Wine pH & Acidity

1. Wine pH & Acidity Concepts and chemistry of pH, organic acids, buffer capacity and wine quality implications of pH

2. Effects of pH on wine • 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 • oxidation rate – increased at higher pH • 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 • tartrate stability – dissociation of tartaric acid is pH dependent • Overall palatability is affected by wine pH

4. Definition of pH • pH is related to the concentration of the H+ ion in solution • pH = -log[H+] • 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) • HA  H+ + A-

5. pH versus Titratable Acidity • pH is a measure of [H+] only • 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 • pH and TA are not the same thing, nor do they have a linear relationship!

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

7. Weak Acid Dissociation in Wine • The degree of dissociation is specific to each acid • denoted by the dissociation constant (Ka) Ka = [A-][H+] [HA] • Diprotic and triprotic acids have a K value for each hydrogen ion (K1, K2 etc) • In wine, K values are typically around 10-5 • This represents only about 1% dissociation • Tartaric acid is the ‘strongest’ acid – 50% dissociation of first H+ at pH 3.14

8. Dissociation Constants of Organic Acids in Wine

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 • 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

11. Mechanics of Wine Buffer Chemistry • Simple buffer equilibrium (weak acid buffer) HA  H+ + A- • 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+  H2O • 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 Ka values

12. Measuring Buffer Capacity in Wine versus Juice Buffer capacity in juice or wine can be assessed empirically in the following manner: Determine the amount of hydroxide or hydrogen ions required to change the pH (either up or down) by 1 pH unit If measured on both the juice and the resulting wine, it can be shown that the wine has a reduced buffering capacity

13. Wine Acidity Titration Curve • Weak acid vs. Strong Base, therefore endpoint is >pH 7 • 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 • Therefore we only see the one inflection point • Titratable acidity is not the same as total acidity, as an endpoint of pH 8.2 does not encompass dissociation of all organic acid protons, or phenolic compound protons

14. Effect of Potassium Ions on Wine Acidity • Titratable acidity values will vary with potassium ion content • Potassium is a significant component of grape juice • Potassium ions modify the dissociation equilibrium of organic acids • 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.

15. Effect of Alcohol on Wine pH and acidity • 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 • Wine has a lesser buffer capacity than juice

16. Consequences for Winemaking • 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 • 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 • Only slight changes in pH during normal fermentation

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

18. Tartrate Stability – pH Graph

19. Tartrate Equilibrium Equations Species in solution are descibed by the following equations: HT- + K+  KHT (solid) H2T  H+ +HT- HT-  H+ +T= T=+H2OHT- +OH-

20. 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 H2T 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

21. Consequences of KHT Precipitation • 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 • At lower pHs, [H2T] is dominant species • H2T equilibrium shift produces more H+ than OH- produced by T= equilibrium shift • therefore pH is decreased • At higher pHs,[T=] is dominant species • T= equilibrium shift produces more OH- than H+ produced by H2T equilibrium shift • therefore pH is increased • In all cases of KHT precipitation, titratable acidity decreases

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