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.

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