Читать книгу Handbook of Enology, Volume 2 - Pascal Ribéreau-Gayon - Страница 23

The concept of pH often appears to be an abstract, theoretical one, defined mathematically as the base ten logarithm of the concentration of hydronium ions in an electrically conductive solution, such as must or wine:

Furthermore, the expression of pH shows that it is an abstract measure with no units, i.e. with no apparent concrete physical significance.

The concepts of total or volatile acidity seem to be easier to understand as they are measured in milliliters of sodium hydroxide and expressed in grams of sulfuric or tartaric acid per liter. This is rather paradoxical as the total acidity in a wine is, in fact, a complex function with several variables, unlike pH, which refers to only one variable, the true concentration of hydronium ions in must and wine.

The idea that pH is an abstract concept is even less justified since this physicochemical parameter is based on the dissociation equilibrium of the various acids, HA, in wine, at fixed temperature and pressure, as shown below:

The emission of H₃O⁺ ions defines the acidity of the HA molecule. Dissociation depends on the value of the equilibrium constant, K_a, of the acid:

(1.1)

The usefulness of pH, also known as true acidity, is supported by the fact that its value rather accurately matches the impressions of acidity frequently described as “freshness” or even “greenness” and “thinness,” especially in white wines.

A wine's pH is measured using a pH meter equipped with a glass electrode after calibration with two buffer solutions. It is vital to check the temperature.

Owing to the presence of tartaric acid (a strong acid), the pH values of wines range from 2.8 to 4.0. It is surprising to find such low nonphysiological values in a biological fermentation medium such as wine. Indeed, life is only possible thanks to enzymes in living cells, and the optimum activity of the vast majority of enzymes occurs at much higher intracellular pH values, close to neutral, rather than those prevailing in extracellular media, i.e. must and wine. This provides some insight into the role of cell membranes and their ATPases in regulating proton input and output.

On the other hand, it is a good thing that wines have such low pH values, as this enhances their microbiological and physicochemical stability. Low pH hinders the development of microorganisms while increasing the antiseptic fraction of sulfur dioxide. The influence of pH on physicochemical stability is due to its effect on the solubility of tartrates, in particular potassium bitartrate but, above all, calcium tartrate and the double salt calcium tartromalate.

Iron casse, or iron haze, is also affected by pH. Indeed, iron (III), or ferric iron, establishes soluble complexes with molecules such as citric acid. These complexes are destabilized by increasing pH to produce insoluble salts, such as ferric phosphates (see “white casse”) or even ferric hydroxide, Fe(OH)₃.