an unpleasant muddy aroma It’s most often encountered in bottom-feeding fish, especially catfish and carp that are raised in ponds dug directly in the earth The chemical culprits are two compounds that are produced by bluegreen algae, especially in warm weather (geosmin and methylisoborneol) These chemicals appear to concentrate in the skin and the dark muscle tissue, which can be cut away to make the fish more palatable Geosmin breaks down in acid conditions, so there is a good chemical reason for traditional recipes that include vinegar and other acidic ingredients Fishiness 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 saltwaterbalancing compound TMAO (p 188), 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 freshsmelling 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 Fortunately, the fishiness of fish past its prime can be greatly reduced a couple of simple treatments TMA on the surface can be rinsed off with tap water And acidic ingredients — lemon juice, vinegar, tomatoes — help in two ways They encourage the stale fragments to react with water and become less volatile; and they contribute a hydrogen ion to TMA and DMA, which thereby take on a positive electrical charge, bond with water ... DMA (dimethylamine), which smells weakly of ammonia Fortunately, the fishiness of fish past its prime can be greatly reduced a couple of simple treatments TMA on the surface can be rinsed off with tap water... rinsed off with tap water And acidic ingredients — lemon juice, vinegar, tomatoes — help in two ways They encourage the stale fragments to react with water and become less volatile; and they contribute a hydrogen ion to...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 freshsmelling fragments (aldehydes)