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 blue-
green 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 saltwater-
balancing compound TMAO (p. 188), which
bacteria on the fish surfaces slowly break
down to smelly TMA. Freshwater fish