Sulfite Chemistry in Wine

Case Study for Chemical EquilibriumCase Study for Chemical Equilibrium

Sirromet Wines Pty Ltd850-938 Mount Cotton Rd

Mount Cotton Queensland, Australia 4165www.sirromet.com

Courtesy of Jessica Ferguson

Assistant Winemaker & Site ChemistDownloaded from seniorchem.com/eei.html

What is sulphur dioxide?

Sulfur dioxide is a pungent gas used as Sulfur dioxide is a pungent gas used as preservative in winemakingpreservative in winemaking

May be added as a liquefied gas or as a May be added as a liquefied gas or as a sulfite saltsulfite salt

Various species of sulphites exist in Various species of sulphites exist in equilibrium in wineequilibrium in wine

Very effective preservative as it acts in Very effective preservative as it acts in several different ways:several different ways:

(i) Antimicrobial properties

Inhibits development of micro-organismsInhibits development of micro-organisms Generally more effective on bacteria than Generally more effective on bacteria than

yeastyeast At low concentrations, protection may be At low concentrations, protection may be

transitorytransitory High concentrations may destroy a High concentrations may destroy a

percentage of microbe populationpercentage of microbe population Effect is highly pH dependentEffect is highly pH dependent

(ii) Antioxidant Properties

Binds with dissolved oxygen in the presence of Binds with dissolved oxygen in the presence of catalysers:catalysers:

SOSO2 2 + ½O+ ½O22 SO SO33

This reaction is slow – protects from some This reaction is slow – protects from some chemical oxidation but not microbial or enzymatic chemical oxidation but not microbial or enzymatic oxidationsoxidations

May appear to be antioxidant as it can bleach or May appear to be antioxidant as it can bleach or hide oxidised compounds (but can’t reverse these hide oxidised compounds (but can’t reverse these reactions)reactions)

(iii)Antioxidasic properties

SO2 inhibits the functioning of oxidation enzymesSO2 inhibits the functioning of oxidation enzymes Can also destroy some of these enzymes over timeCan also destroy some of these enzymes over time Used in must prior to beginning fermentation for this Used in must prior to beginning fermentation for this

reasonreason Particularly prevents browning in white juiceParticularly prevents browning in white juice

(iv) Binding properties

The bisulphite ion readily binds to various The bisulphite ion readily binds to various compounds in winecompounds in wine

Most significant is acetaldehyde, and the complex Most significant is acetaldehyde, and the complex formed is very stableformed is very stable

Other binding compounds include sugars, Other binding compounds include sugars, pigments, phenolics and some spoilage pigments, phenolics and some spoilage compounds caused by bacteriacompounds caused by bacteria

Of any addition of SOOf any addition of SO22 to wine, some will be to wine, some will be ‘bound SO‘bound SO22’ and some remain as ‘free SO’ and some remain as ‘free SO22’’

Free SO2 vs. Bound SO2

Unstable binding is somewhat reversible under certain Unstable binding is somewhat reversible under certain conditions (equilibrium will shift if free SOconditions (equilibrium will shift if free SO22 is consumed) is consumed)

Binding with acetaldehyde is permanent – stable complexBinding with acetaldehyde is permanent – stable complex However this is a positive as excess acetaldehyde will give However this is a positive as excess acetaldehyde will give

bruised apple and sherry characteristics to winebruised apple and sherry characteristics to wine

Chemical Equilibrium of Free SO2 HH22O + SOO + SO22 HSO⇌ HSO⇌ 33

-- + H + H++ SO⇌ SO⇌ 33== + 2H + 2H++

decreasedecrease pHpH increaseincrease

Equilibrium is pH driven as shownEquilibrium is pH driven as shown

Ka Values for SO2 in wine

Molecular SO2 Minimum safe levels

critical level for effective critical level for effective protection is around 0.8mg/L protection is around 0.8mg/L molecular SOmolecular SO22

Levels of molecular SOLevels of molecular SO2 2

directly affected by pH directly affected by pH the free SOthe free SO22 requirement to requirement to

obtain 0.8ppm molecular SOobtain 0.8ppm molecular SO22 will vary with pHwill vary with pH

As seen on this table, lower As seen on this table, lower wine pH is desirable as free SOwine pH is desirable as free SO22 is far more efficient at lower pHis far more efficient at lower pH

