2014 Eastern Winery Exposition: Tartrate Stabilization

We attended the 2014 Eastern Winery Exposition held at the Lancaster County Convention Center from March 3-6. My husband and I split the schedule so that we could cover the topics that were relevant and interested us. So, for the first session on Tuesday, I attended two talks given by Tom Payette on "Lab Testing Cost Analysis: In-House or Outsource" and "Cold Stabilization: Reducing Time & Energy Costs". Tom's talks reminded me of the U.C. Davis online course that I took taught by Grady Wann, called "Wine Stability". The second talk that Tom gave was about cold stabilizing tartaric acid. Tom mentioned something about "bifurcation" at a certain pH and his talk reminded me of how much I have already forgotten. So this morning, I dug into my pile of notes and located the lecture on tartaric acid. I think it is really important to know the chemical structure of tartaric acid:

Tartaric acid is a dicarboxylic acid as shown by the red circles in the diagram above. There are 2 titratable hydrogens on the carboxylic acid groups in tartaric acid which sets up the chemical equilibrium shown below:

What we really care about is what is happening to the tartaric acid in the pH range that we typically find in wine, that is between pH 3-4. The species of tartaric acid which exists in wine is pH dependent. The chemical equilibrium shows that between pH 3.1-3.5 both the free acid and its first conjugate base can occur. This equilibrium can also be visualized by the diagram shown below:²

From the diagram above, the maximum concentration of the bitartrate ion, HT^- exists at pH 3.6. If potassium is present, the hydrogen tartrate (HT^-) is present as the potassium salt, KHT. There is in wine, a high concentration of potassium (0.8 to 1.5 g/L), making wine a supersaturated solution of KHT. For tartaric acid, a diprotic acid, the crossover point is at pH 3.65, meaning that hydrogen tartrate, HT^- as it's potassium salt, KHT is at it's maximum concentration at this pH.³ The KHT (potassium tartrate) will precipitate out at its lowest solubility level at a pH between 3.4-4.0.
The pH of the wine prior to cold stabilization will have an impact on the final pH:

• If the pH is < 3.65 loss of too much hydrogen tartrate as the potassium salt will lower the pH, which is a good thing for wines.
• If the pH is at the pK of the KHT, there is no net effect
• If the pH is greater than 4, there will be a rise in the pH

The loss of 2.51 g/L of KHT through precipitation, will lower the TA by 1 g/L.
Enough chemistry! But important, don't you think. And I've figured out what Tom Payette meant by "bifuration point". Next blog will be about how to remove the KHT from the wine.

References:
1. The chemical structure of tartaric acid was drawn by the freely available drawing program from ACD Labs called ACD/ChemSketch Freeware.
2. Bruce Zoecklin, A REVIEW OF POTASSIUM BITARTRATE STABILIZATION OF WINES, Virginia Cooperative Extension, Publication 463-013, 1988.
3. Organic Acids in Wine.