Most people know that this means that sulphur in some form has been added to the wine and some controversy surrounds this. Many suggest they are allergic to this particular chemical and look for preservative free wine.
But there is not a great understanding about the use of sulphur or more properly, sulphur dioxide (SO2), in wine.
Let’s start by saying sulphur was used by the Romans as a wine preservative. It is also a natural by-product of fermentation and wine will always contain some, albeit in its "bound" form, a term we will come to later.
But why add SO2?
Quite simply it has both antibacterial and antioxidant properties, is relatively non toxic and its excess can be detected by smell.
In other words, SO2 can be used in very small quantities to reduce oxidation of grape juice during wine production and to inhibit oxidative and bacterial spoilage of wine during storage.
In Australia the maximum legal addition limit in dry wines is 250 parts per million (ppm) or 250mg per litre of wine. Therefore at its maximum usage a bottle of wine would contain 0.1875g of SO2.
SO2 is generally added to wine in the form of potassium or sodium metabisulphite.
Both these chemicals produce approximately 50% by weight of SO2 in a solution.
In wine it exists in two forms, free and bound ie. some becomes bound to phenolics eg. anthocyanins (wine pigments), flavour compounds and tannins, and some remains unbound ie. free.
The free form is the more effective component.
In turn, the free form consists of molecular (unionised) sulphur dioxide, the bisulphite anion and the sulphite anion with the proportions of these depending mainly on the pH of the wine.
Molecular SO2 is the most germicidal.
The more acid the wine the higher the proportion of molecular sulphur dioxide.
It has been shown that a level of at least 0.8mg/L of molecular SO2 is required in white wine to inhibit oxidation and bacterial growth. To achieve this it is necessary to maintain varying levels of free SO2 not only according to pH but also alcohol content. The higher the alcohol content of a wine the less molecular SO2 is required.
So, as an example, to maintain a 0.8ppm molecular SO2 level in a 12% alcohol white wine of pH 3.4 requires a free content of 32ppm.
SO2 is added on a continual basis throughout the wine’s production, bulk storage and bottling phases so it is necessary to continually monitor the free SO2 level.
In the ‘old days’ we used the ‘Rankine’ or Oxidative/Aspiration Method of determining free, bound and total SO2 but now relatively cheap laboratory machines automatically do it for you.
Now to red wine. As mentioned SO2 binds to phenolics such a wine pigments and tannins. Obviously red wine has a high concentration of these compounds and a considerable amount of SO2 is bound. Measurement of free SO2 in red wine is virtually impossible due to the aspiration of bound SO2 during analysis giving false high readings. Instead recommended SO2 levels for red wine are given as 50-150 ppm of total SO2, depending on circumstances and the pH of the wine.
Sometimes too much SO2 can be added by mistake (believe me this can happen!) and it needs to be removed. This is done by the addition of hydrogen peroxide in very minute quantities. The reaction is the same as that used in the Rankine/Aspiration method for determining sulphur dioxide levels ie. the hydrogen peroxide oxidises the sulphur dioxide to form sulphate and hydrogen ions: H2O2 + SO2 => 2H+ + SO4=
To calculate the required volume of hydrogen peroxide, you have to first determine the reduction in total sulphur dioxide required. The volume H2O2 required is related to the strength of the peroxide solution, which is typically 35%, therefore this must be known. Finally, the volume of juice/wine must be determined. The volume of Z %w/w hydrogen peroxide (in mL) required to reduce the sulphur level by the required amount in (mg/L) in a given volume of wine (L) is calculated as follows: Volume H2O2 = 0.049/Z x (SO2 reduction) x (Volume juice/wine). So that's your wine chemistry fix for the month and maybe even for the year!