1. Wine pH & Acidity Concepts and chemistry of pH, organic acids, buffer capacity and wine quality implications of pH Sirromet Wines Pty Ltd 850-938 Mount Cotton Rd Mount Cotton Queensland, Australia 4165 www.sirromet.com Courtesy of Jessica Ferguson Assistant Winemaker & Site Chemist Downloaded from seniorchem.com/eei.html

2. Effects of pH on wine biological stability – spoilage organisms are generally inhibited at lower pH, whereas high pH may favour them biological stability – spoilage organisms are generally inhibited at lower pH, whereas high pH may favour them colour - particularly of reds, lower pH wines exhibit more purple and ruby tones, higher pH wines more brick and orange tones colour - particularly of reds, lower pH wines exhibit more purple and ruby tones, higher pH wines more brick and orange tones oxidation rate – increased at higher pH oxidation rate – increased at higher pH protein stability – lower pH tends to foster more rapid precipitation of unstable proteins protein stability – lower pH tends to foster more rapid precipitation of unstable proteins

3. Effects of pH on wine (cont) effectiveness of preservatives – the active (molecular) forms of sulphites and sorbic acid exist at higher levels at lower pH effectiveness of preservatives – the active (molecular) forms of sulphites and sorbic acid exist at higher levels at lower pH tartrate stability – dissociation of tartaric acid is pH dependent tartrate stability – dissociation of tartaric acid is pH dependent Overall palatability is affected by wine pH Overall palatability is affected by wine pH

4. Definition of pH pH is related to the concentration of the H + ion in solution pH is related to the concentration of the H + ion in solution pH = -log[H + ] pH = -log[H + ] pH in fruit juices ranges from around 2 in lemon juice to around 4 for warm climate grapes pH in fruit juices ranges from around 2 in lemon juice to around 4 for warm climate grapes Hydrogen ions are produced by the dissociation of acids in solution (under equilibrium) Hydrogen ions are produced by the dissociation of acids in solution (under equilibrium) HA  H + + A - HA  H + + A -

5. pH versus Titratable Acidity pH is a measure of [H + ] only pH is a measure of [H + ] only pH in wine depends on both the concentration of acids present and their relative degrees of dissociation pH in wine depends on both the concentration of acids present and their relative degrees of dissociation Titratable acidity measures free [H + ] plus all undissociated acids that can be neutralised by a base Titratable acidity measures free [H + ] plus all undissociated acids that can be neutralised by a base pH and TA are not the same thing, nor do they have a linear relationship! pH and TA are not the same thing, nor do they have a linear relationship!

6. Organic acids in wine Diprotic acids: Diprotic acids: Tartaric acid Tartaric acid Malic acid Malic acid Succinic acid Succinic acid Triprotic acids: Triprotic acids: Citric acid Citric acid Monoprotic acids: Acetic acid Lactic acid Acetic, Lactic and Succinic acids are products of fermentation

7. Weak Acid Dissociation in Wine The degree of dissociation is specific to each acid The degree of dissociation is specific to each acid denoted by the dissociation constant (K a ) denoted by the dissociation constant (K a ) K a = [A - ][H + ] [HA] [HA] Diprotic and triprotic acids have a K value for each hydrogen ion (K 1, K 2 etc) Diprotic and triprotic acids have a K value for each hydrogen ion (K 1, K 2 etc) In wine, K values are typically around 10 -5 In wine, K values are typically around 10 -5 This represents only about 1% dissociation This represents only about 1% dissociation Tartaric acid is the ‘strongest’ acid – 50% dissociation of first H + at pH 3.14 Tartaric acid is the ‘strongest’ acid – 50% dissociation of first H + at pH 3.14

8. Dissociation Constants of Organic Acids in Wine Acid KAKAKAKA pK in Wine (12% Alc, 20 deg) Tartaric (1) 9.1 x 10 -4 (2) 4.26 x 10 -5 3.144.32 Malic (1) 3.5 x 10 -4 (2) 7.9 x 10 -6 3.555.05 Citric (1) 7.4 x 10 -4 (2) 1.74 x 10 -5 (3) 4.0 x 10 -7 3.234.64- Acetic 1.76 x 10 -5 4.79 Succinic (1) 6.16 x 10 -5 (2) 2.29 x 10 -6 4.295.56 Lactic 1.4 x 10 -4 3.96

9. Example: Distribution of tartaric acid species at various pH

10. Wine is a Chemical Buffer System! A buffer solution resists changes to pH when addition of acid or base is made A buffer solution resists changes to pH when addition of acid or base is made Buffer solutions consist of a weak acid and its conjugate base (or vice versa) in chemical equilibrium Buffer solutions consist of a weak acid and its conjugate base (or vice versa) in chemical equilibrium The buffer capacity of wine is a result of the combined effects of different organic acids in both their dissociated and salt forms The buffer capacity of wine is a result of the combined effects of different organic acids in both their dissociated and salt forms

11. Mechanics of Wine Buffer Chemistry Simple buffer equilibrium (weak acid buffer) Simple buffer equilibrium (weak acid buffer) HA  H + + A - HA  H + + A - Upon addition of acid, free H + consumed by A - : A - + H +  HA Upon addition of acid, free H + consumed by A - : A - + H +  HA Upon addition of base, OH - reacts with H + to produce water: OH - + H +  H 2 O Upon addition of base, OH - reacts with H + to produce water: OH - + H +  H 2 O Limited change in pH will occur, due to these interactions Limited change in pH will occur, due to these interactions In each case, the original equilibrium will be re- established at the new pH based on K a values In each case, the original equilibrium will be re- established at the new pH based on K a values

