
Grapefruit zest is the coloured outer layer of grapefruit peel, the flavedo, scraped or cut from the rind above the bitter white pith.
Aromatic and citrus-bright from the peel oils, with a faint bitter edge; the oil carries the scent rather than any sweetness or acidity.
An aromatic garnish rather than a volume ingredient, expressed over the surface of a finished drink by bending the peel so its oil cells rupture and release scent, then often dropped in. Taken as a shallow strip of coloured rind only, since cutting into the pith transfers bitterness.
Another citrus zest supplies the same kind of oil-borne aromatic lift, though it cannot reproduce the specific grapefruit scent when a drink is built around it.
Zest is the aromatic oil-bearing outer peel, contributing mainly aroma with essentially no acidity or sugar; grapefruit juice contributes tart acidity, bitterness and volume but lacks the concentrated peel-oil aromatics.
Grapefruit peel/twist including pith
True zest is only the flavedo; a strip cut with the white albedo beneath carries the bitter pith, which shifts a drink's balance toward bitterness.
The coloured outer rind of the fruit, the flavedo, holds the oil sacs that carry grapefruit's aromatic essential oils, while the white pith beneath holds little oil and is bitter. It is prepared from fresh fruit by scraping or cutting only the coloured layer with a zester, peeler or paring knife, ideally after any surface wax is removed and while avoiding the pith.
Cut from fresh, ripe fruit for the most oil; old or dried fruit yields far less aroma.