Sulfite Chemistry in Wine Case Study for Chemical Equilibrium 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

What is sulphur dioxide? Sulfur dioxide is a pungent gas used as preservative in winemaking May be added as a liquefied gas or as a sulfite salt Various species of sulphites exist in equilibrium in wine Very effective preservative as it acts in several different ways:

(i) Antimicrobial properties Inhibits development of micro-organisms Generally more effective on bacteria than yeast At low concentrations, protection may be transitory High concentrations may destroy a percentage of microbe population Effect is highly pH dependent

(ii) Antioxidant Properties Binds with dissolved oxygen in the presence of catalysers: SO2 + ½O2 SO3 This reaction is slow – protects from some chemical oxidation but not microbial or enzymatic oxidations May appear to be antioxidant as it can bleach or hide oxidised compounds (but can’t reverse these reactions)

(iii)Antioxidasic properties SO2 inhibits the functioning of oxidation enzymes Can also destroy some of these enzymes over time Used in must prior to beginning fermentation for this reason Particularly prevents browning in white juice

(iv) Binding properties The bisulphite ion readily binds to various compounds in wine Most significant is acetaldehyde, and the complex formed is very stable Other binding compounds include sugars, pigments, phenolics and some spoilage compounds caused by bacteria Of any addition of SO2 to wine, some will be ‘bound SO2’ and some remain as ‘free SO2’

Free SO2 vs. Bound SO2 Unstable binding is somewhat reversible under certain conditions (equilibrium will shift if free SO2 is consumed) Binding with acetaldehyde is permanent – stable complex However this is a positive as excess acetaldehyde will give bruised apple and sherry characteristics to wine

Chemical Equilibrium of Free SO2 H2O + SO2 ⇌ HSO3- + H+ ⇌ SO3= + 2H+  decrease  pH  increase  Equilibrium is pH driven as shown

Ka Values for SO2 in wine

Molecular SO2 Minimum safe levels critical level for effective protection is around 0.8mg/L molecular SO2 Levels of molecular SO2 directly affected by pH the free SO2 requirement to obtain 0.8ppm molecular SO2 will vary with pH As seen on this table, lower wine pH is desirable as free SO2 is far more efficient at lower pH

Red wine bleaching by SO2 Anthocyanins are the compounds responsible for colour in red wines Free anthocyanins are relatively unstable and will react with SO2 to form a colourless complex Young red wines are particularly susceptible In older red wines, anthocyanins have formed condensation products with tannins, which are less reactive with SO2

SO2 use at bottling At bottling the dissolved oxygen level is critical The bottling process itself has the potential to increase dissolved oxygen Dissolved oxygen at bottling can cause browning and flavour oxidation in the bottle 4mg of FSO2 required to eliminate every 1mg O2 Sulfite (SO3=) ion will scavenge dissolved oxygen but the reaction is very slow due to the low concentration of SO3= at wine pH Other forms of sulfite do not react directly with O2 but only with its oxidation products

Winemaking Implications Winemakers need to be extremely vigilant with maintaining safe levels of SO2 All wines need to be tested frequently Emerging problems with some spoilage yeast that can tolerate very high molecular sulphur dioxide levels Need to maintain high level of cellar hygiene and good winemaking practice

SO2 - Health Concerns By law, wine must be labelled as containing sulphites No wine is sulphite free – some SO2 is produced by fermentation (10-50ppm) Many people blame sulphites for various ill-effects caused by wine such as headaches, flushing or hives Many studies have failed to show that SO2 in wine causes headaches – no link could be found Headaches in wine drinkers may be caused by biogenic amines (such as histamine) – research continues One study shows that red wines that have gone through malo-lactic fermentation may contain higher levels of these amine compounds

SO2 – Health Concerns (cont) Headaches are most commonly caused by overindulgence! The side effects of alcohol metabolism can be very rapid (within an hour of drinking) and is underestimated White wines, both sweet and dry, contain higher sulphite concentrations than red wine Around 0.1% of the population do have a serious allergy to sulphites and cannot drink most wines Some asthma sufferers are sensitive to high levels of sulfites – can trigger an attack

References Zoecklin, Fugelsang, Gump & Nury, Production Wine Analysis, Van Nostrand Reinhold, ©1990 Ribéreau-Gayon, Glories, Maujean & Dubourdieu, Handbook of Enology Vol 2, Wiley ©2000 Rotter, B: http://www.brsquared.org/wine/ Sulphur Dioxide article Iland, Ewart, Sitters, Markides & Bruer, Techinques for Chemical Analysis and Quality Monitoring during Winemaking, 2000