SO2 Management and Tannins

This is probably the most important topic connected withmany aspects in winemaking: the overall oxidation-reduction management.

It is a multifaceted and highly complex subject, closely intertwined with numerous factors: grape health, harvesting methods, pressing, gas blanketing, turbidity, fermentation, lees contact, metals, élevage and ageing, storage conditions, vessel type, microbiological activity, low-intervention winemaking, organic and biodynamic practices, fining, filtration, dissolved oxygen, temperature, pumping, bottling, corking or capping, and closure specification.

There is so much to understand—and still so much we do not—that oxidation management alone could easily occupy an entire year of study.

For now, I will focus only on the most important practical aspects of two remarkable natural additives.

As with most things in winemaking, there is no single correct approach. Their use depends entirely on conditions and objectives. That said, the fundamental criteria that truly make the difference remain surprisingly consistent. While tasting wines, I am still struck by how many of the issues encountered in UK wines relate to oxidation–reduction management. Regrettably, many of these wines are also medal winners.

When to use SO₂ and tannins

The best moment to use them is, quite simply, when they are needed—and that depends on the wine’s purpose, style, and condition.

I will inevitably make some generalisations, but it is worth remembering that some wines are genuinely unique. I stopped using SO₂ in juice approximately eight years ago for several reasons, including my use of tannins, yeast-inoculation strategies, pressing criteria, gas blanketing, ethanal production, and the resulting SO₂ levels in finished wines.

In my trials, gallic tannins have consistently been the preferred choice in juice. They act more rapidly than SO₂, have a shorter lifespan, and are far less influenced by sugar content and pH (certainly less so than SO₂). Importantly, they do not bind ethanal in the early stages of alcoholic fermentation, unlike SO₂. On the downside, tannins reduce glycerol production, and their antimicrobial effect is minimal at commonly used dosages.

My first sulphur dioxide addition for white and rosé wines usually takes place shortly after alcoholic fermentation, once the wine has tested dry in the laboratory. My preferred quick test is Fehling’s solution, which takes approximately 15 minutes. For red wines, I prefer to delay the first SO₂ addition by a few weeks. This allows for some ethanal production, aiding early colour and tannin stabilisation, while also delaying malolactic fermentation (MLF) by a few months. Delayed MLF can lead to improved colour stability and lower ethanal levels in the finished wine, though that is a subject in its own right.

At the time of the first SO₂ addition, I also introduce tannins for a second time. At this stage, I often use a blend of gallic, condensed, and ellagic tannins.

After four years of trials across three different wines, some tannins performed surprisingly well when compared to control wines treated solely with SO₂.

It is important to remember that there are many different types of tannins, each with specific targets. Even tannins of the same category can behave quite differently depending on the supplier, extraction method, and composition.

Depending on the amount of SO₂ used initially, the second addition must account for an even broader range of variables: pH, temperature, alcohol content, turbidity, vessel type, volume, ullage, gas blanketing—and ultimately, the desired outcome.

From this point onward, the only meaningful SO₂ measurement is the molecular fraction, not free or total SO₂ (with some exceptions). Bound SO₂, particularly over the long term, provides valuable insight into the wine’s oxidative status, age, and overall handling.

By combining sulphur dioxide and tannins thoughtfully, it is often possible to reduce total SO₂ usage. However, this approach is not suitable for every wine, and trials are essential.

How to use SO₂ and tannins

I often recommend a single, relatively large initial addition of sulphur dioxide (50–70 ppm), rather than multiple smaller additions. This approach takes advantage of the temporarily higher concentration of free molecular SO₂, which for a few hours is less dependent on pH. When applied with the correct criteria, there is no risk of overdosing at this stage.

From the second addition onward, molecular SO₂ becomes critical. Target levels should be determined according to the wine’s purpose, intended storage duration, and storage conditions—typically ranging from 0.6 to 1.2 ppm depending on application. Above 1 ppm, SO₂ may become perceptible on the nose; however, if the wine is not yet intended for sale, such levels are perfectly acceptable for storage, particularly when monitoring intervals are extended.

Tannin usage is more complex. Proper application requires bench trials, systematic testing, and extensive tasting experience—especially as bottling approaches.