No food goes bad faster than fresh fish. I knew that. But until a recent gathering of curious San Francisco fish aficionados, I never stopped to wonder why fish should be more fragile than beef or chicken, or what - if anything - could be done to coddle it into keeping for an extra day or two.
A few casts into the seafood literature snagged the first answer. Fish spoil quickly because they are creatures of the water and therefore of the cold. Deep ocean water is only a few degrees above freezing, and surface waters seldom exceed 70 degrees.The microbes and body enzymes of cattle, pigs and chickens are accustomed to operating above 90 degrees. In a typical refrigerator at 40 degrees, they're nearly paralyzed.
But the bacteria and enzymes of many ocean fish feel right at home and happily proceed to break apart the fish's proteins and fats into smaller, smellier molecules. The spoilage agents in warm-water fish do feel chilled and slow down - somewhat.
Cold waters also give fish their highly unsaturated fats. Such fats have an irregular structure that leaves them fluid and biologically useful at low temperatures, but also more susceptible to attack by oxygen, which speeds spoilage.
So fish spoil faster than meats, and fatty fish from cold waters spoil the fastest. I was relieved: Fish scientists had already done the chill-and-sniff tests.
Kept in ice, under controlled laboratory conditions, lean warm-water carp and tilapia last about 20 days, trout for 15, cod and salmon for eight and oily ocean-going mackerel and herring for just five. In the real world of fishing boats, distributors and markets, these numbers
are wildly optimistic. By the time we see fish, it doesn't have much quality time left.
Stretching that time seemed pretty hopeless, until I found a 1990 report on carp spoilage from intrepid researchers at the Kimron Veterinary Institute in Haifa, Israel. What's the difference between a refrigerator at 40 degrees and icy slush at 33 degrees? It doesn't sound like much, but for carp edibility it's 10 days. Creatures of the cold last twice as long in ice as they do in the refrigerator.
It turns out that the key to preserving the quality of fresh fish is ice. Carry the fish directly home in a bag of ice. Rewrap it in watertight plastic, and bury it in a pan of ice. Cover the pan, and keep it in a cold back corner of the refrigerator. But not too long.