Vol. XIX · Fish and Seafood Plate 2 of 9fresh fish, choosing fish, storing fish, ice, shelf life, spoilage, smell of fish, trimethylamine, eyes, gills, frozen fish, refrigerationST

Choosing and Storing Seafood.

Fish does not smell fishy when it is fresh. The smell is the first sign of the bacteria that will spoil it.

The bacteria on a fish come from cold seas and grow perfectly well at fridge temperatures; the fish's own enzymes, too, work in the cold. So fish spoils far faster than meat (XIII.1). Its life is set mostly by temperature: the spoilage rate rises roughly with the square of the degrees above about −10 °C, so fish kept on melting ice at 0 °C lasts about twice as long as at 5 °C and about three times as long as at 8 °C. Fishmongers bury fish in ice for that reason, and the best thing a home cook can do is the same: keep it at 0 °C, not 5.

Freshness can be read. A whole fresh fish has clear, bulging eyes; bright red, moist gills; shiny skin with firm scales; flesh that springs back when pressed; and a clean smell of the sea or of nothing. As it ages the eyes cloud and sink, the gills turn brown, the flesh softens and keeps a dent, and the smell turns "fishy" — trimethylamine, made by bacteria from a compound fish use to balance salt — and then sour. Fillets should be moist and translucent, not gaping or dull. Frozen-at-sea fish, frozen within hours of catch, is often fresher than "fresh" fish several days old. The workshop sets the storage temperature.

Below: the days a lean white fish keeps at the storage temperature you choose (the relative-rate-of-spoilage model, about two weeks on melting ice), and the signs of freshness to look for.

What you will learn
Workshop
Relative rate of spoilage: proportional to (T + 10)², the square-root model with Tmin −10 °C used for chilled fresh fish; about 14 days at 0 °C for lean white fish, the course's reference. Time already spent between catch and shop counts against these days.

Keep it on ice, use it soon

In this model lean white fish keeps about 14 days on melting ice, 6 days at 5 °C, 4 at 8 °C and only 2.2 at 15 °C. Most of those days have usually gone before the fish reaches the shop. Buy fish last on a shopping trip, carry it cold, and at home put it in a bowl or tray on ice in the coldest part of the fridge, covered, draining the meltwater; cook it within a day or two. Oily fish go rancid as well as stale (IX.4) and keep less well.

Frozen fish keeps for months at −18 °C (XIII.3); thaw it overnight in the fridge or sealed in cold water, never on the counter. Live shellfish — mussels, clams, oysters — need air, not water or a sealed bag: keep them in the fridge under a damp cloth and cook them within a day or two. Prawns are usually sold frozen or thawed; do not refreeze thawed ones.

At the stove

At the fish counter, look at the eyes and gills of three whole fish and press the flesh of each. Ask when each was landed. Buy the best, and smell it again when you get home.

Fish use trimethylamine oxide to balance the salt of sea water; bacteria turn it into trimethylamine, the "fishy" smell. Freshwater fish have little of it, which is one reason they smell less fishy as they age — and acid (lemon, vinegar) binds the amine and takes the smell away.

Terms

Shelf life
How long a food keeps before it spoils.
Trimethylamine
The compound behind the "fishy" smell of stale fish.
Frozen at sea
Fish frozen on the boat within hours of catch.
Spoilage bacteria
Bacteria that make food unpleasant, not necessarily unsafe.
Fish spoils fast because its bacteria like the cold; every degree counts. Keep it on ice at 0 °C — about twice as long as at 5 °C. Read freshness in eyes, gills, flesh and smell.

Sources

  1. Relative rate of spoilage of chilled fresh fish: the square-root model of D. A. Ratkowsky et al. (J. Bacteriol. 149, 1982), rate proportional to (T − Tmin)² with Tmin about −10 °C for fresh fish (T. A. McMeekin and T. Ross; P. Dalgaard, Int. J. Food Microbiol. 26, 1995): fish keeps roughly twice as long at 0 °C as at 5 °C. Lean white fish such as cod keeps roughly two weeks in melting ice.
  2. U.S. FDA, Fish and Fishery Products Hazards and Controls Guidance, 4th ed. (2020), ch. 13 and 16: hot-smoked fish held at an internal temperature of 62.8 °C (145 °F) for at least 30 minutes; cold-smoked fish relies on salt, refrigeration and control of Listeria and C. botulinum.
  3. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.