Instructions

1. Scrub the fruit in hot water before you zest it, even if you plan to eat the flesh afterwards. Nearly all shop citrus is waxed, the wax comes off with the zest, and it does not dry.
2. Zest with a microplane and stop the moment you see white. The pith underneath is bitter, holds water far longer than the coloured layer, and one pass too deep spoils the batch.
3. Zest straight onto the lined tray rather than into a bowl. The oil is released the instant the skin is broken, and every transfer leaves some of it on the sides of something.
4. Spread the zest thinly with a fork and break up the clumps that come off in one piece. Piled zest mats together and dries into a solid disc.
5. Dry at 55 °C for 2 hours. This is a short run because there is very little water in zest; what you are really protecting is the oil, which leaves with heat.
6. Open the machine, stir the zest with a fork and break apart anything that has fused, then continue at 55 °C for 2 hours. The pieces underneath have had no air at all until this point.
7. It is finished when the zest lies loose and dusty in the tray and a piece crushes to powder between finger and thumb instead of squashing. Colour is no guide at all here — zest looks dry hours before it is.
8. Let it cool and then jar it as flakes rather than grinding it. Ground zest loses its scent within weeks; left whole in a dark airtight jar it stays fragrant for a year, and you can grind what you need as you go.