Chemistry Friday; TA

October 14, 2011

An important parameter that we measure frequently during harvest is TA, or Titratable Acidity. This is an approximate measure of the concentration of acid molecules in juice or wine. Acids are responsible for the sour components of a wine’s flavor, so this time of year we generally measure TA in vineyard samples to make sure that the acid in the grapes will not be too high at harvest and leave us with an overly tart wine. We also examine the sourness of the samples by tasting them, but sometimes the high sugar content of the grapes can make it difficult to perceive sourness accurately, so TA gives us a quantifiable way of looking at the issue.

The TA of juice at harvest is usually somewhere between 5 grams / liter and 9 grams / liter, and the TA in finished wine is usually a little bit lower because tartaric acid (the predominant type of acid in grapes) falls out of solution during fermentation.

The way we measure TA is with a potentiometric titration.  We take a known volume of the juice, and add a base with a known concentration until the pH of the sample reaches 8.2 (this is what is known as the titration endpoint, the pH at which, theoretically, all of the acid molecules have been neutralized by base). Using the strength and volume of the base it took to neutralize the acid in the sample, we can then calculate how many molecules of acid must have been present.

TA is quick and easy, but it is also only an approximate measure of the concentration of acid in juice or wine. The analysis assumes that all of the acid in the wine is tartaric acid, which has a titration endpoint close to 8.2, but in fact wine and juice have a complex mixture of acids with a variety of titration endpoints. Still, tartaric acid is the most common acid in grapes, so the results of the analysis are close enough. 

To visit the Amapola Creek Winery main site, please click here.

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