Sulfur

“Sulfur” is present in many forms in the vineyard and winery and has been used for its antiseptic properties as far back as the ancient Greeks, who noticed that sulfur spewing from volcanos helped control local rat populations (1). Sulfur itself is an element found on the periodic table that can combine in many forms to make many compounds; most importantly in grapes, sulfur is essential to make proteins. Elemental (uncombined) sulfur is sometimes used in the vineyard as a spray to prevent rot (2). Sulfur can also form compounds by sharing electrons with other elements to form bonds. 

Sulfides

If sulfur bonds with another element in a way that the electrons spend more time near the sulfur atom than the bonded partner, then sulfur is said to be reduced (since electrons are negative). Sulfides are the reductive compounds of sulfur. These include compounds with negative sensory impressions such as H2S and mercaptans, which smell like rotten egg, skunk and cabbage. They are often formed as a result of nutrient deficiency during fermentation. However, some reduced sulfur compounds have positive sensory effects, such as the thiols (passionfruit, guava, sweet citrus, boxwood) that lend varietal character and aromatic complexity to wine (2). 

Sulfites

When sulfur bonds in a way that loses electrons, this is called oxidation, and sulfites are formed. Most often this occurs when sulfur bonds with oxygen (though not always). Sulfur dioxide is the most common form of sulfite in winemaking (2).

References:

(1) Kate, Emily. From: The History of Sulfite Use in Wine, guest post, The Academic Wino, Sept 25, 2014.
(2) Henderson, J. Sulfur Dioxide and Wine Additives. In Introduction to Enology; Santa Rosa, California.

When used in the winery, SO₂ is most often in a liquid form. When SO₂ dissolves in water, it doesn’t remain only as SO₂. Instead, it interacts with the water in a way that takes three forms: molecular sulfur dioxide, bisulfite and sulfite (see equation). The amount of any one form that is present depends on the pH of the solution, with the equation shifted to the left in low pH environments and to the right in high pH environments. This means that, the higher the pH, the less molecular sulfur dioxide is available (1–3). These relationships are also in dynamic equilibrium, so if SO₂ is removed from solution, say by entering into a microbial cell, sulfite ions will give up another hydrogen ion to replenish the “missing” molecular SO₂. Likewise, if a bisulfate anion binds to an acetaldehyde, a molecular SO₂ molecule will gain and O and an H to replace it. This happens only as much as the remaining ions are available such that if the balance is 20%, 60%, 20%, they will always be in this balance, regardless of what concentration that means for each constituent. (At wine pH, this is more realistically 2%, 98%, 0.1%). Also, if SO₂ is added to the solution, even if it is added in the form of molecular SO2, it will quickly redistribute to its various forms based on pH (1–3). 

The figure below gives a graphical representation of the relationship between pH and the forms of sulfur dioxide. Between pH of 3 and 4, which describes most wine, bisulfite is the predominant form of sulfur dioxide while molecular sulfur and sulfite are scarce.  At a pH of 3.0, molecular sulfur dioxide makes up 5.6% percent of the SO₂, bisulfite makes up 94.4% and the sulfite ion is makes up only 0.006%. At pH of 4, these numbers are 0.585%, 99.4%, and 0.06%, respectively (3).  This balance of forms is important because not all forms of sulfur dioxide have the same activity.

from Rotter n.d. (4)

References

(1) 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.
(2) Zoecklein, D. B. Sulfur Dioxide (SO2). Enology Notes Downloads, 16.
(3) Boulton, R.; Singleton, V. L.; Bisson, L. F.; Kunkee, R. E. Principles and Practices in Winemaking; Chapman and Hall, Inc: New York, 1996.
(4) Rotter, B. Sulfur Dioxide. Improved Winemaking: advanced theory, practical solutions and opinions.

What do the different forms of SO₂ really do? Why does this matter? The activity of SO₂ in grape juice and wine depends on the chemical form it is in. Undersanding the rold of each form can lead to better management decisions. Following is a summary of  the activities of each form.

