Tim F
Well-Known Member
- Joined
- 28/1/08
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Hey all,
With wine making season nearly here again I've decided I want a better understanding of using SO2 in wine - exactly what it does, when to use it and how much. I want to be able to work these out on the fly as opposed to flipping through crusty old books on the day and trying to convert campden tablets per gallon to ppm etc
I've spent the day going through a few of my books, reading up online and talking to an organic chemist in the family. I've written up some notes below for myself and think I've wrapped my head around most of it. I know there are some knowledgable winemakers on the forum so if anyone has the time to read through my notes and point out anything that is wrong it'd be greatly appreciated.
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The legal maximum limit for Sulphur Dioxide (SO2) in Australia is 250-300mg/l
Mg/l = ppm
When added to wine, some SO2 becomes bound to other compounds in the wine and some remains unbound. Some bound SO2 is permanently bound to other compounds (aldehydes and proteins), and some forms less stable compounds which may turn back to the free form when the existing free SO2 in the wine is reduced.
With additions early in the wine making process when total SO2 additions are less than 50mg/L, roughly half of additions remain free and half immediately becomes bound. Later, when total additions are above about 60 ppm, 80-90% of any further addition remain as free SO2.
Free (unbound) sulphur dioxide in wine takes 3 forms: SO2 (referred to as molecular SO2) the sulphite anion (SO32-) and the bisulphite anion (HSO3-). Of these, molecular SO2 has the sole (or at least by far the major) anti microbial action.
When measuring wine, free SO2 (unbound SO2) is usually measured rather than total SO2 (eg bound plus free SO2). The ratio of molecular SO2 to the sulphite anions is determined by the pH of the wine, with a lower pH causing more molecular SO2 to be present.
In a winery actual free SO2 can be measured. In home wine making without being to check how much of the total added SO2 is bound vs free this is more difficult so wine makers may just add small amounts of SO2 (15-30ppm subject to pH) when racking, bottling etc.
SO2 can be added to wine with SO2 gas (or liquefied gas), as sulphurous acid or a solution of SO2 in water, or as salts of SO2 (sodium(Na2S2O5) or potassium(K2S2O5) metabisulphite). Potassium metabisulphite comes as a powder or in 'Campden' tablets. 1 Campden tablet contains 0.5 0.55g of Potassium metabisulphite.
Potassium metabisulphite in practice adds around 50% of its weight in SO2 to the wine. For example adding 10mg of potassium metabisulphite to 1l of wine should give 5mg/L or 5ppm SO2. (As per the chart below, 5ppm SO2 in wine with pH 3.4 would give 0.125ppm molecular SO2 )
A minimum of 0.8 mg/L of molecular SO2 is needed in white wine to stop oxidation/bacterial growth. A figure of 0.4 0.6 mg/L is given for red wine but this may be subject to the condition of the grapes, with 0.825 mg/L needed to kill brettanomyces and wild yeasts. Malolactic fermentation is inhibited when total (free and bound) SO2 exceeds 35mg/L
[codebox]
Wine pH Bisulphite Molecular SO2 Total free SO2 (mg/L) needed to give 0.8mg/L molecular SO2
3.0 94% 6% 13
3.2 96% 4% 21
3.4 97.5% 2.5% 32
3.6 98.5% 1.5% 50
3.8 99% 1% 80
4.0 99.4% 0.6% 130
[/codebox]
With wine making season nearly here again I've decided I want a better understanding of using SO2 in wine - exactly what it does, when to use it and how much. I want to be able to work these out on the fly as opposed to flipping through crusty old books on the day and trying to convert campden tablets per gallon to ppm etc
I've spent the day going through a few of my books, reading up online and talking to an organic chemist in the family. I've written up some notes below for myself and think I've wrapped my head around most of it. I know there are some knowledgable winemakers on the forum so if anyone has the time to read through my notes and point out anything that is wrong it'd be greatly appreciated.
------
The legal maximum limit for Sulphur Dioxide (SO2) in Australia is 250-300mg/l
Mg/l = ppm
When added to wine, some SO2 becomes bound to other compounds in the wine and some remains unbound. Some bound SO2 is permanently bound to other compounds (aldehydes and proteins), and some forms less stable compounds which may turn back to the free form when the existing free SO2 in the wine is reduced.
With additions early in the wine making process when total SO2 additions are less than 50mg/L, roughly half of additions remain free and half immediately becomes bound. Later, when total additions are above about 60 ppm, 80-90% of any further addition remain as free SO2.
Free (unbound) sulphur dioxide in wine takes 3 forms: SO2 (referred to as molecular SO2) the sulphite anion (SO32-) and the bisulphite anion (HSO3-). Of these, molecular SO2 has the sole (or at least by far the major) anti microbial action.
When measuring wine, free SO2 (unbound SO2) is usually measured rather than total SO2 (eg bound plus free SO2). The ratio of molecular SO2 to the sulphite anions is determined by the pH of the wine, with a lower pH causing more molecular SO2 to be present.
In a winery actual free SO2 can be measured. In home wine making without being to check how much of the total added SO2 is bound vs free this is more difficult so wine makers may just add small amounts of SO2 (15-30ppm subject to pH) when racking, bottling etc.
SO2 can be added to wine with SO2 gas (or liquefied gas), as sulphurous acid or a solution of SO2 in water, or as salts of SO2 (sodium(Na2S2O5) or potassium(K2S2O5) metabisulphite). Potassium metabisulphite comes as a powder or in 'Campden' tablets. 1 Campden tablet contains 0.5 0.55g of Potassium metabisulphite.
Potassium metabisulphite in practice adds around 50% of its weight in SO2 to the wine. For example adding 10mg of potassium metabisulphite to 1l of wine should give 5mg/L or 5ppm SO2. (As per the chart below, 5ppm SO2 in wine with pH 3.4 would give 0.125ppm molecular SO2 )
A minimum of 0.8 mg/L of molecular SO2 is needed in white wine to stop oxidation/bacterial growth. A figure of 0.4 0.6 mg/L is given for red wine but this may be subject to the condition of the grapes, with 0.825 mg/L needed to kill brettanomyces and wild yeasts. Malolactic fermentation is inhibited when total (free and bound) SO2 exceeds 35mg/L
[codebox]
Wine pH Bisulphite Molecular SO2 Total free SO2 (mg/L) needed to give 0.8mg/L molecular SO2
3.0 94% 6% 13
3.2 96% 4% 21
3.4 97.5% 2.5% 32
3.6 98.5% 1.5% 50
3.8 99% 1% 80
4.0 99.4% 0.6% 130
[/codebox]
