Malic Acid in Grapes: Part 2 Chemical Structure and pKa

Recently, I wrote a blog about Malic Acid in Grapes: Part 1. In this blog, I'm going to take one step back and spend a little time on the chemistry of malic and tartaric acids.
The chemical structure of malic and tartaric acids looks like this:

Malic and tartaric acids are dicarboxylic acids. The red circles indicate the hydrogens on each molecule that ionizes in solution to yield free hydrogen. The ionizable hydrogens are part of the hydroxyl group of the carboxylic acid [-COOH] moieties. A solution’s pH is really just a measure of the concentration of these free hydrogen in the solution.
The strength or weakness of an acid, or how readily the hydrogens ionize from their parent molecule is defined as the pka, each acid’s pKa value(s) provides some indication of the degree to which each acid is ionized.
Malic and tartaric acids are weak acids. The pka of these weak acids refer to the conditions under which these molecules will allow the separation of hydrogen into the solution, in our case, we are talking about wine being the solution.
Here is a chart of the pka values for malic and tartaric acids. The blue line represents the fraction of the acid that is unionized, the red line represents the singly-ionized acid, and the yellow line represents the doubly-ionized acid.

Since malic and tartaric acid both have two protons which may be lost to the solution, there are two pKa values for each acid. For malic acid, the pka values are 3.4 and 5.2 and for tartaric acid, the pka values are 2.95 and 4.25. The chart shows that acids with different values of pKa will have differing effects on pH.
At the lowest levels of pH, the acid exists in solution in the un-ionized state, but at pH levels between the two pKa values, the singly-ionized state is the most populated, and then at pH levels much above the higher pKa value most of the acid is present in the doubly-ionized state.
At a typical wine pH of 3.4 (the value of pKa1 for malic acid) half of the malic acid would be un-ionized and therefore contribute nothing to the pH of the wine. However, in the case of tartaric acid at pH 3.4, most of the acid will be in the singly-ionized state, and there will also be some un-ionized and doubly-ionized states present, contributing to the pH of the wine.
References:
1. Rochester Area Home Winemakers.
All structures were drawn by the freely available drawing program from ACD Labs called ACD/ChemSketch Freeware.