Lemon or orange zest turns bitter because of the white pith sitting just beneath the coloured outer layer, known as the albedo. This white layer naturally contains bitter compounds such as naringin and limonin, and if grating goes deep enough to reach it, that bitterness spreads into the rest of the zest.
What decides how bitter it gets?
How deep the grater goes is the biggest factor; taking only the thin, coloured outer layer leaves almost no bitterness, while pressing hard enough during grating to touch the white pith noticeably increases it. Using a coarse grater also tends to dig deeper than a fine one, so it inevitably picks up more bitterness.
Boiling the zest or leaving it in a hot liquid for a long time also releases more of the bitter compounds, which is why recipes like syrup or jam that simmer zest for a while need to keep the amount balanced. The type of fruit matters too; grapefruit peel carries some of the highest concentrations of these compounds, while orange zest is usually milder.
How can bitterness be avoided?
Taking only the coloured surface while grating, and being careful not to reach the white layer, is the most basic precaution; a fine grater or a dedicated citrus zester makes that control easier.
How ripe the fruit is also affects the level of bitterness; zest from unripe fruit, or fruit that has sat in cold storage for a long time, can taste a little more bitter than zest from fresh, ripe fruit.
Using the zest right after grating rather than letting it sit is also recommended; grated zest left exposed to air for a while loses its volatile aromatic oils and can leave behind a relatively more bitter taste. That is why zest is usually grated fresh right when it is needed.
How much zest a recipe uses also decides how noticeable the bitterness is; a small pinch of zest in a sweet cake batter stays too mild to notice, while the same amount used in a delicate glaze or a sauce with few other ingredients can make the bitterness far more obvious. That is why recipes with fewer ingredients tend to use zest more sparingly.
How the zest is stored can affect bitterness too; grated zest kept in the freezer holds most of its aroma, while zest left out at room temperature for a long time loses its aroma and can leave behind a relatively flatter taste.