Zesting citrus provides concentrated flavor without the bitter white pith found beneath the peel, resulting in a brighter, cleaner citrus taste in your recipe.
When you zest citrus fruit, you're only removing the outermost layer of the peel, which contains the fragrant oils responsible for the characteristic citrus flavor. This outer layer, called the zest, is packed with flavor and aroma. Sliced citrus peel, on the other hand, includes the white pith, the layer between the zest and the fruit. The pith is quite bitter and can impart an unpleasant taste to your dish. By using zest, you get all the delicious citrus flavor without any of the bitterness. Furthermore, zest disperses more evenly throughout a recipe than sliced peel, ensuring a consistent citrus flavor in every bite. The small size of the zest particles also integrates better into batters, doughs, and sauces.
When zesting, avoid pressing too hard. Only the colored part of the peel should be removed. If you see white pith, you're pressing too hard and will introduce bitterness. Rotate the fruit as you zest to ensure you're only getting the flavorful outer layer.