Potassium sorbate is used to inhibit molds and yeasts from growing in wine.  It acts as a preservative as well as a stabilizing agent.  It is generally added after fermentation has completed, or is added to sweetened wines to stop fermentation.  Sorbate should not be used in any wine that might be exposed to malolactic bacteria;  these bacteria will convert sorbate into hexanedienol (geraniol), a compound with the strong odor of geraniums.

http://en.wikipedia.org/wiki/Potassium_sorbate