Wine pH & AcidityConcepts and chemistry of pH, Concepts and chemistry of pH,

organic acids, buffer capacity and organic acids, buffer capacity and wine quality implications of pHwine quality implications of pH

Sirromet Wines Pty Ltd850-938 Mount Cotton Rd

Mount Cotton Queensland, Australia 4165www.sirromet.com

Courtesy of Jessica FergusonAssistant Winemaker & Site Chemist

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Effects of pH on wine

biological stability – spoilage organisms are biological stability – spoilage organisms are generally inhibited at lower pH, whereas high pH generally inhibited at lower pH, whereas high pH may favour them may favour them

colour - particularly of reds, lower pH wines colour - particularly of reds, lower pH wines exhibit more purple and ruby tones, higher pH exhibit more purple and ruby tones, higher pH wines more brick and orange tones 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 protein stability – lower pH tends to foster more

rapid precipitation of unstable proteinsrapid precipitation of unstable proteins

Effects of pH on wine (cont)

effectiveness of preservatives – the active effectiveness of preservatives – the active (molecular) forms of sulphites and sorbic (molecular) forms of sulphites and sorbic acid exist at higher levels at lower pH acid exist at higher levels at lower pH

tartrate stability – dissociation of tartaric tartrate stability – dissociation of tartaric acid is pH dependent acid is pH dependent

Overall palatability is affected by wine pHOverall palatability is affected by wine pH

Definition of pH

pH is related to the concentration of the HpH is related to the concentration of the H++ ion in ion in solution solution

pH = -log[HpH = -log[H++]] pH in fruit juices ranges from around 2 in lemon pH in fruit juices ranges from around 2 in lemon

juice to around 4 for warm climate grapesjuice to around 4 for warm climate grapes Hydrogen ions are produced by the dissociation of Hydrogen ions are produced by the dissociation of

acids in solution (under equilibrium)acids in solution (under equilibrium) HA HA H H+ + + A+ A--

pH versus Titratable Acidity

pH is a measure of [HpH is a measure of [H++] only] only pH in wine depends on both the concentration of pH in wine depends on both the concentration of

acids present and their relative degrees of acids present and their relative degrees of dissociationdissociation

Titratable acidity measures free [HTitratable acidity measures free [H++] plus all ] plus all undissociated acids that can be neutralised by a undissociated acids that can be neutralised by a basebase

pH and TA are not the same thing, nor do they pH and TA are not the same thing, nor do they have a linear relationship!have a linear relationship!

Organic acids in wine

Diprotic acids:Diprotic acids: Tartaric acidTartaric acid Malic acidMalic acid Succinic acidSuccinic acid

Triprotic acids:Triprotic acids: Citric acidCitric acid

Monoprotic acids:Monoprotic acids: Acetic acidAcetic acid Lactic acidLactic acid

Acetic, Lactic and Acetic, Lactic and Succinic acids are Succinic acids are products of products of fermentationfermentation

Weak Acid Dissociation in Wine

The degree of dissociation is specific to each acidThe degree of dissociation is specific to each acid denoted by the dissociation constant denoted by the dissociation constant (K(Kaa))

KKaa = = [A[A--][H][H++]] [HA][HA]

Diprotic and triprotic acids have a K value for each Diprotic and triprotic acids have a K value for each hydrogen ion (Khydrogen ion (K11, K, K22 etc) etc)

In wine, K values are typically around 10In wine, K values are typically around 10-5-5

This represents only about 1% dissociationThis represents only about 1% dissociation Tartaric acid is the ‘strongest’ acid – 50% dissociation Tartaric acid is the ‘strongest’ acid – 50% dissociation

of first Hof first H+ + at pH 3.14at pH 3.14

Dissociation Constants of Organic Acids in Wine

AcidAcid KKAApK in Wine pK in Wine

(12% Alc, 20 deg)(12% Alc, 20 deg)

TartaricTartaric (1)(1) 9.1 x 109.1 x 10-4-4

(2)(2) 4.26 x 104.26 x 10-5-5

3.143.14

4.324.32

MalicMalic (1)(1) 3.5 x 103.5 x 10-4-4

(2)(2) 7.9 x 107.9 x 10-6-6

3.553.55

5.055.05

CitricCitric (1)(1) 7.4 x 107.4 x 10-4-4

(2)(2) 1.74 x 101.74 x 10-5-5

(3)(3) 4.0 x 104.0 x 10-7-7

3.233.23

4.644.64

--

AceticAcetic 1.76 x 101.76 x 10-5-5 4.794.79

SuccinicSuccinic (1)(1) 6.16 x 106.16 x 10-5-5

(2)(2) 2.29 x 102.29 x 10-6-6

4.294.29

5.565.56

LacticLactic 1.4 x 101.4 x 10-4-4 3.963.96

Example: Distribution of tartaric acid species at various pH

Wine is a Chemical Buffer System!

