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How to Measure Sourness in Beer

We experience sourness in many ways. The puckering sourness of a lemon, that tart Greek yogurt, sour cream, sweet and sour chicken, hot and sour soup, Carolina barbecue sauce, acid rain and, of course, a good Flanders Red Ale, gueuze, gose, Berliner Weisse or fruit lambic. This article will discuss how to go about measuring sourness in a beer, and to help you determine how sour your beer actually is, both in its acidity and in its tasting profile.

Sour beers have been around for centuries. Many modern interpretations are done by “kettle souring,” which uses Lactic Acid Bacteria to sour the wort (pre-fermentation beer). This allows the wort to be boiled after souring and before fermentation, which protects the rest of the brewery from potential bacterial contamination. Kettle souring differs from traditional sour beer styles such as Flanders Beers from Belgium and Adam Bier from Germany, which achieve sourness while fermenting because of the bacteria that is on the equipment, in the buildings, in the air and on the “magic stirring stick” from previous batches.

Detected on our tongue from hydronium (H+) ions, sourness is one of nature’s warnings (along with bitterness – also popular in beer) that a food may be poisonous or dangerous. And yet we crave that acidity and bitterness. Hydronium ions are what makes things acidic: hydrogen chloride is hydrochloric acid, hydrogen acetate is acetic acid (as in vinegar), hydrogen sulfate is sulfuric acid and acid rain is produced from sulfurous acid (H2O water plus SO2 sulfur dioxide).

The pH scale and acid titration are two methods of measuring sourness in beer. 

Apart from the faces we make when sampling increasingly sour beverages and foods, how does one measure the “sourness” of something? There are a couple of easy ways for the homebrewer (or beer connoisseur) to measure sourness. Measuring sourness via the pH scale (power of Hydrogen introduced by Carlsberg Laboratory in 1909) is done via a logarithmic scale from 0 (very acidic – Hydrochloric acid) to 14 (very alkaline – lye and drain cleaner) with 7 as neutral (pure water). In the wine and cider business, acid titration is used. Test kits for both methods are available at your local homebrew store (LHBS).

pH can be measured with Litmus and pH paper strips. Litmus paper turns red with an acidic solution, blue with a basic (alkaline) solution, and does so in various shades that can be compared against a chart of colors. Greater accuracy can be obtained using pH paper strips with specific ranges (I use a 4.6-6.2 strip and a 2.8-4.4 strip) based on some dyes and chemicals that change color in different pH ranges.

However, pH papers are difficult to read accurately. For better pH accuracy, you can buy an inexpensive ($50) pH meter from your LHBS that gives you accuracy to 0.1 pH – but it needs to be calibrated and well cared for. The actual pH drops as temperature increases, so measure the pH of your beer at serving temperature: A beer that is pH 4.0 at 5°C (41°F) will have a pH of 3.955 at 10°C (50°F) and a pH of 3.91 at 15°C (59°F), so there can be a tasting impact (10^-4 = 0.0001; 10^-3.91=0.000123, indicating a 23% increase in acidity). Since pH is a logarithmic scale, each decrease in number represents ten times the acidity, so pH 3 is ten times (1000%) more acidity than pH 4.

Acid titration kits (from your LHBS) are very useful to the homebrewer. The basic idea of titration is to use some indicator dye that changes color based on acidity, add a few drops to your test solution (beer, wine, etc.) then very slowly add a concentrated test solution, repeating until the indicator dye shows a color change. Dark test solutions can be diluted by adding twice the volume of distilled water. Remember that phenolphthalein (LINK) that you played with in high school chemistry? It is bright reddish pink in alkaline solutions, clear in acidic solutions. The “Winemakers Acid Test Kit” that I have uses phenolphthalein to measure the percent acidity tartaric of grape wine. European winemakers measure their products at ppt (parts per thousand, or grams per liter) sulfuric.

There are several types of acid that will affect your beer. Sometimes they are appropriate for the style of beer; sometimes they are a flaw that produces undesired flavors and aromas.

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