pH Without Pain
By J. E. Underhill
This article was published in the October 1989 Grapevine.
It has taken all the years since Granny was a girl for us to learn that, in winemaking, we really have to pay attention to specific gravity and total titratable acidity. Behind us a long trail of horrid wines marks the progress of our learning.
In recent years we have developed a positively friendly attitude towards hydrometers and titration.
Now, POW! We are hit upon the head with the pH thing. It is expressed to us in complexities that instill fear and boggle our minds. Small wonder that some of the more conservative amongst us are crying STOP! Enough is enough! Don’t even tell me about pH!
Be of good cheer. The only thing wrong with pH is that we have been trying to understand it.
Like a lot of other things in life, pH is what you make of it. It is easy to learn to use pH, and you don’t really need to understand it. Most of us don’t understand the cars we drive so easily, or the TV sets we so blithely manipulate.
Start by simply recognizing pH as a very important number that can help you to make better wines. True, it doesn’t work like most numbers we grew up with, because what it measures grows greater as the pH number gets smaller.
It does help to have some understanding of what pH measures. It has to do with acids. Musts and wines are complex solutions of weak acids mixed with a variety of other materials. It is essential that we start a wine with its acids just so.
“Total titratable acidity”, which is what you measure with your titration kit, is not a measure of total acid, but of the acid that is available to react with the NaOH solution with which we titrate. It does not tell us how strong that acid is, and acids vary greatly in strength.
pH is a measure of the acid strength in the must or wine. The “p” in pH is an abbreviation for the Swedish word for power.
It is pH, rather than total titratable acidity, which indicates the ability of a must to resist oxidation and invasion by bacteria, and which determines how much SO₂ is needed.
All wine musts that are to produce satisfactory wines have to start with their pH in the range of pH 3.1 to pH 3.55 at the commencement of fermentation. More particularly, white wine grapes and musts should have a pH close to 3.2 (3.1 to 3.3). Red wine grapes or musts should have a pH of 3.3 to 3.4 with pH 3.55 as tops.
Anything started outside the range of pH 3.1 to 3.55 is headed for trouble. Below pH 3.1 the wine will be highly acidic. Above pH 3.55 the must will be gravely at risk of oxidation and of invasion by bacteria, and it will be difficult or impossible to control with metabisulphite.
In high quality grapes, picked at the right point of maturity, the pH, total titratable acidity, and specific gravity will all be in their correct ranges, and no adjustment is required. Fermentation should produce a quality wine that fully shows the potential of the grape variety.
You may usually expect to be able to raise or lower the pH of a must, but in many musts, and in varying degree, the pH is reluctant to change.