Sulphur dioxide has become the most widely used additive in winemaking because it has four important properties:-

Firstly and most important, the main reason for it's use, is the prevention of oxidation due to its strong anti-oxidant properties. The basis for its anti-oxidant capability is because SO₂ readily combines itself with oxygen forming sulphuric acid. Although SO₂ will get rid of any oxygen dissolved in the wine, this reaction does not occur immediately. Every atom of oxygen destroys a molecule of SO₂ resulting in a constant decreasing concentration of SO₂. A delicate moment is in the preparation of wine for bottling, in the addition and fine  adjustment of SO₂ level, because this will preserve the quality and guarantee that the wine does not get oxidised too early.

Secondly is its antiseptic capability. Bacteria such as Acetobacter are aerobic germs that require oxygen to flourish. SO₂ will poison the bacteria at the same time that it removes any oxygen that can act as a nutrient for this germ. The antiseptic property of SO₂ is used in cleaning and disinfecting barrels and other winery equipment, and in trying to prevent malo-lactic fermentation by attacking the lactobacillus that convert malic acid into lactic acid.

The third property is its anti-oxidasic capacity, in relation to enzymes called oxydases that act as catalytic factors which promote oxidation. SO₂ poisons these enzymes, therefore preventing oxidation and helping to preserve the colour of white wines and preventing it browning.

Fourthly is its corrective property, that can help freshen up tired wines which are suffering from a slight degree of oxidation. The reason for this is that one of the main products of the oxidation of alcohol is acetaldehyde. When added to a tired wine, SO₂ combines with acetaldehyde, converting it into an odourless and tasteless compound, removing hints of oxidation and refreshing the wine.

Free SO₂ can exist in more than just one form. It is soluble in water and when dissolved reacts with water to produce sulphurous acid: SO₂ + H₂O = H₂SO₃; which is harmless in small quantities.
Sulphurous acid can exist in a solution or as an ionised form, where the molecule of acid splits into a positive ion and a negative ion. The reaction can also go in the opposite direction, combining the two ions to form un-ionised sulphurous acid, which can decompose into water and SO₂ and it is known as molecular sulphur dioxide. It is only the molecular version of SO₂ that has the antioxidant and antiseptic properties, and not all the free SO₂.
What is really important in regard to this is the fact that the proportion of molecular SO₂ increases as the pH gets lower. Therefore, an acidic wine will require the addition of less SO₂ than those wines with a higher pH.

A disadvantage of SO₂ is that it bleaches the colour from red wine, and can also result in loss of fruit character, although the wine recovers eventually when the level of SO₂ decreases naturally.

In the EU the permitted additives for introducing SO₂ into wine are:-

- sulphur dioxide gas: from a cylinder, via a special measuring attachment.

- wine sulphur': potassium bisulphite and potassium metabisulphite powder: which when really fresh (say within 7 days of manufacture) produces roughly 50% of its weight in free SO₂ once in solution; however, this percentage reduces slowly as the powder ages, to perhaps 35%, so a larger weight of powder may need to be added in order to achieve the desired level of free SO₂. For this reason it is preferable to re-measure the level of free SO₂ after the addition of wine sulphur.

Under EU regulations, since 25th of November 2005, any wine that contains more than 10 mgm/litre of SO₂ has had to state on the label 'Contains Sulphites' or 'Contains Sulphur Dioxide'.

In the human stomach, much of the bound or fixed SO₂ will be released by the stomach's acidity and the warmth of the digestive system, this would become toxic for human health if above the safe limit. This is why the EU and most countries have established legal maximum limits for different types of wines.
It is understood that red wines need less protection because polyphenols are natural antioxidants.
Natural sweet wines will require a much higher amount because sugar and other compounds bind to SO₂, for example, TBA or Sauterne wines can need up to 400 mgm/litre.

European Commission regulations (EC 1493/1999 and EC 753/2002) specify a maximum total SO₂:-

Prior to the 2009 vintage:-

1. For wines, other than sparkling wines and liqueur wines, of:-
(a) 160 mgm per litre for red wines.
(b) 210 mgm per litre for white and rosé wines.
(c) 185 mgm per litre for Quality Sparkling wines.

