Cider vinegar — apple cider vinegar — is a cloudy, amber liquid made by fermenting apple juice twice over: yeasts first turn the apples' sugars into alcohol (hard cider), then acetic-acid bacteria oxidise that alcohol into acetic acid. The result is sharply sour but rounder and fruitier than harsh distilled white vinegar, carrying a clear apple tang. It is a kitchen workhorse in dressings, pickles, marinades and slaws, and the most fashionable of folk-remedy tonics.