Vol. XIX · Fish and Seafood Plate 1 of 9fish flesh, flakes, myotomes, myocommata, fish collagen, lean fish, oily fish, salmon, cod, overcooking fish, done temperature, translucentMH

Fish Flesh.

A cod fillet goes from moist to dry in about the time it takes to set the table. Fish is not fragile meat; it is a different kind of muscle.

A fish swims in water that holds its weight, so its muscles need no heavy skeleton of connective tissue. They are built in short blocks of fibres (myotomes), stacked like the pages of a book and held by thin sheets of collagen (myocommata). That collagen is fragile: it softens and begins to dissolve near 45–50 °C, which is why cooked fish falls into flakes. The fibres' proteins are adapted to cold water and unfold at lower temperatures than a warm-blooded animal's (VI.8): fish myosin sets around 40–45 °C, and by 60 °C most fish has squeezed out much of its water.

So fish has a lower and narrower target: about 45–50 °C for salmon and tuna that should stay translucent, 50–55 °C for flaky white fish like cod, sea bass and halibut. Oily fish (salmon, mackerel, sardines) have fat between their flakes and forgive a few degrees; lean white fish do not. The workshop roasts a fillet and watches its centre.

Below: the centre temperature of a fish fillet of the thickness you choose in a 200 °C oven (thermal bench), with the band where fish is done and the line where it turns dry, and the minutes between them.

What you will learn
Workshop
Thermal bench: a fillet as a slab of the thickness chosen, from 4 °C, in a 200 °C oven; composition from USDA FoodData Central. "Done" band 45–55 °C; "dry" from 60 °C. Real fillets taper, so their thin ends finish first.

Lower targets, shorter windows

A 25 mm cod fillet in a 200 °C oven reaches 50 °C at its centre in about 17 minutes and passes 60 °C only 6 minutes later; a 12 mm piece reaches 50 °C in about 6. Most of the cooking time is spent getting there and the margin at the end is a few minutes — so fish is cooked by checking, not by the clock: the flesh at the thickest point turns from translucent to opaque and just begins to separate into flakes when pressed, or a thermometer reads 50–55 °C. Take it off a little early; carryover finishes it (II.8).

Fat changes the margin. Cod is about 0.7 % fat, salmon about 13 %: the fat between salmon's flakes keeps it tasting moist several degrees past its ideal, which is why salmon is forgiving and cod is not. Food-safety guidance for cooked fish is 63 °C (XII.6), well above the cook's targets; fish served below it should be fresh, from a trusted source, and — if eaten raw — frozen first against parasites (XIX.6).

At the stove

Cook two pieces of cod of the same thickness in a hot oven: take one out when a thermometer reads 52 °C and the other five minutes later. Compare the flakes: moist and pearly against dry and chalky.

The white flakes of cod and the stripes of white on salmon are the myocommata — the collagen sheets between blocks of muscle. Their zig-zag W shape lets a fish bend its whole body in a smooth wave as it swims.

Terms

Myotome
A block of muscle fibres in fish, seen as a flake.
Myocommata
The thin collagen sheets between myotomes.
Oily fish
Fish with fat in the flesh: salmon, mackerel, sardines, herring.
Lean fish
Fish with fat mainly in the liver: cod, haddock, sea bass.
Fish is built of flakes held by fragile collagen, and its proteins set lower than meat's: done at 45–55 °C, dry by 60 °C. Check it, don't time it; oily fish forgive.

Sources

  1. USDA FoodData Central, SR Legacy (April 2018), public domain.
  2. Differential scanning calorimetry of muscle: myosin denatures around 54–58 °C, collagen shrinks around 60–67 °C, actin around 70–80 °C (A. Wright, I. B. Leach and P. Wilding, "Differential scanning calorimetric studies of muscle and its constituent proteins", J. Sci. Food Agric. 28, 1977; H. McGee, On Food and Cooking, 2004). The course uses midpoints within these ranges.
  3. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.
  4. USDA Food Safety and Inspection Service, "Safe Minimum Internal Temperature Chart" (foodsafety.gov), 2024.