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
Effects of pH on wine
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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
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
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Definition of pH
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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-
pH versus Titratable Acidity
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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!
Organic acids in wine
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Diprotic acids:
Tartaric acid
Malic acid
Succinic acid
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Triprotic acids:
Citric acid
Monoprotic acids:
Acetic acid
Lactic acid
Acetic, Lactic and
Succinic acids are
products of
fermentation
Weak Acid Dissociation in Wine
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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
Dissociation Constants of Organic Acids
in Wine
Acid
Tartaric
KA
(1)
(2)
Malic
(1)
(2)
Citric
(1)
(2)
(3)
Acetic
Succinic
(2)
Lactic
9.1 x 10-4
4.26 x 10-5
3.14
4.32
3.5 x 10-4
7.9 x 10-6
3.55
5.05
7.4 x 10-4
1.74 x 10-5
4.0 x 10-7
3.23
4.64
-
1.76 x 10-5
(1)
pK in Wine
(12% Alc, 20 deg)
6.16 x 10-5
2.29 x 10-6
1.4 x 10-4
4.79
4.29
5.56
3.96
Example: Distribution of tartaric
acid species at various pH
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
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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 reestablished at the new pH based on Ka values
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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
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Wine Acidity Titration Curve
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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
Effect of Potassium Ions on
Wine Acidity
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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.
Effect of Alcohol on
Wine pH and acidity
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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
Consequences for Winemaking
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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
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
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Tartrate Stability – pH Graph
H2T
Distribution of tartaric acid species at typical
wine pH range
HTT=
% concentration of total
80
70
60
50
40
30
20
10
0
2.8
3
3.2
3.4
pH
3.6
3.7185
3.8
Tartrate Equilibrium Equations
Species in solution are descibed by the
following equations:
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HT- + K+  KHT (solid)
H2T  H+ + HT HT-  H+ + T=
 T= + H2O  HT- + OH
Tartaric Acid Dissociation in Wine
(Simplification, ignores effects of other weak acids)
pH
2.8
3
3.2
3.4
3.6
3.7185
3.8
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H2 T
67.41
56.19
44.06
32.29
22.04
16.99
14
HT31.61
41.75
51.89
60.26
65.19
66.02
65.63
T=
0.98
2.06
4.05
7.45
12.77
16.99
20.37
New pH pH change
2.53
-0.27
2.68
-0.32
2.84
-0.36
3.05
-0.35
3.36
-0.24
3.7185
0
4.51
0.71
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
Consequences of KHT Precipitation
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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
Potassium Bitartrate Stabilisation
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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