Instructions

1. Choose a long even sweet potato rather than a fat round one. You want slices of similar diameter so they finish together, and a tapered vegetable gives you chips in four different sizes.
2. Cut the slices as thin as the mandolin will allow. Sweet potato is dense and sugary, and the thickness is what decides whether these come out crisp or chewy — nothing later in the recipe can fix it.
3. Rub the oil and lemon juice into the slices with your fingers rather than tossing them with a spoon. Every face needs a film on it, and the lemon juice is there to stop the cut surfaces greying before they dry.
4. Crush the rosemary between your palms, then add it with the sea salt and toss again. A whole dried needle stays sharp and is unpleasant to bite on a thin chip.
5. Lay the slices out so none are touching and dry at 74 °C for 10 hours. This is a hot setting and oiled sugary slices will brown at the edges, which is part of the point here.
6. Look at them from partway through rather than waiting out the clock. Sugary slices go from crisp to burnt in a short window, and the thin ones at the edge of a tray get there first.
7. If they still flex when the time is up, give them a couple more hours and test again. A cooled chip that snaps and rattles on the plate is done; one that folds is still carrying water.
8. Turn the machine off and leave the chips inside with the door shut for a few minutes, then move them to a rack. They crisp appreciably as the heat leaves them.
9. Store them airtight for three weeks. They soften faster than plain dried sweet potato because of the oil, and once soft they will not come back the way an apple chip does.