Instructions

1. Bake the sweet potatoes whole at 204 °C for about 1 hour, until a skewer meets no resistance. Baking rather than boiling keeps water out and concentrates the sugar, which is half of why a dog will do anything for these.
2. Score the skins and rub them off once the potatoes are cool enough to handle. The skin is fibrous and stays leathery all the way through drying.
3. Use sardines packed in water rather than oil or brine, and tip the tin in with its liquid. The soft bones go in too — they are the calcium, and after drying they crush to nothing between the teeth.
4. Mash the fish and potato together until the fish has broken down completely. Any whole fillet left in the mix stays wet at its centre long after the patty around it has gone hard.
5. Form patties sized for the dog that is eating them, keeping every one the same thickness. A thick patty and a thin one on the same tray will never finish together.
6. Lay them on non-stick mats rather than bare mesh. Fish protein glues itself to plastic and you will lose half the batch scraping it off.
7. Dry at 57 °C for 13 hours. Fish carries a great deal of water, and these are done only when a patty is hard right through with no soft give at the middle.
8. Snap one in half and look at the cross-section. A darker damp band through the centre means the batch is not finished, and fish spoils faster than any other treat you will make.
9. Cool them fully and seal them in bags — two to four weeks in a cool cupboard, six months in the freezer. The smell alone will tell you when a batch has turned.