Campden Tablets to a starting must.
These sulfites are added to the juice 24 hours before adding the
yeast as a sterilizing process. The sulfites are released into the
juice destroying any molds or other contaminants that may be
present, wild yeast included. This allows the winemaker to start
his fermentation off with a clean slate, so to speak.
Once the sulfur has been added to the juice, it slowly starts to
dissipate into the air in the form of a gas and eventually all
goes away. This process usually takes between 18 and 24 hours.
After the 24 hour waiting period, the winemaker can then safely
add fresh, domesticated wine yeast without fear of it being
destroyed by the sulfites.