Molecular SO₂

Molecular sulfur (SO₂) is prized because of its antimicrobial activities. As the only form of sulfur dioxide that is not charged, molecular sulfur can penetrate the cell membranes of microbes and cause cellular damage and death. Once inside the cells of yeast and bacteria, which themselves have a pH around 6.5, molecular sulfur converts to the bisulfite form and denatures proteins, disrupts cell membranes, and ends cell functioning (1–3). Many Saccharomyces cerevisciae have a special cellular pump to rid the cell of sulfide (4), which means they are less affected by SO₂ than other organisms. Molecular SO₂ also has antioxidant properties as it binds to hydrogen peroxide, a main component of oxidative cascades in wine (3). This form is volatile, and in high enough concentration, can cause negative sensory aromas in the headspace of wine. It is also the one way that free SO₂ is lost during aging, as it diffuses into the headspace of barrels and is dissipated (3). The ability of SO₂ to volatilize is used to measure the concentration of SO₂ during the aeration oxidation test, when strong acid is used to convert all free SO₂ in solution to the molecular form, which is bubbled to a catchment vessel and titrated with base (3).

Bisulfite 

The bisulfite form of sulfur dioxide (HSO₃^-) dominates at wine pH. Bisulfite is a potent inhibitor of enzymes such as tyrosinase (aka polyphenoloxidase) that causes enzymatic browning in juice and wine, though it is somewhat less effective against laccase, the oxidative enzyme produced by Botrytis (2). The activity of bisulfite is limited by the fact that it binds many constituents in the wine including acetaldehyde, anthocyanins, and sugars (1–3). Once bound, it no longer acts as an antioxidant. Bisulfite binding to acetaldehyde forms a compound without sensory impact, improving the nutty or bruised apple aroma that accumulates during malolactic fermentation (2,3). Binding of bisulfide to anthocyanins in red and Rose wines can cause color bleaching, as the bound form is colorless. This binding is reversible, so anthocyanins may be released if sulfite is bound up and the equilibrium shifts, however, binding also blocks the polymerization reactions with tannins that would instead stabilize anthocyanins for longer term aging (1,2). Bisulfite also binds sugar. This is the primary fate of most of the sulfur dioxide added to grape must (1,2). New oak barrels have many un-bound sugars that get bound up by bisulfite, a primary reason new barrels tend to require higher SO2 additions than older barrels.

Sulfite 

At the pH of wine, the sulfite ion (SO₃^-2) is nearly non-existent. This form of sulfur dioxide binds directly with oxygen in solution in a slow reaction, so it does have antioxidant properties. However, this reaction is very slow so only has an effect during long bottle aging, where it can be important in phenolic maturation (3).

References

(1)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.

(2)Zoecklein, D. B. Sulfur Dioxide (SO2). Enology Notes Downloads, 16.

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

(4) Bisson, L. F. Geographic Origin and Diversity of Wine Strains of Saccharomyces. American Journal of Enology and Viticulture 2012, 63 (2), 165–176. 

https://www.goodfreephotos.com

Most wineries manage SO₂ by measuring free SO₂, then determining an addition rate to achieve a specific target. But how should that target be determined? Sulfur dioxide is used in wine as an antioxidant and an antimicrobial agent. Each form of SO₂ in solution contributes to the antioxidant function. The molecular form will react with H₂O₂, quenching a cascade of oxidation set off when phenols are present. The bisulfite form binds with quinones, also quenching this cascade as well as inactivating oxidizing enzymes, the fastest way to browning. The sulfite form reacts with oxygen directly, but slowly (1,2). 

 The antimicrobial properties, however, are mostly due to the molecular form, which is why this is the focus of most SO₂ management decisions. Several studies have been done to determine the rate of SO₂ needed to inhibit or kill microbes. Often these rates are reported as ranges because the effective dose is dependent on other factors, such as the alcohol and pH of the wine, or the stage of growth of the microbe. For example, yeast are more susceptible to SO₂ in the early stages of population growth than in the later stages (2). Also, care must be taken as some levels are reported to inhibit growth, but those microbes may still be present in the wine in a dormant state that will re-animate when SO₂ levels fall.

