How canned tuna is made

Tuna is cooked twice, once whole before cleaning and again in the sealed can, and the can does the sterilizing.

stations: 7
things upstream: 38
raw sources: 19
steps from the ground: 6

The short version

Tuna caught by purse seine arrive frozen at a cannery and are thawed, then precooked whole for 1 1/2 hours to 10 hours depending on size. Workers clean the loins of bone, skin and dark meat, pack the pieces into cans with oil, water or brine, and seam the lid on. The sealed cans are cooked under pressure in a retort.

Net the school

Most tuna is caught with purse seine nets, which encircle a school of fish and are drawn shut underneath like a bag. The catch is frozen on board and unloaded at the cannery.

Goes in
Tuna
Thaw and sort

Frozen fish are thawed in tanks and sorted by species and size. Tuna is often caught far from where it is canned, so poor handling in between can cause spoilage.
Precook the whole fish

The fish are gutted and steamed in large cookers, from about 1 1/2 hours for small tuna to 8 to 10 hours for big ones. The cook firms the meat so it can be cleaned, and the fish lose roughly 22 to 26 percent of their weight as moisture runs out.

1 1/2 to 10 hours
Clean the loins

Cooled fish are cut into loins and cleaned by hand. Workers take off the skin, bones and dark lateral blood meat, which is often separated and used in cheaper products.
Pack the cans

Machines cut the loins to can height and press them into steel cans. Then oil, water or brine is added, and in some cases sauce.

Goes in
Tinplate Vegetable oil Table salt Water
Seam the lids on

A lid is rolled onto each can in a double seam that locks the metal edges together, so nothing can get in after sterilization.
Sterilize in a retort

Retort baskets of sealed cans go into a pressure cooker. Low-acid foods like fish need temperatures of 116 to 130 °C, which cannot be reached without pressure. This kills bacteria that would otherwise grow in the can.

116 to 130 °C under pressure

End of the lineCanned tuna

Worth repeating

Nicolas Appert, the father of canning, began experimenting with preserving fish in jars in 1795.
The international standard sets a maximum histamine level of 200 milligrams per kilogram in canned tuna.
Tuna loses roughly a quarter of its weight in the precooker, before it is ever canned.