Titratable Acidity
Titratable Acidity
Many sour beer producers use pH to help determine how "sour" their beer is in relation to a set goal, previous batches, or commercial examples.
pH
In chemistry, pH is the negative log of the activity of the hydrogen ion in an aqueous solution. Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline. Pure water has a pH of 7.
The pH scale is traceable to a set of standard solutions whose pH is established by international agreement.[1] Primary pH standard values are determined using a concentration cell with transference, by measuring the potential difference between a hydrogen electrode and a standard electrode such as the silver chloride electrode. Measurement of pH for aqueous solutions can be done with a glass electrode and a pH meter, or using indicators.
pH measurements are important in medicine, biology, chemistry, agriculture, forestry, food science, environmental science, oceanography, civil engineering, chemical engineering, nutrition, water treatment & water purification, and many other applications. [1]
pH is best tested in sour beers using a PH Meter and is most useful for biological parameters. Cells live or die based on pH, not TA. This means pH should be used when testing sanitizer, sour worting, starter cultures, etc.
Why TA?
When attempting to use pH for sensory parameters - how sour something tastes - the measurement falls short. Humans perceive acidity from tasting acids, not H+ ions. In a strong acid, the amount of acid is always proportional to the amount of H+ ions. However, lactic acid - the primary acid in sour beer, is a weak acid. For various reasons a weak acid does not completely ionize / dissociate.
Any acid not dissociated (in other words, an acid still holding onto its H+) does not affect pH. On the other hand, humans will perceive an increased in "sourness" based upon the amount of acid in a beer - dissociated or not. The best way to test for total acidity, regardless of dissociation, is using a test called "Titratable Acidity" (sometimes also called Total Acidity, which is not entirely an accurate name).
Kara Taylor at White Labs has done a reasonable amount of professional sensory panels showing the correlation between TA and perceived sour taste, as well as showing no correlation between pH and sour taste. Her full presentation, where most of this information was sourced, is available to any attendee of the 2015 Craft Brewer's Conference.
Testing for Titratable Acidity in Sour Beer
Equipment Requirements:
1. pH Meter with Automatic Temperature Correction
2. Buffer/Testing Solutions for pH Meter
3. Buret (100mL recommended) with Stopcock
4. Buret stand (can be DIY)
5. 250ml - 500ml Beaker (or pint glass, etc)
6. 0.1N Sodium Hydroxide (NaOH)
7. Accurate way to measure 50mL (can use the buret for this, but I recommend a serological pipette or high-grade graduated cylinder)
8. Coffee filter or some other way to decarbonate beer (shake and vent repeatedly, pour through filter, etc)
9. Hydrometer
Procedure
1. Calibrate pH meter
2. Pipette 50mL of decarbonated beer into beaker
3. Place pH probe and ATC thermometer into beaker, submerge tip in beer
4. Fill buret with 0.1N NaOH
5. Open stopcock and add NaOH to the beer until pH meter reaches pH 7.0
6. Upon reaching 7.0, go slowly until pH meter reads exactly pH 8.2.
7. Record total amount of NaOH used to reach pH 8.2
8. Use the following formula to find Titratable Acidity:
Titratable Acidity = (mL 0.1N NaOH * 0.9) / (mL beer * S.G.)
The number you get is TA in grams/L.
Uses in Sensory & Further Reading:
It may be best to compare the ratio of Titratable Acidity / Final Gravity as sweetness counter-acts acidity in sensory experiments.[2]
More information on this procedure is available from the American Society of Brewing Chemists, who publish a similar set of procedures under the name "Total Acidity with Potentiometer".
Jim Crook of Firestone Walker has created a presentation about blending sour beers using TA. Contact him for a copy.
References: