Acidulated water is water where some sort of acid is added — often lemon juice, lime juice, or vinegar — to prevent cut or skinned fruits or vegetables from browning so as to maintain their appearance.
Some vegetables and fruits often placed in acidulated water are apples, avocados, celeriac, potatoes and pears. When the fruit or vegetable is removed from the mixture, it will usually resist browning for at least an hour or two, even though it is being exposed to oxygen.
Acidulation prevents oxidative browning by inhibiting the action of the enzyme polyphenol oxidase, both by altering the pH of fruit surface, and by placing the cut surface in an oxygen-poor underwater environment.