One of the reasons that grapes have been used to make wine for thousands of years is that they are one of the few fruits in the world that contain large concentrations of tartaric acid. The strength of acids is measured by their ability to shed protons – or more specifically, hydrogen ions (H+). Without going too deep into a chemistry lecture (which I’m sure will lose most of you in a few sentences), when you measure the pH of your wine, you are measuring the concentration of these ions – that’s what the big ‘H’ in pH stands for. The tricky thing to remember is that while pH is a measurement of H+, the formula for its calculation causes the pH to be inversely proportional to the H+ concentration. Thus, as the H+ concentration increases, your pH decreases.
So what is the big deal about pH?
Because tartaric acid is relatively strong, it works to keep a wine’s pH near 3.0, which in turn keeps the wine stable against microbes. This is one of the reasons why wine made from grapes has flourished around the world: it doesn’t spoil easily, and acts as an antiseptic. The combination of ethanol and the acidic environment are extremely inhospitable to most microbes. In an indigenous yeast fermentation, after the wine hits 5-6% alcohol, one yeast will dominate the fermentation:Saccharomyces cerevisiae or S. bayanus. After the sugar is depleted, there isn’t much left in the wine to act as a food source for microbes that are capable of surviving in those harsh conditions. Lactic Acid bacteria, if present, will begin to consume the malic acid (transforming it to lactic acid), while Acetobacter species are capable of turning ethanol into acetic acid (vinegar).
However, Acetobacter needs oxygen in order to do this, so as long as you keep your containers full, you don’t need to worry much about them.
This year, like in 2010, we saw problems with high pH in many of our wines, but we saw it especially in Marquette. The most likely explanation is that Marquette grown under certain conditions has an excess of potassium, which can drive up the pH. Malic acid concentration likely also plays a role in increasing the pH, since it is a weaker acid that in turn is converted to an even weaker acid (lactic acid) in red wine vinification. In any case, the high pH is worrisome and steps need to be taken to ensure that the wine remains stable.