The use of sulphur dioxide in grape must and wines
(based on information from Alex Marchal for DUAD and Emile Peynaud Travail et Connaissance du Vin)
All wines contain sulphur dioxide in various forms, collectively known as sulphites. Even in completely unsulphured wines, sulphur dioxide is present at concentrations of approx 10 milligrams per litre, because it is produced by yeasts during fermentation (up to 30mg/l can be formed, depends on yeast strain and general wine environment such as amount of solid particles suspended in the wine and/or pH etc). Commercial wines often contain 10x that amount.
The reasonable use of SO2 (annhydride sulfureux/sulphur dioxide/sulphur anhydride, sulphurous gas - all terms can be used) is the base of wine conservation. But it is also a controversial subject, particularly in light of the natural wine movement.
Sulphur dioxide (SO2) is a gas that forms when you burn sulphur. It's a very old conservation technique, dating back at least to the 18th century and the 'Dutch match'. With the exception of oxidative-style wines, its use is indispensable. But the correct rules of using sulphur remain a fairly recent discovery. Essentially as little as possible, but not too little, at the right moments.
What you want is for SO2 to contribute to the establishment of a low oxidation-reduction potential, favouring wine aroma and taste development during storage and ageing.
What is certainly true is that it was through the discovery of how to use sulphur that we were able to make 'modern-style wines'. It was the ability to preserve wines, to stop them from spoiling within months, that allowed the creation of the wines we know today. Without protection against the potentially noxious effects of bacteria, yeasts and oxygen, wine becomes vinegar (at best). Without conservation of some kind, they are not able to be transported to be sold, certainly not in export, and will not preserve their fruit.
The amount of sulphur to use is really a question of mathematics. If too much is added, the wine can give off a burning sensation that burns the back of your throat and nose, and has a distinctive sulphur smell (and sensation). The formation of H2S and unpleasant mercaptans from extended lees ageing can increase under excessive SO2 use. And even levels that are not obviously smelly can dampen fruit aromas if they are too high.
Too little, however, and the wine can oxidise, or unwanted micro-organisms can grow, refermentation can set off in bottle (particularly sweet wines). This is because SO2 is an antiseptic and it slowly combines with dissolved oxygen in the wine (under the presence of certain catalysts) which then prevents oxidation. It's ability to combine with ethanal (acetaldhyde) also has this affect.
Having said all that, clearly it is essential to limit sulphur only to what is strictly necessary. Toxicity reports on SO2 show reactions are fairly limited - about same number of reported allergies in the US as those to bicarbonate of soda (6 cases per year). Recommended maximum dose is 0.7mg per kg of weight - so 42-56mg maximum allowable per day for someone weighing 60-80kg.
One half bottle of red wine probably contains around 28mg and one half white wine perhaps 39mg, so both within the daily amount. Although many other food products use sulphur as a preservative, so it is possible to head over comfortable limits.
Excess SO2 can lead to B1 vitamin deficiency, stomach issues, and other health problems (including hangovers and allergic reactions, or more serious).
Asthmatics more frequently have reactions to low doses.
Sulphur content now has to be recorded on wine labels (contains sulphites is most usual wording). Under EU law, any wine containing more than 10mg/l of sulphur dioxide must be labelled as such.
Permitted levels under EU law:
150mg/l in red wine (100 for organic wines)
200 mg/l white/rosé (150 for organic wines)
400mg/l sweet (370 for organic wines)
Usually a typical Bdx chateau will add perhaps 65mg at end of harvest, 30mg at end of AF and maybe 50mg at beginning of ageing - can quickly reach the EU allowable levels.
Useful properties of Sulphur (bearing in mind some is free sulphur, some is combined with other substances in the wine):
1) Controls yeasts
The fraction of sulphur that is found in the gas SO2 (free sulphur/SO2 libre) has a strong effect on yeasts. The fraction that is found in bisulphite (ie bound with other elements in the wine) has less of an affect.
Lactic bacteria are highly sensitive to free SO2 but also (to a lesser extent) to combined SO2.
The antioxidant properties of sulphur are due to its reductive ability. It stockpiles the oxygen and prevents browning and 'maderisation''. Again it's the free SO2 that has the effect.
4) Improves fresh aromas
By reacting with the acetaldhyde, and blocking it by combining with it (in the form of stable not free sulphur), it improves the taste and smell of wine. It allows the development of a positive reductive character and reductive ageing, which is the basis of most fine wines.