Red wine bleaching by SO2 Anthocyanins are the compounds responsible for colour in red winesAnthocyanins are the compounds responsible for colour in red wines Free anthocyanins are relatively unstable and will react with SOFree anthocyanins are relatively unstable and will react with SO22 to to

form a colourless complexform a colourless complex Young red wines are particularly susceptibleYoung red wines are particularly susceptible In older red wines, anthocyanins have formed condensation products In older red wines, anthocyanins have formed condensation products

with tannins, which are less reactive with SOwith tannins, which are less reactive with SO22

SO2 use at bottling

At bottling the dissolved oxygen level is criticalAt bottling the dissolved oxygen level is critical The bottling process itself has the potential to increase The bottling process itself has the potential to increase

dissolved oxygendissolved oxygen Dissolved oxygen at bottling can cause browning and Dissolved oxygen at bottling can cause browning and

flavour oxidation in the bottleflavour oxidation in the bottle 4mg of FSO4mg of FSO22 required to eliminate every 1mg O required to eliminate every 1mg O22

Sulfite (SOSulfite (SO3=3=) ion will scavenge dissolved oxygen but the ) ion will scavenge dissolved oxygen but the reaction is very slow due to the low concentration of SOreaction is very slow due to the low concentration of SO3=3= at wine pHat wine pH

Other forms of sulfite do not react directly with OOther forms of sulfite do not react directly with O22 but but only with its oxidation products only with its oxidation products

Winemaking Implications

Winemakers need to be extremely vigilant Winemakers need to be extremely vigilant with maintaining safe levels of SOwith maintaining safe levels of SO22

All wines need to be tested frequentlyAll wines need to be tested frequently Emerging problems with some spoilage Emerging problems with some spoilage

yeast that can tolerate very high molecular yeast that can tolerate very high molecular sulphur dioxide levelssulphur dioxide levels

Need to maintain high level of cellar Need to maintain high level of cellar hygiene and good winemaking practicehygiene and good winemaking practice

SO2 - Health Concerns - Health Concerns

By law, wine must be labelled as containing sulphitesBy law, wine must be labelled as containing sulphites No wine is sulphite free – some SONo wine is sulphite free – some SO22 is produced by is produced by

fermentation (10-50ppm)fermentation (10-50ppm) Many people blame sulphites for various ill-effects caused by Many people blame sulphites for various ill-effects caused by

wine such as headaches, flushing or hiveswine such as headaches, flushing or hives Many studies have failed to show that SOMany studies have failed to show that SO2 2 in wine causes in wine causes

headaches – no link could be foundheadaches – no link could be found Headaches in wine drinkers may be caused by biogenic amines Headaches in wine drinkers may be caused by biogenic amines

(such as histamine) – research continues(such as histamine) – research continues One study shows that red wines that have gone through malo-One study shows that red wines that have gone through malo-

lactic fermentation may contain higher levels of these amine lactic fermentation may contain higher levels of these amine compoundscompounds

SO2 – Health Concerns (cont)

Headaches are most commonly caused by Headaches are most commonly caused by overindulgence! The side effects of alcohol metabolism overindulgence! The side effects of alcohol metabolism can be very rapid (within an hour of drinking) and is can be very rapid (within an hour of drinking) and is underestimatedunderestimated

White wines, both sweet and dry, contain higher White wines, both sweet and dry, contain higher sulphite concentrations than red winesulphite concentrations than red wine

Around 0.1% of the population do have a serious Around 0.1% of the population do have a serious allergy to sulphites and cannot drink most winesallergy to sulphites and cannot drink most wines

Some asthma sufferers are sensitive to high levels of Some asthma sufferers are sensitive to high levels of sulfites – can trigger an attacksulfites – can trigger an attack

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

Rotter, B: Rotter, B: http://www.brsquared.org/wine/ Sulphur Sulphur Dioxide articleDioxide article

Iland, Ewart, Sitters, Markides & Bruer, Iland, Ewart, Sitters, Markides & Bruer, Techinques for Techinques for Chemical Analysis and Quality Monitoring during Chemical Analysis and Quality Monitoring during WinemakingWinemaking, 2000 , 2000