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, Potassium Metabisulfite (A.K.A. “sulfite”, “meta” & “SO2”) 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 the more sulfite-tolerant, cultured yeast strains that we recommend using. If your grapes are in good condition, free of mold etc., add 50 ppm (‘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 100 ppm. However, be aware that levels of SO2 above 50 ppm will inhibit a Malolactic Fermentation if you choose to do one (a full explanation of MLF can be found in our 5 Steps to a Malolactic Fermentation, and our Guide to Malolactic Fermentation). The 50ppm dosage rate at the time of the crush is usually fine. For each 5 gallons: 50ppm equates to 1/4 tsp or 1.6 g SO2.
* SO2 is used extensively in winemaking to control oxidation and microbial contamination. This is the first of several points in the process where you will be adding SO2 to protect and preserve the wine.
To best protect the delicate white wine juice from oxidation it is best to add the sulfite as soon as the grapes have been broken open. Therefore, the timing for when you add the SO2 will be different depending on the type of juice processing technique you choose to employ. Either way it is recommended to sulfite a little at a time as the juice is coming out of the crusher or press to start protecting the juice as soon as possible:
Note: The sulfite addition made during the crush usually becomes entirely “bound-up” by the end of the alcoholic fermentation. During the wines’ ageing and storage, only the “free” portion of the SO2 addition is actually contributing to the protection of the wine. Therefore, keep in mind that this first addition isn't part of the sulfite level that will be later needed to protect the wine during its storage and ageing.
For more comprehensive information on SO2, see our Guide to SO2 Management.
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