The bisulfite form of sulfur dioxide cannot enter cell membranes but does make a small contribution to microbial stability in two ways. Lactic acid bacteria have been shown to be inhibited by free as well as bound (bisulfide) forms of sulfur dioxide, with as little as 10 mg/L inhibiting growth and 30 mg/L inhibiting viability. Some lactic acid bacteria such as Leuconostic consume acetaldehyde. If the acetaldehyde is bound by bisulfide, this form is then released as SO2 back into solution, which further inhibits growth (1–3). The bisulfite form of sulfur dioxide also binds to thiamine and destroys it. Thiamine is an essential micronutrient for microbes. Destruction of this one micronutrient may limit population growth in spoilage microbes such as Brettanomyces and Lactobacillus. However, overuse of SO2 at crush can limit thiamine for Saccharomyces, and lead to difficulty in primary fermentation (Bruce Zoecklein, personal communication).

When managing SO₂ in wine production, free and total sulfur dioxide are the most common measurements used. Free SO₂ refers to any form of SO₂ in wine that is not bound to another molecule. As mentioned above, the bisulfite form of SO₂ binds readily to many common constituents of wine, which also means it is no longer available for use as an antioxidant. Total sulfur dioxide, then, is the sum of the free sulfur dioxide (which includes all unbound molecular, bisulfite and sulfite forms) and bound sulfur dioxide (mostly bisulfite bound to its many targets) (Figure 2)(1,2,4). 

Figure 2: Forms of free and bound sulfur dioxide in wine. From Zoecklein (4)

Since the proportion of SO₂ in any one form in wine is pH dependent, the concentration of molecular SO₂ can be calculated using the value for free SO₂ and the pH of the wine. Many textbooks and online sources provide graphs and tables for this calculation. The Wine Adds website (www.wineadds.com) is a good resource that calculates the molecular sulfur as well as how much SO₂ should be added to reach a given target. For example, if a winemaker has a Chardonnay at pH = 3.4 that he/she would like to age at 0.8 ppm SO2, the free SO₂ needed to hit this target would be 32 ppm. If that same Chardonnay had a pH of 3.25, only 22 ppm free SO₂ would be needed. So, a small acid addition may allow for a much lower SO₂ addition. The importance of pH in SO₂ management becomes even more pronounced for a Viognier at a pH of 3.6 (50 ppm). To age a Petit Verdot with a pH of 3.8 at a molecular sulfur of only 0.5 ppm, he/she would still need to carry 49 ppm of free SO₂!

 The most common technique for measuring free sulfur dioxide in the winery laboratory is the aeration oxidation (AO) method. However, there are a few cautions when using this test. In the first step of AO testing, all of the free SO₂ is converted to the volatile molecular form by the addition of acid. The volatilized SO₂ is then transferred through tubing to a catchment vessel with hydrogen peroxide that converts it into liquid sulfuric acid, which is then titrated with base to determine the total amount of acid. With this method, bisulfite that is loosely bound may be released by the strong acid used in the first step, causing many AO measurements to overestimate the amount of free SO₂ in the wine. This is especially true in red wines where bisulfite is bound loosely to anthocyanins, which are released and counted as free SO₂ in the analysis but have been shown not to have meaningful activity in the wine (8). If one is using free SO₂ measurements by AO to determine molecular SO₂ in the wine, the wine is left less protected against microbial spoilage (5). 

The attached report gives a summary of target SO2 levels for antimicrobial and antioxidant targets from several sources. 

Many guidelines used in winemaking regarding how much SO₂ to add to the wine, are based on known quantities needed to inhibit oxidation or microbial growth. Table 1 (insert link) provides a compilation of these quantities from various sources. In many cases, ranges are given rather than firm numbers. Microbes are not a single entity, and different strains will have different tolerance under different conditions. The effectiveness of SO₂ on a given population depends on the starting population, so good cellar practices that limit microbes, such as settling after pressing, racking, and filtering, increase overall effectiveness of SO₂ additions. Generally, stress is cumulative, so tolerance to SO₂ is lower if other stressors such as high ethanol or low pH are also present. SO₂ binding also occurs slower in lower pH solutions. However, microbes can acquire tolerance over time. Most targets for antioxidant effects will be met if one is also managing for antimicrobial effects (4). 

