The fastest way to lower Total SO2

Winemakers are often looking for ways to limit the use of SO₂. Once overall grape quality and cellar hygiene have been addressed, the best, least risky, way to limit SO₂ is to limit the bound fraction of SO₂ and maximize the unbound fraction, so that each SO₂ addition is more effective. When SO₂ is added to wine, it binds to several components including acetaldehyde, thiamine, enzymes, phenolics, and sugars^1,2. In white wines, 80% of the bound SO₂ is bound to acetaldehyde² whereas in red wines, most bound to either acetaldehyde or anthocyanins³. In sweet wines, glucose also binds SO₂. No matter the wine, reducing acetaldehyde will reduce the bound fraction of SO₂ and help limit total SO₂.

Acetaldehyde is formed by yeast as an intermediate in alcohol production during fermentation. Ethanol in wine can also be oxidized to acetaldehyde during aging ^2,4. It has sensory properties of its own, with a nutty, oxidized aroma sometimes compared to bruised apple^2,5. Bisulfite binding converts acetaldehyde to a heavier , non-volatile compound with no sensory impact³. Bisulfite binding to acetaldehyde is relatively strong and rarely reverses, scavenging most SO₂ in solution. So ,if there is free SO₂, then the acetaldehyde is entirely bound up, packing the bound sulfur fraction^1,3,4.

There are opportunities to control acetaldehyde production at crush, at the end of fermentation, and during long cellar aging. Saccharomyces cerevisciae produces more acetaldehyde than non-Saccharomyces yeast, with a fairly uniform production rate among strains. The most important variable in acetaldehyde production is the addition of SO₂ at crush. SO₂ is toxic to Saccharomyces as well as other microbes, so when it is added, cells take action to detoxify it. In addition to employing cellular machinery to pump SO₂ out of the cell⁶, cells enhance acetaldehyde production. SO₂ will bind to acetaldehyde before it has a chance to enter their cells and shut down cellular function^2,4. Based on this mechanism, limiting SO₂ at crush limits acetaldehyde production.

Acetaldehyde production by yeast peaks during early fermentation after which time the balance shifts and yeast begin to consume it in the production of ethanol². A strong fermentation with adequate nutrition produces a larger number of viable cells at the end of fermentation, which are more likely to consume any remaining acetaldehyde. Other factors that affect acetaldehyde consumption include warmer temperatures (less acetaldehyde is produced) and contact with lees after completion of fermentation². 

Malolactic bacteria consume acetaldehyde simultaneously with and after completion of malolactic fermentation². They also consume other compounds that bind SO₂ including pyruvic acid and alpha keto glutarate². Therefore, allowing full malolactic fermentation reduces SO₂ use. Waiting 1-2 weeks after the completion of malolactic fermentation allows time for the consumption of acetaldehyde (as well as diacetyl if it is present) by malic acid bacteria⁵.

A special note should be made at this point regarding sweet wine production. Due to the response of Saccharomyces cerevisciae to SO₂ (the overproduction of acetaldehyde), trying to stop primary fermentation with SO₂ will be very difficult and produce a wine with a large amount of bound SO₂ (due to both acetaldehyde and residual sugar). The winemaker will get a much better result by stopping the fermentation first with cold or centrifugation^2,4. If chilling is not available, Ribereau-Gayon et al (2006)⁴ recommend adding 100ppm SO₂, then waiting 5-24 hours to see a decrease in activity. They say 1.2 mg/L may be needed to ensure proper storage of a wine with high levels of residual sugar!

Acetaldehyde continues to form during aging as long as oxygen is present. According to Zoecklein³, most acetaldehyde in wine results from microbial oxidation of ethanol under aerobic conditions. Oxygen can be introduced through any number of means including un-topped tanks, racking and barrel storage. Switching out fermentation bungs for solid bungs, topping regularly, gassing with inert gas, and limiting racking can all diminish acetaldehyde formation during aging. Generally, SO₂ can’t compensate for poor cellar practices⁷.

References

(1) Boulton, R.; Singleton, V. L.; Bisson, L. F.; Kunkee, R. E. Principles and Practices in Winemaking; Chapman and Hall, Inc: New York, 1996.

(2) Jackowetz, N.; Li, E.; de Orduña, R. M. Sulphur Dioxide Content of Wines: The Role of Winemaking and Carbonyl Compounds. Appellation Cornell 2011, 3, 1–7.

(3) Zoecklein, B. W. Sulfur Dioxide: Science behind This Antimicrobial, Antioxidant, Wine Additive. Practical Winery and Vineyard Journal 2009.

(4) Ribereau-Gayon, P.; Dubourdieu, D.; Doneche, B.; Lonvaud, A. Handbook of Enology Volume 1: The Microbiology of Wine and Vinifications, 2nd ed.; John Wiley & Sons: West Sussex, England, 2006.

(5) Stamp, C. How Much SO2 to Add and When. Wines and Vines 2011.

(6) Bisson, L. F. Geographic Origin and Diversity of Wine Strains of Saccharomyces. American Journal of Enology and Viticulture 2012, 63 (2), 165–176. https://doi.org/10.5344/ajev.2012.11083.

(7) Stamp, C. Methods for Calculating SO2. Wines and Vines 2011.