1. Wine pH & Acidity
Concepts and chemistry of pH, organic acids, buffer capacity and wine quality implications of pH
Sirromet Wines Pty Ltd
Mount Cotton Rd
Mount Cotton Queensland, Australia 4165
Courtesy of Jessica Ferguson
Assistant Winemaker & Site Chemist
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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. Organic acids in wine 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

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 WineKA
pK in Wine
(12% Alc, 20 deg)
Tartaric
9.1 x 10-4
4.26 x 10-5
3.14
4.32
Malic
3.5 x 10-4
7.9 x 10-6
3.55
5.05
Citric
7.4 x 10-4
1.74 x 10-5
4.0 x 10-7
3.23
4.64 -
Acetic
1.76 x 10-5
4.79
Succinic
6.16 x 10-5
2.29 x 10-6
4.29
5.56
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
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 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