 When determining how much SO₂ to add, it is important to keep in mind that some of the added SO₂ will be bound up by components of the wine. It is estimated that 1/3 to ½ of the SO₂ addition to finished wine will be bound up within the first few days of addition (1,3,6). The proportion is higher in younger wines because more unbound components exist. Botrytis and acetic acid bacteria are both known to produce high levels of compounds that bind SO₂, so infected wines will also bind more SO₂ (1,7). If the free SO₂ is already relatively high, less binding is expected (4). SO₂ binding takes time; 3-5 days should be allowed prior to re-testing (4,6). SO₂ will be lost faster in barrel storage (up to 5ppm per month) as well as any time the wine is exposed to oxygen (such as during racking, filtration or storage in untopped tanks)(6). When fine tuning SO₂ additions for bottling, Zoecklein (3) recommends adding 5-6 ppm to offset the oxygen in the headspace of the bottle while Stamp (6) recommends adding 8-10 ppm to offset all of the processing steps leading up to bottling. 

 The decision of how much SO₂ to add to a wine is a good demonstration of the goldilocks principle: one wants to add just enough but not too little or too much. 

Injudicious use can lead to:

• loss of color due to anthocyanin bleaching (8,10)
• reduced rate of tannin polymerization and thus maturation in red wines (9)
• neutralization of other aromas of the wine (1)
• negative odors itself such as a metallic, harsh pungent aroma4, wet wool or burning characteristic (1). Sensory thresholds for these negative aromas depend on the temperature and pH of the wine, with 10ppm in air and 15-40 mg/L in wine common ranges. They are more perceptible in low pH wine at higher temperature (2).

Judicious use can:

• preserve the freshness and fruit character of the wine by protecting it from oxidation (8)
• reverse the nutty, oxidative character of wines caused by acetaldehyde (8)
• increase extraction of color in red wines (9,10)
• prevent browning of white and red wines during aging (2,9,10)
•  help prevent microbial spoilage leading to ethyl acetate and acetic acid (8)
• prevent malolactic fermentation in crisp white wines (8)

There is no single right target SO₂ for aging wine. Rather, each winemaker must decide the target based on the history, future plans, and winemaking goals for each wine. When deciding on a target free SO₂, the best decisions rest on consideration of the disease load of the grapes coming in, the pH and alcohol of the wine, and the amount of time you will be aging the wine before bottling. Other decisions include when to make that first addition, as well as how much to add each time (fewer larger additions or multiple smaller additions). 

References

(1)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.

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

(3) Zoecklein, D. B. Sulfur Dioxide (SO2). Enology Notes Downloads, 16.

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

(5) Howe, P. A.; Worobo, R.; Sacks, G. L. Conventional Measurements of Sulfur Dioxide (SO2) in Red Wine Overestimate SO2 Antimicrobial Activity. Am J Enol Vitic. 2018, 69 (3), 210–220. 

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

(7) Margalit, Y. Concepts in Wine Chemistry, 3rd ed.; The Wine Appreciation Guild LTD: San Francisco, California, 2012.

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

(9) Gomez-Plaza, E.; Gil, R.; Lopez-Roca, J.; Adrian, M. Effects of the Time of SO2 Addition on Phenolic Compounds in Wine. Vitis -Geilweilerhof- 2001, 40, 47–48.

(10) Picinelli, A.; Bakker, J.; Bridle, P. V. Model Wine Solutions: Effect of Sulphur Dioxide on Colour and Composition during Ageing; 2015.

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.

Sulfonation of wine procyanidins occurring in wine, from: "Influences of Storage Conditions on the Composition of Red Wines

In addition to its effects on preserving fruit flavors and aromas, preventing oxidation and spoilage, and potentially muting flavors, SO₂ has other impacts on the sensory perception of red wines through its interactions with phenolic groups. In red wines, bisulfite binds mainly to acetaldehyde and anthocyanins. When bisulfite binds to anthocyanins, it turns these pigments colorless, decreasing color intensity (1,2). Bisulfite also binds in the same location on the anthocyanin molecule that tannins would bind, therefore the presence of SO₂ may delay or diminish the formation of more stable polymeric pigments (1). Polymeric pigments that have already formed resist bleaching by SO₂ for this same reason (1). Binding of SO₂ to anthocyanins is reversible, so as free SO₂ decreases over time, anthocyanins may be released and gain color (3,4), however at wine pH, the bound form is highly favored (2). 

