Filleting: the principles, not the recipe

There is no shortage of filleting videos and there is very little explanation of why the knife goes where it does. The explanation is short, and it is the part that lets somebody who has filleted a bass deal sensibly with a gurnard they have never seen before.

A FILLET IS THE MUSCLE BLOCK ON ONE SIDE OF THE BACKBONE, and every method is three cuts: one down to the bone behind the head, one along the top of the backbone from head to tail, and one releasing the fillet from the rib cage. Everything else is variation.

THE KNIFE FOLLOWS BONE. The single technique that separates neat work from wasteful work is keeping the blade in contact with the skeleton and letting the bone steer it, rather than cutting through flesh at a guessed angle. The faint resistance of bone through a thin blade is the feedback the whole operation runs on, which is why a flexible filleting knife works and a stiff kitchen knife fights you.

SKELETONS DIFFER, AND THAT IS WHAT MAKES SPECIES DIFFER. A round fish has two fillets and a rib cage curving down around the cavity. A flatfish has four — two per side, separated by the backbone running down the middle — and the fillets lift almost flat, which is why flatfish are the easiest fish to fillet and look the hardest. Fish with heavy pin bones, like the shads and many of the carp family, have a row of intramuscular bones that no filleting technique removes; they are dealt with by the cook or accepted, and pretending otherwise is where a lot of freshwater fish acquire an unfair reputation.

SKINNING IS A SEPARATE OPERATION with one trick: the blade goes between skin and flesh at the tail end and is then held still at a shallow angle while the SKIN is pulled. Sawing at the skin with the knife is the common error and is why it tears.

WHAT MATTERS FOR FOOD SAFETY AS OPPOSED TO NEATNESS. Two things. First, cleanliness — a fillet has an enormous surface area relative to its volume compared with a whole fish, and the FDA notes that histamine formation is "particularly problematic when the surface-to-volume ratio of the exposed fish muscle is large". Filleted fish therefore needs to be cold faster, not slower, than whole fish. Second, the belly flaps: the FDA records that trimming them is a recognised way of REDUCING parasite numbers while noting it does "not completely eliminate the hazard". That is a fair summary of what trimming achieves.

AND FISHINGHQ STOPS HERE. What to do with the fillet is cookery, and cookery is outside this catalogue's boundary. What is inside it is that the fish is now portions with a large exposed surface, and that they belong on ice or in a fridge within minutes rather than on a board while the next fish is done.