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The complete guide to understanding sulfites in wine

The term ‘sulphites’ and ‘SO₂’ have become increasingly part of winespeak. So, what exactly are they and why are they relevant? 

Use of sulphur dioxide in winemaking has been around for centuries. It was produced on the spot by burning a piece of sulphur, empty casks were sterilised by lowering the burning sulphur into the cask.
A little basic chemistry; a sulphite is a compound that contains the sulphite ion, which is an electrically charged particle. SO₂ or Sulphur Dioxide, is not a sulphite but a closely related chemical oxide. This is formed when the element sulphur is burned in air.

The simplest way of obtaining sulphur dioxide is by using potassium metabisulphite, a white crystalline substance used as an antioxidant. This substance releases sulphur dioxide when dissolved in an ‘acid aqueous liquid’, and our favourite acid aqueous liquid is wine! Sulphur dioxide can also be used in its liquified gas form. It is toxic in large doses, but generally accepted as harmless when used correctly and is widely used as a preservative in many foodstuffs such as dried fruit, fruit juices and sausages. However it is known to cause allergic reactions in people with asthma.

More relevant to this subject however is the World Health Organisation’s recommended maximum daily intake, 0.7mg per kilo of bodyweight. For a man of average weight this is less than one third of a bottle of conventional white wine. Regular consumption of conventional wines could therefore mean quite regularly exceeding the RDA by a large margin.

Use of SO₂ can result in a loss of fruit and generally, good winemakers use it very sparingly. The principle use of SO₂ is as an antioxidant, ie. a preservative. It is also used as an antiseptic or antimicrobial agent as it kills bacteria in wine, in particular acetobacter which turns wine into vinegar. It keeps lactic-acid bacteria under control as well as Brettanomyces, often referred to as ‘brett’, a yeast that can cause spoilage in wine. Sulphur dioxide is in addition used as a corrective to freshen wine that has been poorly handled and begun to oxidize.

Under EU regulations, any wine that contains more than 10 milligrams per litre (mg/l) must be labelled ‘contains sulphites’ or ‘contains SO₂’, but here is where the story gets interesting; virtually all wines contain naturally present sulphites, as traces are produced by yeasts during fermentation, and usually at a higher amount than 10 mg/l, so even most natural wines will have ‘contains sulphites’ on the label. White wines generally require more SO₂ than reds as they do not have the benefit of protective tannins.

So what’s all the buzz about? Why are people asking for wines without added SO₂?

This shows a very wide variation in permitted levels, especially considering the base-line is sixteen times less than the highest permitted level for dry red wine.
Various organic organisations impose differing limits on certified and labelled products:
Nature&Progres and Demeter - 70, 90 and 210 mgs/l for red, white and sweet respectively.

The natural wine movement eschews the use of what are seen as unnecessary additives in wine, sulphur dioxide being high on the list. Winemakers who are part of this movement want to create a pure expression of their product. It takes immense skill to make a wine without any preservatives that shows the individuality of varietals and place. It is also understandable that if a consumer wants to limit their exposure to chemicals, by buying organic food and wine for example, that they would like to drink wines with less preservatives added. On a personal level, sticking to the WHO recommended daily limits for SO₂ would make a big dent in consumption, and that’s no fun at all.

There is a distinction between free SO₂ and total SO₂. When it combines with other natural substances such as sugars, aldehydes and ketones, sulphur dioxide is no longer protective and is known as the ‘combined, bound or fixed’ SO₂. There are higher amounts of aldehydes and ketones in poorly made and oxidized wines, therefore a greater proportion of the SO₂ becomes combined leaving less available for its primary purpose of protecting the wine. The ‘free’ SO₂ and the ‘combined’ equals the’ total’ SO₂.Combined SO₂ is released by stomach acid and is free to do its damage if above the safe amount.
Just to clarify; although most white or sweeter wines will have higher doses of SO₂, it is only the combined SO₂ that will usually become a problem and this will be higher in poorly made wines, so drinking a top class Riesling Auslese with a maximum of 350 mg/l will be better than a poorly made red wine with a maximum of 160 mg/l, but it will still make quite a dent in the RDA!

There are other substances in wine that can cause unpleasant reactions. Histamine and Tyramine are produced by lactic acid bacteria during malolactic conversion. There are higher amounts in red wines and they have been implicated in triggering headaches and ‘red wine intolerance’. Indeed, tannins themselves can also cause headaches.
White wines are more delicate and require higher protective amounts of SO₂. In addition they have a higher acidity which increases the release of SO₂ from solution and may be a cause of ‘white wine intolerance’.

As we know, white wines are more delicate due to less protection from skin tannins. The natural wine movement has found some degree of solution with the trend in ‘skin contact’ wines, white wine grapes allowed to macerate for varying lengths of time adding these protective substances to the wine thus enabling a far less interventionist regime. These winemaking techniques bring differing complexities to what is seen as usual for a particular variety. Amber coloured Pinot Grigios, Gewürztraminers with the most lusciously deep honeyed depth and complexity, these are wines that open up a whole new flavour profile.

Beaucastel, widely acknowledged as one of the world’s great wines, has been the subject of of a case study suggesting that the most successful vintages have been marked with high levels of Brettanomyces. It seems like a little bit, properly managed, can add to a wines character. Granted this is quite a specific type of wine, robust varietals and blends that can be positively enhanced, not delicate, soft wines where the highly noticeable brett characteristics would overpower.

However, it leads to an interesting and quite relevant supposition; if major players in the wine industry cannot agree if a bit of brett enhances a wine or not, the whole idea of ‘faulty’ wine comes into question. Obviously unpalatable stinky odours and sour flavours are one end of the spectrum, but a hint of this or that, who is to say whether it is not part of the whole wonderful idiosyncratic world that is wine? It is such a subjective area, all of our smelling and tasting history as individuals has such a bearing on how we perceive a wine. A possible reason for the acceptance of what purists would call faulty wine is the explosion of the craft beer industry in parallel with the natural wine trend. The flavour profile of some natural wines, with their unfettered yeasty aromas and flavours, could be described as similar to a pale ale. The intensely beautiful ‘Haggis’ by maverick Australian winemaker Patrick Sullivan has definite characteristics that would appeal to anyone who had a fondness for complex beers.

Limiting or eliminating the involvement of SO₂ in winemaking gives cause for the use of quality techniques, the winemaker has to ensure exceptionally high standards lest their whole vintage may spoil. Ingenuity, best practice and superior production can minimise intervention, creating wines of particular nuance and individuality.

Below is a list of the pros and cons of using sulphur dioxide in winemaking:

Whilst there is no excuse for winemakers to release faulty wines just because they are ‘natural’, the perception of fault can be highly individual. If the wine is pleasant to drink and expresses what the winemaker wants it should be enjoyed as just that.

A possible move forward would be for EU permitted levels and possibly the combined SO₂ to be displayed on bottles giving consumers the choice. Food labelling is far more rigorous so it seems only fair that we are given proper information about what is actually in our wine. Certified organic and biodynamic wines will have far less SO₂ added so maybe until then experiment with more naturally made wines, broaden your horizons and indulge in new experiences.

Author: Ruth Farnon

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