2. Notwithstanding paragraph 1, for wines with a residual sugar content of not less than five grams per litre, a maximum of:-
(a) 210 mgm per litre for red wines.
(b) 260 mgm per litre for white and rosé wines.
(c) 300 mgm per litre for white quality wines psr originating in the United Kingdom described and presented in accordance with British legislation by the term 'botrytis' or other equivalent terms, such as 'noble harvest', 'noble late harvested' or 'special late harvested'.

From the 2009 vintage onwards:-

1. For wines, other than sparkling wines and liqueur wines, of:-
(a) 150 mgm per litre for red wines.
(b) 200 mgm per litre for white and rosé wines.
(c) 185 mgm per litre for Quality Sparkling wines.

2. Notwithstanding paragraph 1, for wines with a residual sugar content of not less than five grams per litre, a maximum of:-
(a) 200 mgm per litre for red wines.
(b) 250 mgm per litre for white and rosé wines.
(c) 300 mgm per litre for white quality wines psr originating in the United Kingdom described and presented in accordance with British legislation by the term 'botrytis' or other equivalent terms, such as 'noble harvest', 'noble late harvested' or 'special late harvested'.

The amounts of wine sulphur to add to a wine is not a simple matter. A set of scales are needed to weigh accurately to 1 gram.

White wines, rosé and sparkling wines, are treated differently from red wines. Free SO₂ is the important measure for these, only total SO₂ is measured for red wine.

For white or rosé wine:-

The simple approach is: add 10 grams of wine sulphur per 100 litres of presses juice/must, to try to 'sterilise' the must and ensure that malo-lactic fermentation does not take place later. This SO₂ is consumed during the fermentation process.
More experienced winemakers use much less wine sulphur, unless the grapes are in poor condition (or the temperature is high): add 4 to 8 grams per 100 litres of mash; pectolytic enzymes reduce the need for wine sulphur.
For Rosé wine, less wine sulphur should be added as it removes colour.

Then after the fermentation is complete, when racking - put into the receiving tank a calculated amount of wine sulphur; 8 to 10 grams per 100 litres of wine, i.e. sufficient to give about 40 to 50 mgm/litre free SO₂, which will help to prevent oxidation of the wine, until the free SO₂ level falls below about 20 mgm/litre (depending on the pH of the wine).

Different vine varieties 'consume' SO₂ at different rates, so ideally it is necessary to measure the SO₂ every month or two with a test such as the Sulfacor.

At bottling it is usual to adjust the free SO₂ level to between 30 and 35 mgm/litre (slightly more if it is a sweet wine).
I find that sterilising the bottles with 2% free SO₂ solution, some hours prior to filling, adds about 5 to 8 mgm/litre free SO₂ to the wine.

The total SO₂ content must be below the upper limit set by European Commission regulations, as stated above.

For red wine:-

Immediately after de-stemming/crushing: to stop any 'browning', add wine sulphur at 2.5 to 6 grams per 100 litres of mash.
Wine sulphur takes out some colour so add as little as necessary.

Then when first racking, add a calculated amount of wine sulphur: 3 to 7.5 grams per 100 litres of wine; providing about 30 to 75 mgm/litre total SO₂, according to whether you hope malo-lactic fermentation will take place or not.
For malo-lactic fermentation to take place the total SO₂ must be less than 50 mgm/litre, although it can take place at higher levels.
Put the calculated amount of wine sulphur into the receiving tank before pumping. This is better for the wine than rousing it in afterwards.

When malo-lactic fermentation takes place it consumes at least 50 % of all SO₂, so after it is complete (it takes a few months) the total SO₂ should be measured and increased to 50 to 75 mgm/litre.

The total SO₂ level at bottling should be 50 to 75 mgm/litre; well below the upper limit set by European Commission regulations, as stated above.

For Sparkling Wine:-

After primary fermentation, when first racking, add a calculated amount of wine sulphur: to raise the free SO₂ to 17 mgm/litre.

The level of free SO₂ always needs to be as low as possible, maximum 20 mgm/litre, just to preclude any oxidation occurring before it is bottled for secondary fermentation in bottle.

After secondary fermentation, no wine sulphur is added. The CO₂ pressure in the bottle prevents oxidation.

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