Zest is the colorful, outermost part of the peel of citrus fruit. It is distinguished from the white pith which is bitter. The zest contains rich aromatic oils that are used to add flavor to food. Zest is removed from fruit by one of several methods. It can be grated on a box-shaped grater or removed with a tool known as a zester. The verb zest refers to the technique of removing zest from citrus fruit.