Winegrowers Supplies  -  The use of Sulphur dioxide (SO2)

Sulphur dioxide is a gas formed according to the equation: S + O2 = SO2.
It is important to distinguish between free, bound and total SO2.
SO2 is a reactive substance that will combine not only with oxygen but also with other natural compounds in wine such as sugars, aldehydes and ketones. When SO2 is combined with these substances it is known as bound or fixed SO2. The part that is not combined is known as free SO2 and it is this that possesses all of the properties that protect wine. The total SO2 will be the sum of both bound and free SO2.

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 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 rosť 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 rosť 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 rosť 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 rosť 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, rosť 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 rosť 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 Rosť 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 30 to 35 mgm/litre (slightly more if it is a sweet wine).
I find that sterilizing 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-stalking/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:-

 

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