SO2 in Wine

SO2 is a naturally-occurring preservative which is very effective against wine spoilage organisms. Some wine yeasts will naturally produce small amounts of SO2 during fermentation as a means to control the growth of other yeasts.

SO2 exists in wine in several forms. Winemakers are most often concerned with the level of Free SO2 (FSO2) in a wine. FSO2 can be easily measured in a lab and is considered to be the SO2 that is available to participate in chemical reactions (like protecting against spoilage organisms.)

The portion of FSO2 responsible for protecting against spoilage organisms is called Molecular SO2 (MSO2). MSO2 makes up a tiny fraction of the FSO2, so it is difficult to measure directly. Instead we can calculate it by knowing the FSO2 and the pH of the wine.

The final form of SO2 of concern is Total SO2 (TSO2), which is a combination of FSO2 and bound SO2 (SO2 that has bonded to other chemicals in the wine such as aldehydes, proteins and pigments.)

Why add SO2?

It will keep your wine from turning into vinegar. A winery is not usually a sanitary environment (barrels being a good example), but it's not a problem if you use modest amounts of SO2. The low pH and high alcohol in wine will do most of the heavy lifting involved in keeping things from living in the wine, but there are still some nasties that can ruin wine if the SO2 is not managed effectively.

To fully understand what levels of SO2 you should have in your wine, you'll need to know your wine's pH as well to figure out the molecular SO2.

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Why reduce SO2?

Elevated SO2 levels will bleach the color of young red wines and cover up most of the nice aromas with the aroma of Sulfur. Reducing the SO2 in wine can be done but it carries the potential of severe oxidation of the wine (because you're adding a powerful oxidizing agent, Peroxide). With that in mind, you should carry out lab trials first and observe the long-term effects of the Peroxide before adding it to the wine.

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What is molecular SO2?

Molecular SO2 (MSO2) is the form of SO2 that protects your wine from the spoilage organisms that want to do it harm.

Typically, 0.8 ppm MSO2 is considered the magic number for keeping your wine free of spoilage organisms. If you have a microbially stable wine (RS Dry and ML Dry), you might live dangerously and consider 0.5 ppm MSO2 your magic number.

Calculate MSO2 »

Why bother with SO2 solutions?

SO2 solutions are great for adding SO2 to small volumes of wine, like barrels or small tanks.

Make an SO2 solution »

Why bother with SO2 trials?

SO2 trials are not useful in the same way as acid or RS trials, because there should be little to no aroma or flavor differences between your SO2 adjustment levels. Otherwise, aim for your preferred molecular SO2 target and go.

Make an SO2 trial »