As the points above show, sulphur takes two main forms in wine Free SO2 + Combined SO2 (bisulphite) = together they from Total SO2.
Free SO2 - this is the fraction that will be effective as an antioxidant and antiseptic. Most sulphur in wine is found in acid salts or bisulphites, and so has combined with other elements. At this point it is HSO3 (bisulphite). Some HS03 can also be in free form, and this does not have an odour, but can impart a bitter flavour. It is the free SO2 (also called molecular SO2) that has the disagreeable smell and taste of burnt matches etc.
The amount of sulphur that combines with other substances, or that remains 'free' depends on the wine's pH. The more acidic the wine (so lower the pH) the more free SO2 there will be. Above a pH of 4, there is practically no free SO2, so it ceases to have an antiseptic effect.
A wine contains different molecules reacting to the SO2 in different ways:
A small part of SO2 libre (most important antiseptic) - not linked to any other molecule, 'sulphur' smell
A larger part of HSO3 libre (antioxidant, stops the browning of the juice) - not linked to any other molecule, odourless, but salty, bitter taste
A larger part of HSO3 combined (antioxidant and antioxydasique, but unstable combinations)
A smaller part of HSO3 combined with ethanal (organolptic, aldehydes) - stable
Red wines have natural antioxidants in their skins so require less sulphur dioxide (or no according to some winemakers, but not the bdx faculty of oenology).
White wines do not stay in contact with skins so don't get this natural protection and need higher levels of SO2
Sweet wines need highest levels to avoid refermentation in bottle.
Rotten grapes need a higher addition of sulphur than healthy grapes because they contain substances (oxidative enzymes) that oxidise sugars more quickly.
The main inconvenience of sulphur dioxyide as an antispectic is that its high level of chemical reactivity makes the majority of its form in wine inactive. So for several milligrams of active, you need several hundred milligrams of inactive.
For some tasters, SO2 becomes apparent at around 30mg/l but for most people it is much higher.
Sulphur can combine with various substances in wine, namely aldehydes and cetones, giving two different combined forms, some more stable than others.
Oxygen in must is consumed by an enzyme called tyrosinase, naturally present on healthy grapes. If a must is not sulfited, this enzyme keeps on scavenging oxygen until it has all gone (like an apple exposed to the air turning brown). But tyrosine is sensitive to sulphur.
Rotten grapes have an oxidation enzyme called laccase that is less sensitive to SO2, to takes longer for SO2 to take effect and need higher quantities.
Alcoholic fermentation will start in both sulfited and non sulfited yeasts (bacteria are more sensitive to sulphur than yeasts), but more yeast strains are competing in non-sulphited must. FA will start a little slower with sulphite, but will go quicker. Because SO2 eliminates some undesirable yeasts; it cleans the environment before SO2. And quick fermentations are more likely to be complete, leaving no residual sugar.
Brett yeasts forming volatile ethyl-phenols more present with lower SO2 dose.
Dubourdieu suggests one addition of 5g/l SO2 during vinif, but not as high as 10g/l (but not to actively fermenting must, as it would immediately combine with ethanal, the intermediary product in the formation of ethanol from sugars, and so would not be effective).
So2 can be added as potassium metabisulphate or sodium metabisulphate (most winemakers prefer to use the potassium form), or indeed as sulphur dioxide gas or liquid solution. The effect is the same, regardless of the form used. But effect always depends on wine pH, and molecules present.
During racking, need careful use of SO2 as air is being introduced, so to guard against oxidation but also again 4ep-4eg. Addition of 2.5g/l would keep things stable, 5g/l better for maintaining protection. (and new barrels 'use up' more oxygen at racking). Bung position on top of barrel is also a good idea for protection.
So2 additions are more effective in clarified juice.
Interesting impact on en primeur/general Bdx winemaking. Recent additions of DO2 can harden mouthfeel of a wine temporarily, so many avoid it, but one effect can be increased ethanal - too far other way.
Plus chateau in organic conversion want to avoid sulphur to sat under the 100mg/l limit, but in high pH vintages such as 2009, tough to avoid brett with SO2 under 100mg/l.
As temperature increases, free SO2 increases and bound SO2 decreases (SO2 bound to acetaldhyde/ethanal remains constant, as this is the most stable part of the bound form of SO2). This is because heating allows partial liberation of the bound sulphur (the part that is bound but unstable). So keeping a wine cold will keep more of the SO2 'bound' (so wines sometimes with a high Total Sulphur content are served very cold to hide the smell that they would otherwise exhibit).