Debating sulfur dioxide in wine: is it a necessary evil?

By Peter Douglas, DipWSET – a wine expert affiliated with VinoVoss 
Tuesday, 11 June, 2024


Debating sulfur dioxide in wine: is it a necessary evil?

The use of sulfur dioxide in winemaking has long been a topic of heated debate within the industry and among wine enthusiasts. A contentious issue, many wonder whether sulfur dioxide is an indispensable tool for preserving wine quality or an avoidable additive that compromises natural wine purity.

Is sulfur dioxide in wine a necessary evil?

Wine drinkers today are increasingly seeking out sulfite-free options. For some, it’s a key factor in their enjoyment of wine. However, others, like the Romans before them, view sulfur dioxide (SO2) as a vital tool for preserving wines. The rise of natural wine producers has brought a wave of acclaim for wines made without added sulfur, often touted as the ‘healthier’ or ‘more natural’ option. But here’s the thing: sulfites are a regulated allergen, and the amount allowed in wine is tiny — just a tenth of what you’d find in dried fruit. Strangely, no one seems to be complaining about headaches from all those sulfites in their raisins!

What is sulfur dioxide?

Sulfur dioxide (SO2) is a naturally occurring compound found in grapes and can be added to wine in powder, liquid or gaseous form. Winemakers wear protective gear when adding concentrated SO2 to prevent eye irritation. It binds with the wine and creates two forms: bound SO2 (BSO2) and free SO2 (FSO2). The total amount of both is called total SO2 (TSO2).

When a winemaker talks about a maximum level of 20 mg sulfites in wine, they likely refer to 20 milligrams per litre (mg/L) or 20 parts per million (ppm) of total SO2. Legal limits always refer to TSO2 levels.

Sulfur dioxide (SO2) is a naturally occurring byproduct of fermentation. When using wild yeasts, it’s not uncommon to see levels reach 10 mg/L of total SO2 (TSO2).

In the US, organic wine regulations limit TSO2 to a maximum of 10 mg/L. This creates a point of contention when compared to the European Union (EU), where standard dry white wines can have up to 200 mg/L and reds up to 150 mg/L. Organic wines in the EU are consistently 30 mg/L lower than their conventional counterparts.

[The Australian limit for sulfites in wine is a maximum of 250 ppm in dry wine and up to 300 ppm for sweet wines.]

The effectiveness of SO2 depends on the wine’s pH. Lower pH wines, like sparkling wines (pH 2.9–3.1), have a higher proportion of free SO2 (the active form) compared to high-bodied reds (pH 3.6–3.9). This increased chemical reactivity allows lower pH wines to function effectively with less total SO2. Therefore, maintaining a lower pH can be a strategy to reduce overall SO2 levels.

What does it do?

Sulfur dioxide can be used at nearly every stage of the winemaking process, from grape reception all the way through to the bottling process, and even for cleaning barrels.

It has four key properties:

  1. Antioxidant
  2. Antioxidasic
  3. Antimicrobial
  4. Last-minute correction

Most importantly, sulfur dioxide acts as an antioxidant, preventing oxidation. Winemakers follow a rule of thumb: four oxygen molecules can bind with one molecule of free sulfur dioxide (FSO2). This means FSO2 is constantly consumed. This plays a crucial role in allowing wine to age and mature gracefully, both in barrels and bottles. The amount of added SO2 ultimately influences a wine’s shelf life.

As an antioxidasic, it also keeps specific enzymes in check. The group of polyphenoloxidases (PPOs), which includes laccase and tyrosinase, can promote enzymatic oxidation. This turns the wine brown and contributes to aromas of bruised apple and tired fruit.

Beyond its antioxidant properties, it also serves as a disinfectant. This means it can deactivate microbial activity, particularly from naturally occurring lactic acid bacteria (LAB) like Oenococcus oenilactobacillusleuconostoc and pediococcus. This deactivation can block malolactic fermentation, which is a process that contributes the buttery characteristics to some high-end chardonnays.

Last but not least, it serves as a corrective measure. As a reductive agent, it can reverse oxidation to a limited degree. This means it can act as a corrective measure, rejuvenating and freshening up some tired white wines before bottling. This makes it a valuable tool for bottlers worldwide.

Are there alternatives to sulfites?

Currently, there are no real and effective alternatives on a commercial scale. Winemakers can work with a low pH to increase the free SO2 efficiency. In addition, tannins are naturally occurring antioxidants, which can be added to the wine or extracted more readily to reduce the total levels.

In addition, some winemakers add citric acid, also known as vitamin C, to the wine, to reinforce the efficiency of the SO2. However, it can be denatured, reducing the wine’s quality through oxidative notes. A good hygiene and a low pH are currently considered key to produce wines without adding sulfites.

Putting it to the taste test

It is not possible to taste sulfites in wine directly. Aromas like ‘burnt match’ or ‘flinty’ are commonly viewed as ‘minerality’ or ‘sulfur’, but they may not necessarily indicate high sulfite content.

However, those aromas can be offset by reduction. This means when there is insufficient oxygen during fermentation, hydrogen sulfide (with a ‘d’) occurs. For some wine lovers, these are flaws, for others they contribute to complexity.

Sulfur dioxide is an allergen that can affect some wine lovers. As far as we know, sulfur dioxide does not correlate with headaches for people who are not allergic. The levels are heavily restricted and well below the threshold of other foodstuffs.

Commonly, the narrative excludes that wine is an alcoholic beverage, which can cause hangovers and headaches the following day. Drinking water on the side and enjoying wine in moderation remains a key aspect.

Is it a necessary evil?

Sulfites are an important asset for commercial winemakers who aim to maintain a food-grade and safe product across a large quantity of wines. Brands purchased at grocery stores likely utilise sulfur dioxide (SO2) during production. It’s a significant aspect of the business, aiding in standardisation and reducing the chance of refermentation in the bottle through processes like malolactic fermentation.

However, some winemakers opt not to add SO2 to their wine. They envision each bottle as unique, expressing the terroir. This approach is less commercial but suits some wine drinkers.

Whether sulfites are deemed a necessary evil depends on personal preferences and which approach is favoured. It’s important to note that organic doesn’t always equate to better, and adding sulfites does not render wine a toxic beverage.

Peter Douglas, DipWSET, is a wine expert affiliated with VinoVossAI Sommelier wine search engine and recommendation system. He is an experienced wine trade professional with a diverse background in the HORECA industry, specialist stores, purchasing, portfolio management and general wine trade. He also possesses hands-on experience in winemaking, further enhancing his knowledge and understanding of the industry. Peter’s qualifications include the WSET-Level 4 Diploma in Wines and Spirits, and currently, he is in Stage 2 of pursuing the most esteemed and prestigious title in the wine industry, Master of Wine.

Image credit: Scatena