A buffer solution resists changes to pH A buffer solution resists changes to pH when addition of acid or base is madewhen addition of acid or base is made

Buffer solutions consist of a weak acid and Buffer solutions consist of a weak acid and its conjugate base (or vice versa) in its conjugate base (or vice versa) in chemical equilibriumchemical equilibrium

The buffer capacity of wine is a result of the The buffer capacity of wine is a result of the combined effects of different organic acids combined effects of different organic acids in both their dissociated and salt formsin both their dissociated and salt forms

Mechanics of Wine Buffer Chemistry

Simple buffer equilibrium (weak acid buffer)Simple buffer equilibrium (weak acid buffer) HA HA H H++ + A + A--

Upon addition of acid, free HUpon addition of acid, free H++ consumed by A consumed by A--: : A A- - + H+ H++ HA HA

Upon addition of base, OHUpon addition of base, OH-- reacts with H reacts with H++ to produce to produce water: OHwater: OH-- + H + H++ H H22OO

Limited change in pH will occur, due to these Limited change in pH will occur, due to these interactionsinteractions

In each case, the original equilibrium will be re-In each case, the original equilibrium will be re-established at the new pH based on Kestablished at the new pH based on Ka a valuesvalues

Wine Acidity Titration Curve Weak acid vs. Strong Base, Weak acid vs. Strong Base,

therefore endpoint is >pH 7therefore endpoint is >pH 7 Flat areas of curve show Flat areas of curve show

areas of greatest buffer areas of greatest buffer capacitycapacity

Although wine is a mixture Although wine is a mixture of weak acids, it is not of weak acids, it is not possible to separate them by possible to separate them by titration as the pKa values titration as the pKa values are too similarare too similar

Therefore we only see the Therefore we only see the one inflection pointone inflection point

Effect of Potassium Ions on Wine Acidity

Titratable acidity values will vary with potassium Titratable acidity values will vary with potassium ion contention content

Potassium is a significant component of grape Potassium is a significant component of grape juice juice

Potassium ions modify the dissociation Potassium ions modify the dissociation equilibrium of organic acidsequilibrium of organic acids

This is due to binding of organic acid ions This is due to binding of organic acid ions (particularly bitartrate) as the potassium salt(particularly bitartrate) as the potassium salt

Some potassium acid salts react with NaOH Some potassium acid salts react with NaOH during titration, others do not.during titration, others do not.

Effect of Alcohol on Wine pH and acidity

Equilibrium chemistry of Equilibrium chemistry of wine acids and salts is wine acids and salts is modified by presence of modified by presence of alcoholalcohol

Solubility of some species Solubility of some species is lower in alcoholic is lower in alcoholic solution, particularly solution, particularly tartrate saltstartrate salts

Wine has a lesser buffer Wine has a lesser buffer capacity than juicecapacity than juice

Consequences for Winemaking

Difficult to significantly alter high pH levels in Difficult to significantly alter high pH levels in juice or wine by acid additionsjuice or wine by acid additions

Winemakers must judge effect on pH against Winemakers must judge effect on pH against effect on flavour and wine balanceeffect on flavour and wine balance

Buffer capacity of individual wines will vary Buffer capacity of individual wines will vary depending on their organic acid profiledepending on their organic acid profile

Cannot easily predict the effect on pH of a given Cannot easily predict the effect on pH of a given acid additionacid addition

Only slight changes in pH during fermentation Only slight changes in pH during fermentation

Case Study – Tartrate Stability

Unstable wines can precipitate tartrate salts Unstable wines can precipitate tartrate salts over long storage timeover long storage time

Includes potassium tartrate, potassium Includes potassium tartrate, potassium bitartrate and calcium tartrate salts bitartrate and calcium tartrate salts

Particularly likely if wine is stored cold Particularly likely if wine is stored cold Unsightly in bottled white winesUnsightly in bottled white wines

