Acidity is a term referring to sour, tangy, or tart flavors derived from organic acids. Technically, acidity is the state of being acid—having the properties of an acid or the extent to which a solution is acid.
Almost all drinks that are considered refreshing and “drinkable” contain some notable acidity as part of a balance against sweet elements. The total concentration of “acidity” is typically described in the literature as 220–500 parts per million (ppm), adding to the pleasing tartness of beer. However, this seems rather low, because measurements of 0.1% to 0.3% acidity (expressed as lactic) are typical in beer, which would calculate out to 1000 to 3000 ppm. All-malt worts produce higher amounts of acidity in beer than do malt-adjunct worts. It is a generally assumed rule of thumb that most typical malt-adjunct beers will show about 0.1% acidity and all-malt beers closer to 0.2%. Light beers can be as low as about 0.07% acidity (or 700 ppm).
Beer is thus slightly acidic, with 100% barley malt lager beers having a pH in the range 4.00–5.00. Ales vary a little more, typically pH 3.00–6.00. Sour beer styles such as Belgian lambic, Berliner Weisse, and the new generation of craft-brewed sour ales can get as low as pH 3.30. Although the acidity level is largely driven by organic acids, the carbonation level also lends to the acidity content of these beer styles.
Abnormally high acidity can be an indication of bacterial infection of wort and/or beer. Microbial contamination issues (lactobacillus strains, for example) leading to abnormal acidity are usually perceived by the brewer or consumer before any testing would show the defect.