In addition to its affect on color stability, the presence of SO₂ also affects the polymerization of anthocyanins and tannins due to its interaction with acetaldehyde.  Acetaldehyde forms bridges between tannin subunits and anthocyanins during polymerization, stabilizing the complexes until more long-term bonds can form. SO₂binding to acetaldehyde prevents it from forming bridges. Without acetaldehyde, polymerization reactions are slower, including those among catechin molecules (formation of tannins) as well as between catechin and anthocyanins (formation of polymeric pigments)(5).  

The polymerization of tannins is thought to diminish the astringency and bitterness of wine because larger tannins have less interaction with salivary proteins, so less polymerization in the presence of SO₂ would thus lead to more astringent and bitter wines. However, recent work by Arapitsas et al (2018)(6) and the Waterhouse lab have shown this may not be the case.  In a talk at the 69th ASEV conference, Andrew Waterhouse described discovering examples of sulfonated forms of anthocyanins and the monomers that make up tannins during an experiment examining quinone reactions during the phenolic cascade. In these experiments, the molecules were sulfonated at the position where tannins would normally bind. Normally, the bond that holds tannins together is broken by acid hydrolysis during aging, monomers are released, then re-polymerize over time to form less astringent tannins. Waterhouse hypothesized that in the presence of SO₂, the SO₂ binds to the subunits instead of another tannin or anthocyanin molecule, essentially stopping the polymer from growing. He also found tannins modified with sulfonation, meaning the chain of monomers was capped by a sulfur group. Despite the shortening of tannin complexes, Arapitsas et al (2018)(6) found that when tannin, acid and SO₂ were all present, protein precipitation products were abolished, leading to a predicted softening of the astringency of wines. 

Taken together, these studies reveal a new element of tannin evolution during aging that may significantly affect the sensory perception of astringency with wine age. After discussing the current view of tannin softening by tannin polymerization during aging, Arapitsas et al (2018)(6) conclude their work by saying “alongside these reactions, we should now add sulfonated monomeric and dimeric flavenols, which are expected to direct interaction with proteins… sulfonated flavanols, a class of compounds so far neglected, could play a role in improving the sensorial quality of red wine with aging.” 

References

(1) Zoecklein, D. B. Sulfur Dioxide (SO2). Enology Notes Downloads, 16.

(2) Margalit, Y. Concepts in Wine Chemistry, 3rd ed.; The Wine Appreciation Guild LTD: San Francisco, California, 2012.

(3) 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.

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

(5) Picinelli, A.; Bakker, J.; Bridle, P. V. Model Wine Solutions: Effect of Sulphur Dioxide on Colour and Composition during Ageing; 2015.

(6) Arapitsas, P.; Guella, G.; Mattivi, F. The Impact of SO2 on Wine Flavanols and Indoles in Relation to Wine Style and Age. Scientific Reports 2018, 8. https://doi.org/10.1038/s41598-018-19185-5.

(7) Mattivi, F.; Arpitsas, P.; Perenzoni, D. Influence of Storage Conditions on the Composition of Red Wines. ACS National Meeting Book of Abstracts, 2014, Chapter 3.

In 2018 and 2019, Kirsty Harmon from Blenheim Vineyards explored dosage levels and timing of SO₂ addition post-malolactic fermentation in Cabernet Franc and Cabernet Sauvignon. Her studies found that larger initial dose of SO₂ post-malolactic fermentation led to better aromatic complexity and less spoilage than smaller, more frequent doses. Large additions were more likely to allow wine to age at the target molecular SO₂. Also, delaying the initial dose by 2 weeks led to lower total SO₂ overall.

In June of 2020, Virginia Winemakers gathered virtually to discuss SO₂ management and to taste results from an experiment performed by Kirsty Harmon of Blenheim Vineyards. Here is the link to that recording. 

Watch Video Here

Winemakers from around Virginia gathered (virtually) to discuss results of two experiments. In the first, precision and accuracy of two commonly used SO₂ detection techniques (aeration oxidation and the Hanna titrator) were tested and compared by Phil Fassieu (Whitehall Vineyards), AJ Greely (Hark Vineyards) and Rachel Stinson Vrooman (Stinson Vineyards). In the second, Kirsty Harmon (Blenheim Vineyards) compared the chemical, microbiological, and sensory effects of different SO₂ addition rates after completion of fermentation in Chardonnay.

Watch Video Here