One of the keys to a successful fermentation is removing any native wild yeast and bacteria from the must prior to adding your special winemaking yeast. Wild yeast and bacteria can consume sugar from grape juice just as easily as your special yeast can, but generally produce some pretty terrible flavors in the process. In addition, many wild yeasts are less tolerant to high alcohol levels, and may stop fermenting before all of the sugars have been consumed, creating a “stuck” fermentation. If this happens, left-over sugar could be used as a food supply for any spoilage organisms present, and the wine will be compromised. Therefore, sulfite is added immediately after you crush to “clean the slate” of these unwanted guests. The amount used is usually just enough to kill or at least inhibit spoilage organisms, but not enough to bother more sulfite-tolerant, cultured yeast strains that we recommend using. If your grapes are in good condition, free of mold etc., add 50ppm („parts per million‟) of SO2 based on the total volume of the must. If the grapes are not in good condition, add more sulfite to counteract the presence of the mold and bacteria- up to 100ppm. However, be aware that levels of SO2 above 50 ppm will inhibit an MLF (Malolactic Fermentation) if you choose to do one. The 50ppm dosage rate at the time of the crush is usually fine.
*Note: The first sulfite addition made during the crush usually becomes entirely “bound-up” by the end of the alcoholic fermentation. During its aging and storage, only the “free” portion of the SO2 addition is actually contributing to the protection of the wine. Therefore, it is important to keep in mind that this first addition isn't part of the sulfite level needed to protect the wine during its storage and aging.
For more comprehensive information on SO2, see our Guide to SO2 Management.
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