation, bottling?), and what values should we aim for?
DM: The most critical stage for measuring pH is the mash, especially during starch conversion. The malt enzymes α- and β-amylase require a slightly acid pH to do their work. Values in the 5.0–5.7 range are alright, although values toward the lower end (5.1–5.3) are usually considered optimum. It is important to measure pH at room temperature; a phenomenon called displacement will cause pH to read lower at mash temperatures than at room temperature. Cool the mash liquid sample before reading.
The pH of wort in the kettle is also important for a good break reaction and is worth checking. The best range is the same as that for the mash. Wort pH and mash pH are usually similar, but if your sparge water is alkaline the wort pH may be higher. To correct this difference, you can adjust the pH of the sparge water or the wort with acid. I prefer phosphoric acid for pH adjustments. It is safer than other mineral acids (such as hydrochloric) and more stable than organic acids (such as lactic acid).
The pH of fermenting beer drops very rapidly once fermentation begins, usually getting close to its final value after 24 h. In general, ales have a slightly lower pH than lagers; typical values for ale are 4.0–4.5, for lager 4.4–4.7. Each yeast strain, however, has its own characteristics. If you usually work with only a few strains of yeast, you may want to check the pH of your fermented beers to get an idea of what your yeast is giving you. If the pH goes up or down over successive batches, it may be a symptom of contamination and a signal to go to a fresh yeast culture. This can be especially valuable because you may not notice a gradual change in your beer, especially if it is slow and not terribly obnoxious in character.
By the way, carbon dioxide has a great effect on the measured pH of fermented beer. Always degas the sample before measuring. The easiest way to degas beer is with a blender.
The pH of the fermented beer changes very little during lagering or conditioning. It may be worthwhile to check it before bottling if you suspect a slow-working infection (that is, one that asserts itself only after the fermentation is over). Your nose and taste buds, however, will usually give you the answer just as reliably as a pH meter.
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