Instructions

1. Wash the stalks whole before cutting them, working the dirt out of the groove with your thumb. Grit collects in that channel and it is far harder to rinse out once the celery is in pieces.
2. Snap the top off each stalk and pull downwards to strip the strings out. Those fibres do not soften in a soup and they wrap around the blades if you later grind the batch to powder.
3. Slice the stalks across into 6 mm pieces. Cutting across rather than along leaves short fibres rather than long ones, which matters for exactly the same reason.
4. Use the leaves as well, on a separate tray. They carry more flavour than the stalk does and they will be finished in a fraction of the time.
5. Spread the pieces so none are touching. Celery is wet enough that two pieces in contact will stay soft where they meet for the whole run.
6. Dehydrate at 57 °C for 7 hours. Expect the pieces to shrink to about a third and go from pale green to a dull straw colour.
7. They are done when a cooled piece snaps sharply and there is no give at the thickest part of the curve. A bendy piece is holding water in the arch of the stalk, which is the last place to dry.
8. Jar the cooled pieces airtight in a dark cupboard and they hold a year. Grind them to powder only as you need it — celery powder is strongly hygroscopic and a jar of it will set into a solid plug within a couple of months.