Chemistry is often thought of as interesting because of the colourful crystals, solutions or precipitates obtained when different reagents are mixed. Unfortunately, the real connections between the chemistry of what happens and what we observed with our senses of touch, smell and sight was often neglected. I personally feel that there is more to chemistry than just the spectacular explosions or the colourful solutions formed. What fascinates me is how the molecules interact with our senses, such as how did benzaldehyde in almonds give rise to it’s distinct smell.
What causes the ‘fishy’ smell in ‘rotten’ fish
There are many chemical changes that are happening around us which affect our daily lives . When we buy fish from the wet market, one of the indicators of the freshness of the fish is the presence of the fishy odor. It seems like the more ‘fishy’ the fish smells, the less fresh it is. What is the cause of ‘fishy’ odour or the bad smell ?
The Chemistry of the ‘fishy’ smell
The bad smell is a consequence of natural metabolic process which occurs when fish decays. Marine fish, shellfish and some freshwater fish contain trimethyamine oxide (TMAO), a component that is important for maintaining water balance in saltwater. When decomposed upon exposed to bacterial enzymes, (TMAO) decompose to odoriferous products which include trimethylamine and dimethylamine which are the cause of the fishy smell in ‘rotten’ fish.
The enzyme responsible for this conversion is trimethyamine-N-oxide reductase. This enzyme acts on nicotinamide adenine dinucleotide (NADH) and TMAO to form NAD+ , trimethyamine and water. The TMAO acts as an oxidising agent and is reduced, while NADH undergoes oxidation, acting as a reducing agent.
Question for thought. To remove the fishy smell, we can wash with a solution of lemon juice or vinegar? What is the chemistry behind this?
Kerry K. Karukstis, G. R. V. H. (2003). Chemistry Connections: The Chemical Basis of Everyday Phenomena (Second).