Sulfur dioxide is also available in another powdered form, sodium metabisulfite. Solutions of sodium metabisulfite in water make an excellent sanitizing agent for winemaking equipment but it should not be used for making additions to wine. Although it is not toxic, the sodiumin sodiummetabisulfite is not healthful and it is best left out of wine.

By knowing the pH, you can determine the percentage of free sulfur that is in the molecular (SO2) form by using the table in Table I.

For white wines, a level of 0.8 ppm molecular SO2 will slow down the growth of yeast and will prevent the growth of most other microbes. This level of sulfur dioxide will bind up most of the acetaldehyde in a wine and reduce any oxidation aroma considerably. Therefore, 0.8 ppm is a good target level for molecular SO2 immediately prior to bottling and will provide the maximum protection for the finished wine.

However, sensitive tasters will be able to detect a slight burnt match aroma at 0.8 ppm SO2. This is usually not a problem however because few consumers will be able to detect it. Additionally if the wine is bottle-aged for a few months before consumption, the SO2 will decrease as more sulfites react with other chemical constituents in the wine and become bound. Thus, a wine bottled at 0.8 ppm will decrease to a lower level fairly quickly and there would be no detectable sulfur dioxide aroma.

Winemakers who seal their wine with screw caps know that the sulfur levels diminish more slowly after bottling than wines sealed with corks. In this case, 0.7 ppm would be a better target for molecular SO2 at bottling. During storage, after all fermentations have completed, white wines can be adjusted to between 0.5 and 0.8 ppm molecular. If the wine is sweet or if you wish to prevent MLF, the wine should be kept at the high side of this range.

Total SO2 should be kept below 110 ppm for table wines because, at higher levels, the wine can acquire off-flavors. For dessert and fortifiedwines, that are very sweet, it may be necessary to exceed this limit to obtain adequate free SO2.

For red wines, a level of 0.5 ppm molecular SO2 at bottling is an appropriate target. You do not need to keep the molecular SO2 as high on red wines as you do white wines for several reasons:

First, in most cases, MLF is complete in reds so there is no need to try to discourage it.

Second, red wines are less sensitive to oxidation and their flavor is less dependant on fresh fruity aromas so sulfur dioxide’s preservative effects are not as critical.

Third, red wines usually have a higher pH than whites and often it is not possible to adjust the sulfur dioxide to a level that reached 0.8 ppmmolecular SO2 without having too much total SO2.

By knowing the pH of the wine and using the chart in Table I, it is easy to see what level of free SO2 is necessary to obtain a given level of molecular SO2. However, it is more difficult to determine precisely what the ratio of free to bound sulfur dioxide levelwill be.

When adding sulfur dioxide, the amount in ppm that you add should change the total SO2 by the same ppm. However, because some of the sulfur dioxide that you add will become bound, the level of free SO2 will change by a fraction of the amount that is added.

Free SO2 in wine ranges from about 40% to 75% of the total SO2 depending on the amount of compounds that are available in the wine to which the sulfite molecules can bind. To approximate what the ratio will be, use the 40% level for wines that are turbid or sweet and the 75% level for clean, dry wines.

The higher the level of total SO2 in the wine, the higher the ratio will be, because there are fewer unbound compounds available for reacting with additional sulfur dioxide as it is added. Sulfur dioxide is also more affective if it is added less often and in greater quantities because it will be more of a shock to the microbes. For example, one addition of 30 ppm will be more effective at killingmicrobes than two additions of 15 ppm spread several weeks apart.

The formula and calculations for determining how many grams of PMBS you need to add for a given rise in ppm of SO2 is shown in Figures III and IV. For convenience, pre-weighed effervescent tablets are also available for additions to barrels.

Some wineries prefer to use a premixed aqueous solution of sulfur dioxide rather than PMBS. The liquid is typically 5% to 10% SO2 byweight and it can be purchased or made up at the winery by dissolving SO2 gas or PMBS into distilled water. The liquid can be directly added to wine without mixing and the proper amount is measured volumetrically instead of weighed on a scale.