How To Use and Test Free SO2 in Wine
Copyright by The Beverage People 2021
Scheduling SO2 Additions in Wine
Initial pre-fermentation sulfite may be added at 50-65 ppm (parts per million) to grapes or juice that is free of rot or mold. The presence of a lot of mold, or grapes in otherwise bad condition, might require twice that amount. Under average conditions the information that follows should keep about 20 to 30 ppm of free SO2 available throughout the wine’s cycle of production through bottling. If you plan to use ML bacteria, pre-ML sulfite additions should be kept below 50 ppm.
After ML fermentation and complete sugar fermentation, you may make larger additions to achieve your target free SO2 level. Be aware that SO2 additions greater than 50 ppm may have a temporary impact on the color of the wine. The "bleaching" effect is a reversible reaction which will self-correct over a period of months. For a wine that is expected to be consumed within the coming months, you may prefer to avoid additions larger than 50 ppm. Instead, you may spread out the additions over time. For example, you may first add 30 ppm, and five days later add 30 ppm again, and AGAIN one week later. If you are not concerned with a temporary color loss, you could simply add 80 or 90 ppm in one full dosage. Whichever approach you take for your initial post-fermentation SO2 addition, you should get the wine tested for free SO2 soon after the addition(s). The test results may surprise you. The SO2 you have added may have become largely bound to oxygen, tannins, and other compounds in the wine and no longer available in a "free SO2" state to protect your wine. This is the reason that the size of the additions recommended here exceed the target free SO2 levels is the table below.
Above pH 3.5, you will notice that the amounts of free sulfur dioxide required become quite high. It is best to lower the pH by adding tartaric acid early in the fermentation cycle.
Continue testing every 6-8 weeks, adding SO2 as required to keep at least 20-30 ppm available in the wine.
Sources of SO2
SO2 is available as Campden tablets, effervescent Inodose metabisulfite tablets and as powdered potassium metabisulfite. A premeasured Campden tablet equals 65 ppm in one gallon (13 ppm in a five gallon jug) and is very convenient for those making small amounts of wine. Crush the tablet to a powder, dissolve it into a little water or wine, then add it.
The 2 gram Inodose tablets adds 528 ppm per gallon or 9 ppm per 60 gallons. The 5 gram Inodose tablets add 1320 ppm per gallon or 22 ppm per 60 gallons. The tablets can by dissolved in water to accurately dose carboys. Metabisulfite powder is added in a liquid preparation to adequately disperse it, and because it is very potent. This is also the least expensive method and accurate to measure for any size container (see mixing and use rate instructions below)
pH and SO2
It is generally recognized that only a small amount of molecular SO2 (.5 to .8 ppm.) needs to be present to provide bacterial stability in wine, but pH has an important effect on how much free SO2 is needed in order to provide that amount, and this is why both pH and SO2 need to be tested.
Regard the Table of Molecular SO2 below. The amount of free SO2 needed is based on the pH of the wine. A fairly safe amount for protection of the wine is either .5 ppm for red wines or .8 ppm for white wines. If you know the pH (pH meters available here), simply make sure you have the corresponding level of free SO2, or slightly more, present in the wine during storage and bottling.
Molecular SO2 needed for Stability (ppm)
Preparing Metabisulfite Solutions
10% Metabisulfite Solution
Dissolve 100 grams of potassium metabisulfite in 1 Liter of distilled water. Tightly stopper and store labeled: poison. When adding your sulfite additions make sure you measure carefully. Labware is helpful when working with a 10% solution.
Replace your solution every 3 months.
Usage Rate Table - 10% Solution
3% Metabisulfite Solution
Dissolve four ounces of potassium metabisulfite powder in one gallon of distilled water. This is a weaker solution than the 10% solution given above. However, at this concentration, the solution is still quite strong and should be labeled: poison. Household kitchen tools such as tablespoons and measuring cups are generally adequate for using the 3% solution.
Replace your solution every 3 months.
