Determining the pH of wine

The pH in wine influences the taste of the final product, but also its colour, oxidation and chemical stability. This is why measuring it, in order to take action, when necessary, becomes essential.

But what is wine pH, what are the methods for determining it and why is knowing its value important?

Providing information on the activity of hydrogen ions within a solution, the pH of wine is closely related to its acidity. Its formula is determined as follows:  pH = log₁₀ (H⁺). Where H is the concentration of hydrogen ions measured in moles per litre. 

The pH values of wine range in the area between 2.8 and 4, but - the producers - try to keep them between 3 and 3.5 for white wines and between 3.3 and 3.8 for red wines. This, although much depends on the grape variety 
(ranging from 3.3 for Chardonnay and Riesling to 3.8 for Cabernet-Syrah).

In the course of winemaking in red as in white, the pH often varies. The reason? The change in equilibrium constants due to ethanol, the decrease in acid salt levels, the consumption of malic acid and amino acids, the formation of succinic acid and the chemical reaction that tartaric acid undergoes. Wine is in fact an acid-base solution containing weak acids AH, which release hydrogen ions H⁺ in the presence of their salts A^- (which accept protons by acting as strong bases). Acids therefore tend to form salts, with strong acids dominating. However, appreciable variations in pH are countered by the so-called 'buffering capacity', which determines the chemical-physical, microbiological and organoleptic stability of wine, and which - the higher it is - makes it more difficult to alter the pH of wine.

The pH of wine can be modified through the use of acidity correctors. These, with an acidifying or deacidifying action, are permitted by current regulations and are used especially when climatic conditions hinder the proper ripening of the grapes. 

EU Regulation 2019/934, supplementing EU Regulation 1308/2013, sets precise limits for each acidifier/deacidifier: 

• maximum variation in total acidity of 2.5 g/L for tartaric acid;
• maximum variation of 1.35 g/L on musts and 2.25 g/L on wines for malic acid;
• maximum variation of 2.25 g/L on musts and 3.75 g/L on wines for lactic acid;
• maximum decrease in total acidity of 1 g/L for potassium bicarbonate.

Alternatively, revolutionary systems based on pH-Stab 2.0 can be used to lower the pH and for tartaric stabilisation. 

After vinification, in order to make them softer, several red wines and some white wines undergo malolactic fermentation: lactobacteria capable of converting the more acidic malic acid into a milder lactic acid are used.

In fact, it is the acids that determine the freshness a wine offers on tasting: fixed acids (tartaric acid, malic acid, lactic acid and their salts) balance the warmth and softness of the alcohol and sugars, conveying a sensation of freshness of taste. And they thus make it possible to distinguish between flat wines (weakly acidic), fairly fresh wines and acidic wines (very acidic).

Several instruments can be used to determine the pH of wine.

One can use the pH meter, which uses the potential created by the difference in the concentration of H⁺ ions on the two sides of a glass membrane. It is therefore an electrical instrument, which has numerous applications including - precisely - the oenology sector.

Alternatively, it is possible to use pH buffer solutions, available in bottles or single-dose sachets, and pH sensors which - available in a wide range of variants - are suitable for use in the laboratory or in the field, and come in routine and highly professional versions. 

How to choose the sensor? One must base oneself on the characteristics of the liquid sample, considering its homogeneity and the possible presence of particulate matter (which determines the need for a ceramic or glass diaphragm). For wine, it is advisable to use a glass sensor because ceramic is difficult to clean and clogs more easily.