Small amounts of sulphur compounds, usually sulphur dioxide and sorbic acid, are often added to the finished wine to help prevent spoilage by micro-organisms while the wine is maturing.  Sorbic acid was originally obtained from the berries of the mountain ash tree and has been produced commercially since 1954.

    A small amount (about 0.001%) of sulphur dioxide, SO₂,  is effective in inhibiting the growth of bacteria.  This is usually added as sulphite, but is not an effective fungicide.  Sorbic acid (trans,trans-2,4-hexadienoic acid) is a good fungicide but is not much use as an antibacterial agent; hence when added  together the two are very complimentary.
 

What would happen if sorbic acid was added without sulphur dioxide?

Bacteria in the wine would reduce the carboxylic acid group on the sorbic acid to an alcohol.  Wine is acidic and with the ethanol, the conditions allow the sorbyl alcohol to be converted to a foul-smelling ether, trans-5-ethoxy-1,3-hexadiene.