Usage Rate Table - 3% Solution
Methods for Testing Free SO2 in Wine
When you add sulfite to wine, sulfur dioxide ionizes to the sulfite ion, SO3—, and bisulfite ion, HSO3—. A small fraction remains in the “molecular” form, SO2. It is this molecular form that protects the wine from spoilage organisms and oxidation. As sulfite reacts with other wine components, it becomes “bound” to them and is no longer available to participate in producing “molecular” sulfite.
We cannot measure molecular sulfite directly. Rather, we measure “free” sulfite, and use a table of wine pH values to predict the amount of ‘molecular’ sulfite we will achieve. This is why it is so important to frequently measure your free sulfite. No matter how high your total sulfite (within reason), it is only the free sulfite number that really counts. Don’t just guess and toss some sulfite in. Analyze it first, then add it. To this end, we offer some advice on ways to keep up with testing your SO2.
Aeration-Oxidation (AO) Method for Free SO2
This is the original primary laboratory method for sulfite measurement in wine that helps define what “free” SO2 means. In the AO method, a wine sample is placed in a flask and phosphoric acid is added to force the sulfite ion into molecular SO2. A small air pump pushes air bubbles through the sample. Since sulfur dioxide is a gas, it dissolves in the air stream and transfers to a trapping solution. In the trapping solution, hydrogen peroxide oxidizes the sulfur dioxide into sulfuric acid. Also in the trapping solution is an acid-base indicator that changes color as the sample gas accumulates. After the 10 or 15 minute transfer period, the trapping solution is titrated with sodium hydroxide solution to measure the acid formed. The free sulfite level can be calculated from the titration results.
The Beverage People once supported a Aeration-Oxidation test apparatus, but it has been discontinued due to the difficulty our customers experienced in its operation. It is still good to be aware of this test method because it is the standard reference test for "free" SO2 and it can generally be requested at wine labs.
Ripper Method for Free SO2
We sell the 10 pack box of Titrets, based on the Ripper method, but they are only recommended for white wine.
The Ripper method is an iodine titration that is often faster, easier, and cheaper than A/O. Most Ripper method testing is limited by the chemistry involved. Any substance that reacts with iodine—including some tannins—will be measured as sulfite. Further, the acidification of the sample for the titration tends to release some sulfite bound to anthocyanins (color compounds) in red wine, making it appear “free” when it is not.
These Ripper limitations have been largely overcome through a combination of equipment and techniques from Vinmetrica. That company produces proprietary instruments for sulfite analysis (SC-100A) and for sulfite plus titratable acidity (SC-300) that rely on amperometric titration with iodine instead of a visual endpoint or a straight oxidation-reduction (redox) detection. Allowing very rapid titration to overcome release of additional sulfites and showing a very sharp endpoint on the meter to improve precision, they have reduced the discrepancy between AO and Vinmetrica Ripper to only 2 to 3 mg/L (ppm) for most wine samples. Those differences are small enough that the convenience and ease of use will make the Vinmetrica meters attractive choices for many users.
If you would rather not do sulfite analysis yourself but you want to do a good job keeping up with your levels, a wine testing laboratory can do it for you. Find a commercial lab or perhaps a university lab near you to minimize shipping of samples. For those of you who live in Northern California Wine Country, we can recommend a couple local labs with whom we have worked throughout the year. ETS Laboratories of Healdsburg offers a full range of wine testing. In Windsor, Gusmer Enterprises offers two testing panels for home winemakers on their FOSS WineScan---a juice panel and a finished wine panel which includes free and total SO2 results.
Removing Excess SO2
If you ever need to lower your SO2 due to a mistake in calculation try splash racking or stirring vigorously to aerate. If the free SO2 is still too high do the following: for every 10 ppm free SO2 you want to remove, add 1 ml. of 3% hydrogen peroxide per gallon of wine. An oxidative reaction occurs immediately. Use only fresh 3% Hydrogen Peroxide, available at the drugstore. Use this method to remove up to 100 ppm - any more than this and the wine will oxidize and lose its flavor.