By Harold McGee
The moment fish are caught and killed, other aromas begin to develop. The strong smell that we readily identify as “fishy” is largely due to the saltwater-balancing compound TMAO, which bacteria on the fish surfaces slowly break down to smelly TMA. Freshwater fish generally don’t accumulate TMAO, and crustaceans accumulate relatively little, so they don’t get as fishy as ocean fish. In addition, the unsaturated fats and fresh-smelling fragments (aldehydes) produced from them slowly react to produce other molecules with stale, cheesy characters, some of which accentuate the fishiness of TMA. And during frozen storage, the fish’s own enzymes also convert some TMA to DMA (dimethylamine), which smells weakly of ammonia.