Tartrate Stability – pH Issues

EquationsEquations HTHT- - + K+ K+ + KHT H KHT H22T T H H++ + + HTHT- -

HTHT- - H H++ + + TT= = TT== ++ HH22OO HTHT- - ++ OHOH--

Distribution of tartaric acid species at typical wine pH range

0

10

20

30

40

50

60

70

80

2.8 3 3.2 3.4 3.6 3.7185 3.8pH

% c

once

ntra

tion

of t

otal

H2T

HT-

T=

Tartaric Acid Dissociation in Wine (Simplification, ignores effects of other weak acids)

pH H2T HT- T= New pH pH change2.8 67.41 31.61 0.98 2.53 -0.27

3 56.19 41.75 2.06 2.68 -0.323.2 44.06 51.89 4.05 2.84 -0.363.4 32.29 60.26 7.45 3.05 -0.353.6 22.04 65.19 12.77 3.36 -0.24

3.7185 16.99 66.02 16.99 3.7185 03.8 14 65.63 20.37 4.51 0.71

At pH 3.718 the dominant form is HTAt pH 3.718 the dominant form is HT-,-, with the other two forms H with the other two forms H22T T and Tand T== present at equal concentrations present at equal concentrations

Precipitation of KHT occurs when [KPrecipitation of KHT occurs when [K++] and [HT] and [HT--] exceed the solubility ] exceed the solubility product constantproduct constant

HTHT-- concentration decreases dramatically with precipitation of KHT concentration decreases dramatically with precipitation of KHT Equilibria of other tartaric acid species will shift to compensateEquilibria of other tartaric acid species will shift to compensate

Consequences of KHT Precipitation

At pH 3.718, the equilibria shifts result in equal quantities of HAt pH 3.718, the equilibria shifts result in equal quantities of H+ +

and OHand OH- - being produced being produced hence no net change in pH despite loss of KHThence no net change in pH despite loss of KHT At lower pHs, [HAt lower pHs, [H22T] is dominant speciesT] is dominant species HH22T equilibrium shift produces more HT equilibrium shift produces more H+ + than OHthan OH- - produced by Tproduced by T==

equilibrium shift equilibrium shift therefore pH is decreasedtherefore pH is decreased At higher pHs,[TAt higher pHs,[T==] is dominant species] is dominant species TT== equilibrium shift produces more OH equilibrium shift produces more OH- - than Hthan H+ + produced by Hproduced by H22T T

equilibrium shift equilibrium shift therefore pH is increasedtherefore pH is increased In all cases of KHT precipitation, titratable acidity decreasesIn all cases of KHT precipitation, titratable acidity decreases

Potassium Bitartrate Stabilisation

Wine is cooled to force precipitation of saltsWine is cooled to force precipitation of salts Temperature range -2Temperature range -2°C to +2°C°C to +2°C As KHT is less soluble at lower temperatures, wine As KHT is less soluble at lower temperatures, wine

becomes ‘supersaturated’becomes ‘supersaturated’ Formation of crystal nuclei requires energyFormation of crystal nuclei requires energy Winemakers assist by ‘seeding’ the chilled wine with Winemakers assist by ‘seeding’ the chilled wine with

powdered KHTpowdered KHT ‘‘seed’ provides nuclei for crystals to precipitate from seed’ provides nuclei for crystals to precipitate from

solutionsolution Wine is held at low temperature and filtered cold once Wine is held at low temperature and filtered cold once

precipitation is completeprecipitation is complete

References

Zoecklin, Fugelsang, Gump & Nury, Zoecklin, Fugelsang, Gump & Nury, Production Wine Production Wine Analysis, Analysis, Van Nostrand Reinhold, Van Nostrand Reinhold, ©1990©1990

Ribéreau-Gayon, Glories, Maujean & Dubourdieu, Ribéreau-Gayon, Glories, Maujean & Dubourdieu, Handbook of Enology Vol 2Handbook of Enology Vol 2, Wiley ©2000, Wiley ©2000

Harris, Harris, Quantitative Chemical AnalysisQuantitative Chemical Analysis, Freeman ©1991, Freeman ©1991 Delfini & Formica, Delfini & Formica, Wine Microbiology: Science & Wine Microbiology: Science &

TechnologyTechnology, Marcel Dekker ©2001, Marcel Dekker ©2001