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, that simple. If you want to make a wine without SO2 you will need impeccably clean conditions and probably a membrane filter nearby.

A winery is not usually a sanitary environment (barrels being the notable 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 understand what levels of SO2 you should have in your wine, you'll need to know your wine's pH. Knowing the pH will tell you what level of SO2 is required to achieve the holy grails (yes, there are two grails) of SO2: 0.5 ppm and 0.8 ppm molecular SO2.

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Why reduce SO2?

First, it's worth knowing that it's most likely illegal to add Hydrogen Peroxide to reduce the SO2 levels in your wine. Now that we've got that out of the way...

Elevated SO2 levels will bleach the color of young red wines and cover up most of the nice aromas with the aroma of Sulfur. Not only that, but most countries have an SO2 threshold above which they'll reject the wines from their market.

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.

It is usually found in minute quantities in wine (< 1.0 ppm), but a little bit goes along way.

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.

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Why bother with SO2 solutions?

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

They also come in handy if you want to add a little SO2 before sending barrel samples out to a wine writer or competition, and weighing out KMBS to add 10 ppm SO2 to a sample bottle is tedious.

Or maybe you need to standardize the Free SO2 levels in samples for color analysis, or maybe you're just wild about making solutions.

In any case, having a little SO2 solution on hand never hurt.

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Why bother with SO2 trials?

Good question...

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.

SO2 trials are useful for blush wine production where you want to make sure your addition level is not going to excessively bleach the wine.

Otherwise, aim for your preferred molecular SO2 target and go.

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