The control of acidity and pH in the vinification of red wines is often considered a secondary aspect. This is because such wines generally have a lower acidity than whites and frequently undergo malolactic fermentation during ageing. However, an oversimplified view of this process risks underestimating the significant technological implications that acidity and pH can have on the final quality of the wine. In recent years, climate change and the growing trend to produce wines with softer tannins and greater colour stability, in line with consumer expectations, have led to an average increase in pH in red wines compared to what was observed a few decades ago.
The effect of pH on microbiological and physico-chemical stability
Although acidity and pH are often neglected in the vinification of red wines, their role is of fundamental importance. The pH of a wine is closely linked to its microbiological and physico-chemical stability, influencing both the malolactic fermentation process and the natural selection of the microorganisms involved. Furthermore, pH conditions the balance of sulphur dioxide in wine, determining the available quantities of free and molecular sulphur dioxide. In addition to this, pH and acidity, especially tartaric acid, influence the solubility of tartaric salts, playing a decisive role in tartaric stability.
The effect of pH on the colour of red wines
The colour of red wines can be greatly influenced by pH, as the latter regulates the balance between the different forms of anthocyanins and affects the reactions that lead to the polymerisation and condensation of pigments and flavanols.
Organic acids in wine
The main organic acids present in grape juice and wine are tartaric, malic, citric, succinic and lactic acid, with smaller concentrations of compounds such as acetic and pyruvic acid. The concentrations of tartaric, malic and citric acid in grape juice can vary depending on many factors, including grape variety, harvest date, latitude, climatic conditions, seasonal patterns and agronomic practices used. The relative concentration of these organic acids is crucial as it directly affects the pH of the wine, and both concentration and pH can alter the perception of acidity in the final product.
Tartaric acid: the main organic acid in wine
Tartaric acid (H2T), the main organic acid in must and wine, is a typical substance in grapes and occurs in isomeric form. It is a diprotic acid with pKa values of 2.98 and 4.34 at 25 °C, indicating that, at the typical pH of wine, tartaric acid is predominantly in the form of the bitartrate ion (HT2). According to the calculations of Usseglio-Tomasset (1985), at pH 3.50, tartaric acid is present in 23.4 % in the indissociated form (H2T), 67.5 % as bitartrate (HT2), and 9.1 % as tartrate ion. This prevalence of the bitartrate ion, combined with the relatively high concentration of tartaric acid in must and wine (approximately 2.6 g/L), explains the key role of this acid in the formation of insoluble potassium bitartrate salts. Although tartaric acid is microbiologically stable, its concentration may vary following bitartrate precipitation or acidification and deacidification treatments.
Malic acid: characteristics and behaviour
Malic acid is another diprotic acid, with pKa values of 3.40 and 5.11 at 25 °C. At a pH of 3.50, the indissociated form (H2M) accounts for about 47% of the total concentration, while the dissociation of the first carboxylic group (HM2) reaches 51.7%. The second acid function of malic acid, on the other hand, is poorly dissociated at wine pH, with the malate form constituting only 1.2% at pH 3.50. Consequently, the contribution of malic acid to wine pH, both in terms of strength and concentration, depends mainly on its first acid function, as is the case with tartaric acid. Unlike the latter, however, malic acid is physically and chemically stable under normal conditions, but can precipitate as calcium tartromalate under specific circumstances and is subject to microbial transformations, such as malolactic fermentation.
Citric acid: a limited contribution to pH
Citric acid is a triprotic acid with pKa values of 3.13, 4.76 and 6.40 at 25 °C. At a pH close to that typical of red wines (approx. 3.50), the monodissociated form (H2C2) is the most predominant, accounting for 67% of the total, while the indissociated form (H3C) constitutes 29%. Due to the high dissociation constant of the third carboxyl group, this dissociation practically does not occur at wine pH. As a result, citric acid is stronger than malic acid, but is present in relatively low concentrations in the wine. Prior to malolactic fermentation, the amount of citric acid in young wines usually varies between 0.5 and 1 g/L, contributing only to a limited extent to the overall pH of the wine.
Advantages of the combined use of acids in winemaking
The combined use of several acids in different proportions has numerous advantages over the use of a single acid.
Firstly, the combination of acids makes it possible to modulate the pH more precisely, keeping the wine’s total acidity within an optimal range without altering its taste profile too much. Each acid also contributes differently to sensory perception: tartaric acid, for example, brings structure and stability, while malic acid adds freshness and liveliness. Citric acid, on the other hand, can improve aromatic intensity and the sensation of freshness, without excessively influencing the pH. The use of a mixture of acids also ensures greater stability than the use of a single acid, as the various acids act synergistically to maintain the balance of the wine over time. This reduces the risk of problems such as precipitation or sensory imbalances, ensuring a more harmonious and pleasant end product.
Our blends
essedielle offers its customers liquid solution blends containing different acids, which accelerate and simplify acidification operations while improving wine balance and freshness.
Compacid MT: liquid blend of tartaric acid and malic acid, ideal for wines and musts that require an increase in total acidity without compromising acid freshness on the palate.
Compacid ML: liquid blend of malic and lactic acid, capable of imparting a fresher, more floral profile to flat, poorly aromatic wines, while permanently correcting the wine’s final taste.
Compacid Ferm: liquid blend of tartaric, malic and lactic acid, for wines and musts that need more total acidity but at the same time do not want to lose acidic freshness to the taste.