It's fishy

Today, I had some fish and so the question of the day is why are fish fishy? I meant have a strong odor.

Fishy smell is from a lot of different things but one of them is the trimethylamine oxide decomposition. Old fish smell more because the bacterial enzymes that attack the fish triggers an redox reaction.

Redox is an oxidation reduction pair. It's a class of reactions describing electron transfers. It's so important that one of my organic chem professors used to say that there are only two kinds of reactions: redox (oxidation-reduction pair) and not redox -_-. One commonly known redox reaction is the browning of apples. The apples oxidize and the oxygen in the air is reduced. Note that redox reactions must occur in pairs, because it's a TRANSFER of something. Nothing is lost or gained in the process. Some thing loses (and is oxidized) and another thing gains (and is reduced).

Back to fishy smells, trimethylamine oxide is decomposed via an enzyme mediated redox reaction into trimethylamine. It's the trimethylamine that is reduced (notice the loss of the oxygen group) smells fishy.

trimethylamine

trimethylamine oxide

Why do fish have trimethylamine oxide? Apparently it's a common metabolite in animals. Metabolites are substances that are produced during metabolism (biochemical processes occuring in the body for example during generation of energy from food). But in saltwater fish, it serves another interesting function. It's an osmolyte. Particularly in sharks that use urea as osmolyte, trimethylamine oxide also counters the denaturing effects. Osmosis is an interesting topic and I hope to cover it in the near future. In short, think of what happens when you put a cucumber in salt water. The water moves out. Why is that?

Often, lemon or other acidic things are added to fish dishes. This has a chemical basis. Trimethylamine can undergo an acid-base reaction and the trimethylamine is protonated (gains a proton to become positively charged). Due to this positive charge, it becomes less volatile (more difficult to go into gas phase) and thus is not readily detected by our nose.

Next time you eat fish, remember to use lemon or lime to enhance the flavors and think about the new level of understanding you have about this previously encountered event.

ref:
http://www.fao.org/docrep/v7180e/v7180e06.htm
http://www.southernfriedscience.com/?p=8659