"It is a proof that wine is beginning to go bad if a sheet of lead, when dipped in it, turns a different color." (Pliny, Natural History XIV.130)
There was something unique to wine that troubled Roman legal authorities for centuries-the fact that it could turn to vinegar during storage and no one would be aware that change was happening. The vinegary odor and flavor of truly spoiled wine results from a secondary fermentation in the presence of special bacteria (Mycoderma Aceti) which grow by oxidizing the alcohol of the wine into acetic acid and ethyl acetate. The Romans recognized the endpoint of this chemistry well enough; they called it acor, but they had no idea what caused it.
Working in favor of the preservation of the better Roman wines was their sweetness. Vintners would ripen their grapes on the vine as long as possible, to maximize the sugar content. Subsequent fermentation yielded a wine with a high alcohol content that would discourage its deterioration. Legal records indicate that a spoilage level of about 10% was both usual and acceptable.