Welcome to the second part of the series Terroir & the chemistry of wine. In the first part we looked at soil, climate and geology as the frame around the grape. Now we go one layer deeper into the juice itself and look at what gives wine its tension and longevity: the acids and pH.
Acidity is easy to take for granted, because we mostly experience it as freshness on the tongue. But beneath the surface it governs far more than taste. It determines colour, microbial stability and how sulphur works. In this part we tie the chemical understanding together with the decisions made in the vineyard and the cellar.
What you will learn
- Which acids dominate in the grape and in the finished wine, and where they come from
- The difference between titratable acidity and pH, and why they are not the same
- How pH affects colour, stability and taste all at once
- How the vineyard and the cellar are used to manage acidity
The acids of the grape and the wine
Organic acids are one of the largest groups of compounds in the grape after water and sugar. They rarely make up more than around 1 percent of the must's content, but their importance is out of all proportion to that modest quantity.
The two dominant acids are tartaric acid (vinsyre) and malic acid (æblesyre). Together they account for the majority of the grape's total acidity, typically in a ratio that lies between 1:1 and 1:3 depending on the grape and the climate. Tartaric acid is the most important, because it is relatively stable and contributes to the wine's biological stability through its buffering capacity at low pH. Malic acid is the more unstable partner, and it is central to how the wine can develop later on.
Alongside the two main acids we find smaller amounts of citric acid (citronsyre) and traces of acetic acid (eddikesyre). Acetic acid belongs to the volatile acids, and here the winemaker will typically keep the level down, since too much volatile acidity gives an unwanted vinegary character.
The imprint of climate on the acid balance
Climate leaves a clear fingerprint. In cooler areas the grape retains more malic acid relative to tartaric acid, while warmer regions give lower acidity overall. This is one of the reasons the same grape tastes so different depending on where it grows. Even water supply plays a part: a controlled water stress in the vineyard can raise the titratable acidity and the malic acid content while at the same time lowering pH.
Titratable acidity vs. pH
Here is one of the most important points in all of wine chemistry, and at the same time the one most often confused. Titratable acidity and pH do not measure the same thing.
Titratable acidity is a measure of the total amount of acid you can neutralise. Think of it as the stock: how many acid molecules there are in total. It is the quantity that is closely linked to how fresh and lively the wine is experienced on the tongue.
pH, on the other hand, is a measure of the active acidity, that is, the concentration of free hydrogen ions in the solution. It is the electrical charge from the hydrogen ions that determines pH. You can have two wines with the same titratable acidity but different pH, because the acids have different strength and because salts and other substances affect how many hydrogen ions are actually free.
In practice, the pH of wine typically lies in a range around 3.1 to 3.5, and precisely this span helps keep the wine microbiologically and chemically stable. The crucial thing to remember is: titratable acidity tells you something about the taste impression, while pH tells you something about the stability and the chemistry. The two often go hand in hand, but not always.
Why pH governs more than taste
When you really begin to understand pH, you discover that it is a kind of hidden conductor in the wine. It affects three things at once.
Colour
In red wine the colour sits in the anthocyanins, the phenolic colour pigments from the grape's skin. These pigments are pH-sensitive. At low pH the colour appears clearer and more red, while higher pH pulls it towards a more bluish and dull expression. This is why the same amount of colouring matter can look markedly different in two wines with different pH.
Stability
pH is decisive for which microorganisms thrive. At a pH below around 3.5 the growth of typical spoilage bacteria such as Lactobacillus and Pediococcus is inhibited. The higher the pH, the more open the environment becomes to unwanted microbes.
The connection with sulphur sharpens the picture even further. It is the molecular part of sulphur dioxide (SO2) that has the antimicrobial effect, and how large a proportion is in molecular form depends directly on pH. At low pH (around 3) a far greater part of the sulphur is active, while the molecular proportion approaches zero at pH 4. In other words: a wine with high pH requires more sulphur to achieve the same protection. pH and sulphur therefore cannot be thought of separately. We go deeper into sulphur and oxidation in a later part of the series.
Taste
Finally, pH also shapes the taste experience itself. Low pH and ample acidity give a wine that seems taut, fresh and vital, while too little acidity leaves the wine flat and dull. Balance is the whole art. Acidity is essential to the life of the wine, but too much makes it sharp.
Managing acidity in practice
The winemaker has several levers to pull, and they begin already out in the vineyard.
In the vineyard
The choice of grape and location sets the frame, because climate and ripeness determine the starting point for the acid balance. Cooler sites and a well-chosen harvest time give more acidity to work with. As mentioned, water management can also shift titratable acidity, malic acid and pH in a more desired direction. The decision about when to harvest is, at its core, a decision about acidity versus sugar and phenolic ripeness.
In the cellar
The most important natural tool for adjusting acidity in the cellar is malolactic fermentation (MLF). Here lactic acid bacteria convert the sharp malic acid into the softer lactic acid. The process lowers the total acidity and typically raises pH by a few tenths. It is especially relevant for wines from cooler climates, where the malic acid content is high. At the same time MLF provides microbial stability and changes the aroma. This process deserves its own treatment, and it gets it in a later part of the series.
It is worth noting that MLF is also an example of how the levers hang together. When pH rises after MLF, both colour stability and the need for sulphur change. Managing acidity is therefore never an isolated action, but part of a connected whole.
In short
- Tartaric acid and malic acid are the grape's dominant acids, with citric acid and acetic acid in smaller amounts.
- Titratable acidity measures the total amount of acid and is linked to the taste impression. pH measures the active acidity and governs the chemistry.
- pH affects colour, microbial stability and the action of sulphur all at once, not just taste.
- A cool climate gives more malic acid and lower pH. A warm climate gives lower acidity.
- Acidity is managed both in the vineyard (grape, location, harvest, water management) and in the cellar (especially through malolactic fermentation).
Frequently asked questions
Why can two wines with the same acidity taste so different?
Because titratable acidity and pH are not the same. Two wines can have an equal total amount of acid but different pH, because the acids have different strength and because other substances affect the amount of free hydrogen ions. pH helps determine both the taste impression and the stability.
What happens to the acidity during malolactic fermentation?
The sharp malic acid is converted into the softer lactic acid. This lowers the total acidity and typically raises pH a little. The result is a rounder wine that at the same time becomes more microbiologically stable.
Ready for the next step?
Now that the acids and pH are firmly in place, you are ready to look at the other great structure-giving group of compounds. In the next part, Phenols: Tannin, colour and structure, we dive into anthocyanins, tannins and the interplay between colour and structure that pH has already given us a foretaste of.
Do bring your new knowledge along when you look through the range and sense how the acidity plays out in the glass. And remember that the finest combination is always the wine you yourself like, with the food you love.