The Role of pH in Regulating Organic Chemistry: Part 1
Many of the most common organic chemical reactions in wine production are influenced or even regulated by pH. A sound understanding of this relationship can open a range of possibilities and techniques which may not at first seem intuitive.
In simple terms, an acid is just a dissolved substance that can shed a hydrogen ion (H+), i.e. a proton. Now, because the acids in wine are “weak”, some of the hydrogen ions remain bound to them.
We commonly use two measures of acidity; TA and pH. The thing is, they measure quite different aspects of acidity and so it can seem that they’re not that closely related. For example, high TA and low pH both indicate lots of acidity. But whereas high TA indicates that zingy, sour taste in wine, pH is a measure of those disassociated H+ ions that are floating around and it is these free H+ ions that are so influential in controlling the wine’s chemistry and microbiology.
Think of it this way: TA counts the total number of guys in the roadworks gang but pH counts the ones who are actually working.
The strength of an acid is measured by its pKa, (i.e. the pH at which half of the acid is ionized). Lactic acid, for example, has a pKa of 3.8. Below pH 3.8, it’s mostly undissociated but above pH 3.8, it’s mostly ionized meaning it has shed most of those H+ ions, to go do useful stuff.
The bit of the acid molecule that’s left over after the H+ has all buggered off is called the conjugate base. Tartaric acid is handy because its conjugate base, bitartrate, reacts with potassium to precipitate crystals that reduce TA — nice! This effect is maximized at pH 3.6, the peak of the bitartrate curve and this turns out to be significant.
When this reaction occurs in a solution with a pH of less than 3.6, precipitation lowers TA and raises pH, just as you would expect. But counterintuitively, when this reaction occurs in a solution with a pH greater than 3.6, precipitation lowers TA but also lowers pH. The acidity goes down, but also goes up!
This is just one example of a common organic chemical reaction in wine production that is influenced or even regulated by pH. There are many more that we will investigate in subsequent commentaries, especially where they are relevant to the working of Vintech Pacific’s technology services.