To acidulate is to add a drop of vinegar or lemon juice to add piquancy, sharpness or acidity to a dish. Similarly a few drops of lemon or vinegar or citric acid may be added to water to prevent discolouration of foods. Commonly used with artichokes, cut avocados, bananas and other foods which discolour rapidly on contact with the air. It also refers to the process of souring cream by the addition of lemon juice.