The Basics About Acids


The same red wine shows the following chemistry in adjacent years:

'11 pH: 3.65 TA: 6.2 g/l
'12 pH: 3.45 TA: 5.9 g/l

How does the Total Acidity level AND the pH rise at the same time (since higher pH indicates less acidity)?



Dear Ron,

An acid is any dissolved substance that can release a proton.  Since the element hydrogen is just one proton with one electron in orbit around it, we also call this a hydrogen ion (H+).

The acids in wine are weak acids, which means they can dissociate a proton (HA --> H+ + A-) but can also take it back (HA <-- H+ + A-).  We write these together as an equilibrium (HA <=> H+ + A-).

In juice, the two acids involved are tartaric, which is stronger, and malic, which is weaker.  This means that at any given pH, the tartaric acid will be more dissociated than the malic.  Both of these acids contain two acidic protons, so we write H2Ta and H2MA.

The titratable acidity (TA) measures how many dissociable protons there are without regard to whether they are actually dissociated or not at the moment.  In other words, the H's on both sides of the equilibrium.  This is like the cops on the payroll. 

We care about TA because it tells us how the wine tastes.  That’s because when we put wine in our mouth, our glands keep secreting basic saliva until the wine is neutralized completely.  That’s called a titration.  We get the sensation of sourness or tartness, and also the palate-cleansingness of all that saliva.

The pH measures only the free H+.  You can’t taste pH (only its effects, and only if you are very, very good).  But the free protons are the cops on the beat, out there fighting crime.  The pH determines all the chemistry and microbiology of the wine: how fast it ages, how prone to spoilage, how reactive with oxygen, how effective SO2 is – everything.  The sourness of TA has nothing to do with any of this.

pH is regrettably upside down. Higher free H+ means lower pH, because it’s really the negative exponent.  pH 3.0 means the ratio of water molecules to free H+ is 10-3, or one in a thousand, whole pH 4.0 means 10-4, or one in a ten thousand.

The A- parts left over when the H+ departs are called conjugate bases.  In juice, these will be tartrate (Ta2-), bitartrate (HTa-), malate (Ma2-) and bimalate (HMa-).  These can hook up with other positively charged elements besides hydrogen ions (H+), principally potassium (K+). 

The wine’s resistance to pH change is its buffer capacity, which has to do with how much of these potassium salts are around, providing conjugate bases over and above what is paired with the available H+.  The effect of these is to pull protons away from the free pool we measure as pH.  If there’s a lot of buffer capacity, we will have a high concentration of K+ and a high pH even if our TA is also high.

The other thing that matters is the ratio of tartaric to malic.  A pure tartaric acid solution has a much lower pH (higher free H+) than a pure malic acid solution, thus the higher the tartrate component, the lower the pH.  When the pH and TA are both high and the potassium is low, the cause is high malic acid. 

This happens in certain varieties like French Colombard, Petit Verdot and Norton.  It can also happen because of drought. Low soil moisture in winter causes early budbreak and grapes maturing too early in the season, when there is not enough time for the malic acid to be metabolized. 

Hot weather can shrivel the grapes, causing the acid to be more concentrated and the TA to rise.  This has happened the last two years in California.

Think of the cops analogy.  If you have lots of cops on the payroll (high TA) but few on the street fighting crime (high pH), where are the cops?  They’re in the donut shops!  So either you have a whole lot of donut shops (high K+) or just a few very good Krispy Kreme donut shops full of cops (high malate).

More about pH and TA in my blog about acid adjustment.

If you have a real Jones for this kind of stuff, and for anyone wishing to enter winemaking, let me shamelessly plug my Fundamentals of Modern Wine Chemistry course, which tackles this as a foundation and goes on to address much of the stuff you really need to know from an Enology degree from Davis or Fresno.  There’s also a 500 page syllabus loaded with articles on a wide variety of practical winemaking topics, with the entire first 50 pages dedicated to pH and TA.