Instructions

1. Buy tuna packed in water rather than oil. Oil will not dry off, it coats every flake, and it turns rancid on the shelf within a fortnight.
2. Drain the tin and then press the fish against the lid to force the last of the liquid out. Tinned tuna holds far more water than it appears to.
3. Rinse the flakes under the tap if the tin was salted and this is going to an animal. An amount of salt that is unremarkable in a sandwich is a great deal for a small dog.
4. Break the fish into even flakes with a fork and separate any compressed chunks. It is packed under pressure, and the pieces at the bottom of the tin are denser than they look.
5. Spread the flakes over a mesh sheet in a thin single layer. They are small enough to fall through a bare tray and light enough for the fan to move.
6. Dehydrate at 63 °C for 6 hours, stirring the flakes about with a fork partway through so the pieces underneath change places with the ones on top.
7. It is done when a flake crumbles to powder under a fingernail and the tray smells toasted rather than fishy. A flake that squashes instead of crumbling is still wet.
8. Cool it completely and seal it in an airtight jar for up to two months, or vacuum-seal it for longer. Fish oil rather than moisture is the real limit — once it smells sharp instead of savoury, it has gone.