By Harold McGee
Once we’ve obtained good fish, the challenge is to keep it in good condition until we use it. The initial stages of inevitable deterioration are caused by fish enzymes and oxygen, which conspire to dull colors, turn flavor stale and flat, and soften the texture. They don’t really make the fish inedible. That change is caused by microbes, especially bacteria, with which fish slime and gills come well stocked—particularly Pseudomonas and its cold-tolerant ilk. They make fish inedible in a fraction of the time they take to spoil beef or pork, by consuming the savory free amino acids and then proteins and turning them into obnoxious nitrogen-containing substances (ammonia, trimethylamine, indole, skatole, putrescine, cadaverine) and sulfur compounds (hydrogen sulfide, skunky methanethiol).