The few millimetres of liquid, oil or brine or tomato, do not announce themselves as food. The fish goes into the can raw, unless it was grilled or smoked, and the lid goes on before any cooking has happened, so whatever was poured in alongside it is the thing the fish then gets cooked in, kept in, and quietly altered by for as long as the tin sits unopened.

A cannery seals first and cooks second. Once the lid is seamed on, the cans go into a retort, a big pressure vessel, and are held at somewhere around 116 to 121 degrees, long enough to sterilise everything inside. Nothing can be stirred or tasted or adjusted once they are in this state. The heat has to travel to the middle of the fish, and it travels through the liquid, which is why the liquid decides the cook and not just the taste. Brine carries heat faster than oil. In one trial on yellowfin, the brine-packed cans reached holding temperature in about fifteen minutes where the oil-packed took twenty-six.

Then the exchange begins. Fat dissolves fat, so an oily fish in oil gives some of itself up to the oil around it, and analyses of oil-packed tuna have found that pouring that oil away takes a share of the fish's own omega-3s with it. Water-packed fish holds on to more of them, oil and water declining to mix, but the brine takes its own cut in the things that do dissolve in water, the salt-soluble savour that ends up in a liquid nobody keeps. Oil is the one where what is left is worth eating most.

Olive oil is the Iberian and Breton answer, the slowest and the most protective, a medium that shields the fat from turning and lets a good tin go on softening and deepening for years. Brine is the plainest, handing you the fish and very little else, which is what you want when the tin is heading into something that brings its own sauce. Tomato is a cooked sauce in its own right, acid and a little sweet, seasoning the fish rather than protecting it, and it is what most of the world's sardines are packed in, the everyday tin of Morocco and Portugal and the Baltic ports and the British table.

A cannery cannot make its fish better by changing what it pours in, but it can make the tin dearer or cheaper by a wide margin, because good olive oil costs a multiple of sunflower, and that is why the label draws such a careful line between olive oil, vegetable oil, and the blend that says both. Europe's marketing standard for preserved sardines even polices the pack itself: in an oil tin, the watery exudate that seeps out of the fish is not allowed to run past eight percent of the net weight.

Everything a tin becomes between the cannery and your kitchen, it becomes through that liquid. Warm a good one, tip it out over the bread along with the fish, and understand that the pour is not a garnish. It is the other half of what you bought.