# How much SO2 is too much?



## bluecrab (Nov 7, 2020)

I stumbled across some information today that I thought others might find useful. I think many of us have seen this chart for SO2 levels required to protect our wine, as a function of pH. The chart below shows red wine (0.5 mg/l molecular) and white wine (0.8 mg/l molecular).




What I stumbled upon today is the sensory threshold of SO2 (2 mg/l molecular), as a function of pH. I was not aware of the role pH plays in detecting SO2 in wine. So, now I know what the upper limit of free SO2 should be for my wines.



[Improved Winemaking: Sulphur Dioxide]


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## Ajmassa (Nov 7, 2020)

Good find. Probably we all have different thresholds to a degree- but this is a great reference point. Gotta be careful with those low ph wines! Goin by the 1/4tsp per 5-6gal would def put you at the threshold on wines 3.0-3.2 it seems


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## stickman (Nov 7, 2020)

I agree with @Ajmassa, you'll find some conflicting information regarding the sensory threshold of SO2. I've also seen information indicating taste can be affected between 100 and 200ppm total SO2.

A cider study at Long Ashton Research Station was used by Roger Boulton (4) at UC Davis as the basis for establishing 0.8 mg/L molecular SO2 as the recommended maintenance level to suppress microbial activity during cellaring. Once bottled, a target level of 0.5 mg/L has been advocated by Lisa Van de Water of The Wine Lab, which is closer to the sensory threshold. (1)

(4.) Beech, F.W.; Burroughs, L.F.; Timberlake, C.F.; and Whiting, G.C. (1979). Pro-gres recents sur I'aspect chimique et antimicrobienne de I'anhydride suifureux. Bulletin OIV 52(586):1001-1022.

(1) Read more at: https://www.winesandvines.com/columns/section/92/article/97777/Sulfur-Dioxide-Basics-Revisited
Copyright © Wines & Vines, March 2012


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## bluecrab (Nov 7, 2020)

There are more factors involved. Here is a little more information from the article.

*“8. SO2 and Temperature* 
As temperature increases, free SO2 increases and bound SO2 decreases. (SO2 bound to acetaldehyde remains constant.) This is because increased temperatures cause partial dissociation of the bound SO2form, resulting in increased free SO2 and hence increased molecular SO2 concentrations.

For example, a wine containing 68 mg of free SO2 at 0°C (30°F) will contain 85 mg at 15°C (57°F) and 100 mg at 30°C (84°F) [Peynaud, 1984]. Sudraud [1977] showed 64 mg/l of free SO2 at 16°C increased to 120 at 48°C and to 200 at 80°C, as BSO2 was released.

Sometimes, wines with high molecular SO2 levels are served cold to hide the sulphurous aroma they would exhibit with their high SO2 content.


*9. Sensory Threshold *
It is the molecular SO2 form which is responsible for the sensory threshold. Hence, the sensory threshold of SO2 depends on the pH and temperature. There exists considerable variation in threshold within the population. Nevertheless, the sensory threshold is generally considered to be around 2 mg/l molecular SO2.”

[Improved Winemaking: Sulphur Dioxide]


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## bstnh1 (Nov 8, 2020)

The upper limit for commercial wine is 350 ppm!!!!


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## Rice_Guy (Nov 8, 2020)

350 is total, ,,,, 
sulphite is a reactive molecule which produces bound sulphites which are not involved in the pH related production of free SO2. 
The pKa is 1.81 for the disassociation constant of: SO2 + H2O <==> H+ HSO3 ,
, , , , (ie pH where 50% of the reaction has occurred) , , , @bluecrab I was not surprised to see the graphic of pH dependence but I would have guessed the slope of detection of smell to go down as pH increases, , , , an interesting thread



bstnh1 said:


> The upper limit for commercial wine is 350 ppm!!!!


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## balatonwine (Nov 8, 2020)

A few things to consider:

1) the first chart is an "ideal" SO2 level mostly used at bottling. I have had a copy of this chart for a long time. But "ideal" is not necessarily what one should do at different points in the wine making process. For example see (2) below.

2) The second chart seems to give what one should add based on what something like one Campden tablet will do at the ideal pH of 3.4 for a "red" grape at crush (i.e. you have to use more or few tablets as pH changes and that is not obvious from the chart). Ergo, using the "ideal" bottling value from the first chart may be too low at crush.

3) The "upper" threshold for wine of SO2 is defined by local and national law. Check your regulations.

4) The only way to know how much SO2 you actually need, especially at each racking, is to test (classic method is Ripper titration, but there are of course other options available today).

I have attached some additional sources to consider.


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## stickman (Nov 8, 2020)

I just wanted to point out that care needs to be taken when following the molecular SO2 chart with high pH wines.

The following was quoted from the 2005 Gusmer Managing SO2 document above:

"In general, we suggest a lower molecular SO2 for reds than for whites, perhaps around 0.4-0.6 ppm at bottling. Wines with higher pH levels, red or white, may require too high a total SO2 level to achieve desired free SO2 levels. Rather than have excessive bound SO2 (which may give a “chemical” taste), it is best to rely on a combination of factors, including susceptibility to spoilage. Some pH problems can be relieved by adjusting the pH downward with tartaric acid."

What is also interesting is that the above Managing SO2 article from Gusmer indicates "Adapted from: Enology Briefs I (#1), Feb/Mar 1982. University of California Cooperative Extension". Clark was involved with that publication as noted in the references below, and he made a revision in 2012.

Sulfur Dioxide Basics Revisited, Wines & Vines March 2012 by Clark Smith is the article I referenced above; you can read the full article if desired. There are a lot of details, but didn't want to reprint the entire thing here, a few items are noted below.

Clark indicates in the article:

"Thirty-two years ago my first published piece, printed in the inaugural issues of the University of California, Davis, Extension’s Enology Briefs (1)(2), concerned the basics of conventional SO2 management. A table I worked out with pencil and paper in a Shields Library basement can still be found tacked up on winery lab walls throughout the United States. There are omissions I have since regretted, and it is high time for a rewrite. The most important omission in my 1980 article was to point out the folly of applying the table to high-pH wines."

"In low-pH winemaking, we stress the role of molecular sulfur dioxide to control the growth of microorganisms. Since its effectiveness as an inhibitor is greatly lessened at high pH, it is more sensible to forget about molecular SO2 in this zone and instead regulate free SO2 (FSO2)."

"Free SO2, which is substantially all bisulfite, should be maintained to combine with H2O2 as it is formed as a side product of chemical oxidation of diphenols. The reaction of sulfites and peroxide is the fastest reaction known to chemistry, and it may be relied upon to prevent the formation of aldehyde from ethanol oxidation. Since SO2 is depleted by this action and by aldehyde binding, it must be measured by aeration/oxidation and maintained at a reasonable level (20-30 ppm) throughout aging."

References

(1.) Enology Briefs, University of California, Davis, Extension, Vol. 1 No. 1 (1980)

(2.) Enology Briefs, University of California, Davis, Extension, Vol. 1 No. 2 (1980)

(3.) Smith, Clark. Studies on Sulfur Dioxide Toxicity for Two Wine Yeasts In a Wine like Medium and the Mechanism of Cell Death. (1982)


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