12. Wine Acidity Titration Curve Weak acid vs. Strong Base, therefore endpoint is >pH 7 Weak acid vs. Strong Base, therefore endpoint is >pH 7 Flat areas of curve show areas of greatest buffer capacity Flat areas of curve show areas of greatest buffer capacity Although wine is a mixture of weak acids, it is not possible to separate them by titration as the pKa values are too similar Although wine is a mixture of weak acids, it is not possible to separate them by titration as the pKa values are too similar Therefore we only see the one inflection point Therefore we only see the one inflection point

13. Effect of Potassium Ions on Wine Acidity Titratable acidity values will vary with potassium ion content Titratable acidity values will vary with potassium ion content Potassium is a significant component of grape juice Potassium is a significant component of grape juice Potassium ions modify the dissociation equilibrium of organic acids Potassium ions modify the dissociation equilibrium of organic acids This is due to binding of organic acid ions (particularly bitartrate) as the potassium salt This is due to binding of organic acid ions (particularly bitartrate) as the potassium salt Some potassium acid salts react with NaOH during titration, others do not. Some potassium acid salts react with NaOH during titration, others do not.

14. Effect of Alcohol on Wine pH and acidity Equilibrium chemistry of wine acids and salts is modified by presence of alcohol Equilibrium chemistry of wine acids and salts is modified by presence of alcohol Solubility of some species is lower in alcoholic solution, particularly tartrate salts Solubility of some species is lower in alcoholic solution, particularly tartrate salts Wine has a lesser buffer capacity than juice Wine has a lesser buffer capacity than juice

15. Consequences for Winemaking Difficult to significantly alter high pH levels in juice or wine by acid additions Difficult to significantly alter high pH levels in juice or wine by acid additions Winemakers must judge effect on pH against effect on flavour and wine balance Winemakers must judge effect on pH against effect on flavour and wine balance Buffer capacity of individual wines will vary depending on their organic acid profile Buffer capacity of individual wines will vary depending on their organic acid profile Cannot easily predict the effect on pH of a given acid addition Cannot easily predict the effect on pH of a given acid addition Only slight changes in pH during fermentation Only slight changes in pH during fermentation

16. Case Study – Tartrate Stability Unstable wines can precipitate tartrate salts over long storage time Unstable wines can precipitate tartrate salts over long storage time Includes potassium tartrate, potassium bitartrate and calcium tartrate salts Includes potassium tartrate, potassium bitartrate and calcium tartrate salts Particularly likely if wine is stored cold Particularly likely if wine is stored cold Unsightly in bottled white wines Unsightly in bottled white wines

17. Tartrate Stability – pH Issues Equations HT - + K +  KHT H 2 T  H + + HT - HT -  H + + T = T = + H 2 O  HT - + OH -

18. Tartaric Acid Dissociation in Wine (Simplification, ignores effects of other weak acids) At pH 3.718 the dominant form is HT -, with the other two forms H 2 T and T = present at equal concentrations Precipitation of KHT occurs when [K + ] and [HT - ] exceed the solubility product constant HT - concentration decreases dramatically with precipitation of KHT Equilibria of other tartaric acid species will shift to compensate

19. Consequences of KHT Precipitation At pH 3.718, the equilibria shifts result in equal quantities of H + and OH - being produced At pH 3.718, the equilibria shifts result in equal quantities of H + and OH - being produced hence no net change in pH despite loss of KHT hence no net change in pH despite loss of KHT At lower pHs, [H 2 T] is dominant species At lower pHs, [H 2 T] is dominant species H 2 T equilibrium shift produces more H + than OH - produced by T = equilibrium shift H 2 T equilibrium shift produces more H + than OH - produced by T = equilibrium shift therefore pH is decreased therefore pH is decreased At higher pHs,[T = ] is dominant species At higher pHs,[T = ] is dominant species T = equilibrium shift produces more OH - than H + produced by H 2 T equilibrium shift T = equilibrium shift produces more OH - than H + produced by H 2 T equilibrium shift therefore pH is increased therefore pH is increased In all cases of KHT precipitation, titratable acidity decreases In all cases of KHT precipitation, titratable acidity decreases

20. Potassium Bitartrate Stabilisation Wine is cooled to force precipitation of salts Wine is cooled to force precipitation of salts Temperature range -2°C to +2°C Temperature range -2°C to +2°C As KHT is less soluble at lower temperatures, wine becomes ‘supersaturated’ As KHT is less soluble at lower temperatures, wine becomes ‘supersaturated’ Formation of crystal nuclei requires energy Formation of crystal nuclei requires energy Winemakers assist by ‘seeding’ the chilled wine with powdered KHT Winemakers assist by ‘seeding’ the chilled wine with powdered KHT ‘seed’ provides nuclei for crystals to precipitate from solution ‘seed’ provides nuclei for crystals to precipitate from solution Wine is held at low temperature and filtered cold once precipitation is complete Wine is held at low temperature and filtered cold once precipitation is complete

21. References Zoecklin, Fugelsang, Gump & Nury, Production Wine Analysis, Van Nostrand Reinhold, ©1990 Zoecklin, Fugelsang, Gump & Nury, Production Wine Analysis, Van Nostrand Reinhold, ©1990 Ribéreau-Gayon, Glories, Maujean & Dubourdieu, Handbook of Enology Vol 2, Wiley ©2000 Ribéreau-Gayon, Glories, Maujean & Dubourdieu, Handbook of Enology Vol 2, Wiley ©2000 Harris, Quantitative Chemical Analysis, Freeman ©1991 Harris, Quantitative Chemical Analysis, Freeman ©1991 Delfini & Formica, Wine Microbiology: Science & Technology, Marcel Dekker ©2001 Delfini & Formica, Wine Microbiology: Science & Technology, Marcel Dekker ©2001