The Influence of pH
©Copyright Ben Rotter 2005-2008
The pH of must and wine has a profound influence on winemaking practice and wine quality. High quality winemaking should not be ignorant of this winemaking variable. The following article outlines the ways in which varying pH values influence wine and the issues surrounding winemaking. It also notes the influence of pH on various taste factors, typical pH aims in wine types and expected pH increases/decreases during winemaking.
Influences of pH on wine
pH contributes to wine in a number of ways. These are listed below, with a low pH affecting wine in the following ways:
SO2 is more effective as an antimicrobial agent
Favours the most desirable types of lactic acid bacteria (LAB)
Increases microbial stability through increased inhibition of bacterial growth
Enhances production of fruity esters during yeast fermentation
Generally speaking, yeast ferments less efficiently
Shifts colour equilibrium to more red pigments
Increases colour hue
Provides a fresher taste
Increases monoterpene (geraniol, citronellol and neral) concentrations
Increases ageing potential
pH and taste
Sourness: Acid sourness is independently influenced by the concentration, pH, and anion species of acid [Sowalsky and Noble, 1998]. A lower pH at the same titratable acidity (TA) gives more sourness.
Astringency: This is defined as a persistent sensation, increasing upon repeated ingestion. Astringency is affected by pH - as pH increases, astringency decreases [Noble, 1998; Peleg and Noble, 1999].
Bitterness: pH has little or no affect on bitterness [Noble, 1998].
Mouthfeel: higher pH's tend to give a rounder, softer, mouthfeel.
Polymerisation: the higher the pH, the slower the polymerisation and the more unstable the colour.
Volatile acidity: more volatile acidity is produced by yeast at lower pHs.
Acid types and pH
Within the usual pH range of wine, tartaric acid is more highly ionised than malic, which is inturn more so than citric. Tartaric will liberate the most H+ ions yielding a greater shift in pH. Thus, adjusting TA with tartaric causes a larger shift in pH than an equivalent quantity (same TA shift) using malic or citric being added. For maximum pH adjustment with a minimum accompanying TA increase, use tartaric.
The typical pH aims for given wine styles are as follows:
Final pH of white wine: 3.0-3.3
Final pH of red wine: 3.3-3.5
Optimal pH before a MLF: 3.2-4, >2.9
It should be noted that these are typical values only. Deviations may be expected and should not necessarily be seen as a problem.
pH's higher than 4.0 are generally avoided as spoilage is more likely to occur above this level. (The optimum pH for bacterial multiplication is 4.2-4.5.) Many winemakers keep wine pH below 3.65. Many winemakers tend to ignore TA and acidify/deacidify based on pH only. This may be a practical approach when working with quality fruit that is typically high in pH and low in acidity, however it can be unwise to follow this approach in other cases. There is a trend, particularly with warmer climate fruit, to acidify musts in order to bring down what are seen as excessively high pH's (generally > 3.65). The corresponding increase in TA should be noted and kept in check (at least by tasting the must/wine). Surely it is preferable to produce a high pH yet drinkable wine to a lower pH wine whose TA is so high it is difficult to drink?
Typical pH increases/decreases
Addition of acid: Wine is a highly buffered liquid. This means that the corresponding pH decrease for a given addition in TA (added acidity) is not directly proportional. Further, the change in pH for a given TA increase/decrease is unique to each individual wine, since every wine is buffered slightly differently. However, as a general rule, the addition of 0.5-1 g/l acid as tartaric tends to drop the pH by about 0.1 units.
Maceration/pressing: These operations may result in an expected pH increase of 0.05-0.2 units. This is due to the leaching of potassium in the grape (/fruit) skins.
Fermentation: Following fermentation of tartaric-dominant grape wines, an increase in pH of 0.1 unit may be expected. Note that in the case of non-grape wines inwhich tartaric is not a dominant acid, a significant increase in TA and a corresponding decrease in pH may be expected during fermentation.
MLF: Following MLF an increase in pH of 0.1-0.3 units may be expected.
Tartrate precipitation: The precipitation of tartrate (requiring the presence of both tartaric acid and potassium) can cause pH shifts either up or down. It is important to note that there is a pH point around 3.6, above which tartrate precipitation causes a pH increase and below which tartrate precipitation causes a pH decrease. The point varies from wine to wine but generally occurs between pH 3.5 and 3.7.
pH and titration
Titratable acidity is determined analytically by titration. Titratable acidity is not the same as "total acidity", which is defined as the equivalent number of protons that the organic acids would possess if they were not dissociated. Titratable acidity is no more than a measure of the hydrogen ions required to obtain a specific pH end point, and is always lower than total acidity.
The standard titration method advocated by the Office International de la Vigne et du Vin (OIV), and used in France, titrates to the endpoints pH 7. Whilst the method advocated by the Association of Official Analytical Chemists (AOAC), used in the United States and elsewhere, titrates to the endpoint pH 8.2. Given that the dissociation of the organic acids present in wine is weak, neutralisation with NaOH (the common strong base used for acid titration) is likely to occur above pH 7. For this reason, titration to the pH 8.2 endpoint is considered more reasonable. It should be noted that the calculated TA value of the same wine will be different when using a different pH endpoint. Having titrated to a pH 7 endpoint and calculated the TA, the equivalent TA for a pH 8.2 endpoint may be calculated based on the following equations (Darias-Martín et al., 2003):
To determine the TA, titrate to the respective pH and calculate as follows:
TA is the titratable acidity (g/l as tartaric),
N is the molarity of the NaOH,
V is the volume (ml) of NaOH required to titrate to the endpoint,
S is the volume (ml) of the sample titrated.
Darias-Martín, J., A. Socas-Hernández, C. Díaz-Romero and E. Díaz-Díaz. 2003. Comparative study of methods for determination of titrable acidity in wine. Journal of Food Composition and Analysis, 16, 555–562.
Noble, A.C. 1998. Why do wines taste bitter and feel astringent? In Chemistry of Wine Flavor (ed. A.L. Waterhouse and S. Ebeler ) ACS Symposium Series #714, Amer. Chem Soc. Washington, D.C. pp 156-165.
Sowalsky, R. A. and A. C. Noble. 1998. Effect of the Concentration, pH and Anion Species on the Sourness and Astringency of Organic Acids Chem. Senses 23 (3) 343-350.
Peleg, H. and A.C. Noble. 1999. Effect of viscosity, temperature and pH on astringency in cranberry juice. Food . Qual. Pref. 10:4-5, 343-347.