SO₂ in wine

Sulfites, sometimes called sulphates, are important naturally occurring compounds with applications as disinfectants, refrigerants, and lab solvents and reagents. Sulfur dioxide, or SO₂, is a commonly used sulfite which is a colorless, pungent, toxic, corrosive gas at standard temperatures and pressures. It is primarily synthesized by volcanic activity, but also from burning fossil fuels, acid rain, copper extraction, and natural processes occurring in many foods and animals, including inside the human body.

In the food industry, SO₂ is used as a preservative and disinfecting agent. In the following, we will cover how SO₂ improves vinification, or winemaking.

Conventional winemaking

In conventional winemaking, natural sugars found within grapes and other fruits are converted to alcohol (ethanol) via anaerobic fermentation through catalysis using special strains of yeast which are prevalent in fruit skins or artificially added. Fermentation is generally subdivided into two steps, primary and secondary fermentation. Primary fermentation first converts about 60-70% of total fruit sugars to alcohol within the first 3-5 days, and then secondary fermentation converts the last 30-40% of sugars to alcohol within another 1-2 weeks. After fermentation is completed, wines are typically stored and aged, often in wooden barrels, for a few months to many years before their distribution. This aging process helps to better wine’s quality by improving such characteristics as aroma, color, mouthfeel and taste.

Benefits of adding SO₂ in winemaking

A naturally occurring byproduct of fermentation, SO₂ already exists in trace amounts in all wines – even those marketed as “no sulfites added.” Natural SO₂ plays a similar role to added SO₂, reducing competition for sugar between fermentable yeasts, such as saccharomyces, and undesirable types of wild yeasts and bacteria. By inhibiting the growth of wild yeasts while allowing fermentable yeasts to grow, SO₂ helps to establish a predominance of favorable microflorae in wine. Because bacteria are especially sensitive to SO₂, adding more SO₂ helps to reduce the risk of malolactic fermentation as well as the risk of spoilage.

Additionally, SO₂ helps to reduce wine’s oxygen consumption by inhibiting enzymes which catalyze the oxidation of phenolic compounds. At concentrations of 50 PPM or greater of SO₂, wine’s enzyme tyrosinase is inactivated. Moreover, some of the SO₂ binds with acetaldehyde, the main cause of wine’s oxidative smell, negating some of the negative effects of oxidation. Therefore, SO₂ creates an antioxidant like effect which helps to preserve and maintain wine’s freshness.

How much SO₂ is needed?

Before discussing how much total SO₂ to add to a batch of wine, it is important to understand the difference between bound and free SO₂. In wine, SO₂ exists in several free forms, molecular (SO₂) and bisulphite (HSO3), as well as bound forms, such as those attached to sugars, acetaldehyde (MeCHO), and other compounds. The free SO₂ portion acts as the main buffer against microbes and oxidation whereas the bound SO₂ portion is tied to other molecules and is unable to provide additional protection. As a result, total SO₂ levels are less important to wine preservation than are maintaining sufficient free SO₂ levels and maximizing the ratio of free SO₂ to bound SO₂.

There is no exact recommendation for how much total and free SO₂ should be added to any specific wine. Due to variation occurring in individual wines, such as amounts of sugar, PH, concentrations of natural SO₂, and storage conditions, SO₂ requirements will change. Lower PH wines need less SO₂ than do higher PH wines, sweeter wines need more SO₂, white wines need more SO₂ than red wines, and individual wooden wine storage barrels need varying amounts of SO₂. Any wine should have a sufficient amount of protection so long as its free SO₂ level exceeds its specific spoilage requirements and its total SO₂ level is less than the toxicity limit for humans.

Although there are not exact recommendations for SO₂ concentrations needed to fully protect different wines, the US and EU have set regulations establishing SO₂ minimums and maximums. In the US, all wines with SO₂ amounts greater than 10 PPM, either natural or added, must indicate “contains sulfites.” In the EU, the maximum SO₂ concentration in wine is 200 PPM.

Generally, so long as the concentration of total SO₂ falls between about 50-150 PPM and free SO₂ levels exceed 50-70 PPM, most wines will have an appropriate amount of SO₂ for desired results. When the PH value of wine is known, refer to this table for a more specific recommendation of free SO₂ concentration.

Measuring SO₂ in wine

Since the recommended amount of SO₂ in wine will vary for different batches and wines, winemakers must first measure a wine’s PH and its initial concentration of existing SO₂. Then, more SO₂ can be added. These measurements are repeated if SO₂ is added at multiple stages of winemaking.

The most common way to measure SO₂ levels in wine is by titration. In the aeration-oxidation method, SO₂ gas is aspirated out of a sample of wine through a condenser and impinge using an air pump. Flow controllers add additional accuracy to these tests by precisely controlling air flow rates, ensuring that measured concentrations are not higher or lower than actual concentrations.

Adding SO₂ in wine

SO₂ is generally added to wine during both prefermentation and postfermentation. Common methods for adding SO₂ are dissolving potassium or sodium metabisulfate or burning sulfur discs. As adding additional particles to wine are not ideal, a better method involves the precise injection of SO₂ gas.

Relative to other techniques, adding SO₂ gas, the most active and free form of SO₂, maximizes the ratio of free to bound SO₂ and provides the greatest protection to wine. After bottling is completed, wine barrels and corks are sterilized by additional SO₂ gas injection, which enables their use for multiple batches. Anti-corrosive flow controllers specifically designed to work with SO₂ allow for greater repeatability, reliability, and control of the injection process.