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What gives a decaying body its distinctive odour?

Dead bodies give off a distinctive, sickly-sweet odour that’s immediately recognisable and hard to forget. The smell of death can consist of more than 400 volatile organic compounds in a complex mixture. These compounds are produced by the actions of bacteria, which break down the tissues in the body into gases and salts.

The exact composition of the gas mixture changes as decomposition progresses. It also varies slightly according to the exact composition of the bacterial population in and around the body and the interactions between them, the climate of the habitat, and (to a lesser extent) the genetic make-up and diet of the deceased.

The compounds given off can vary, but there may be core compounds with concentrations that change in a consistent way. If so, analyses of the exact composition of the smell given off by a body could eventually help forensics investigators to estimate the time of death more accurately.

Chemists typically analyse the smell of death using a technique called gas chromatography, which enables them to separate the compounds in the mixture and determine the concentration of each.

The two best-characterised components are cadaverine and putrescine, foul-smelling molecules that repel most animals. First discovered in 1885 by a German physician named Ludwig Brieger, they are small molecules produced by the breakdown of the amino acids lysine and methionine, respectively. Several years ago, researchers finally identified the cadaverine receptor in zebrafish.

Necrophagic (or ‘dead-eating’) insects are attracted to the smell given off by rotting flesh and may help researchers to identify other core compounds. Different species colonise a cadaver in successive waves and at different stages of decomposition, using their exquisitely sensitive olfactory systems – which have adapted over millions of years of evolution – to home in on decaying flesh. As a result, different species might identify different components in the smell, which might only be given off during certain stages of decay.  

Some researchers are developing ‘electronic noses’ and other gas sensor systems capable of detecting many of the compounds in the smell of death. Such devices could one day be used to locate the bodies of people who die in natural disasters, such as buried earthquake victims, or murder victims whose bodies have been buried in shallow graves.

They could find applications beyond forensics, too, particularly in the food industry. Devices that are sensitive to the smell of decay may, for example, help with estimating how long fish or meat have been kept in storage or help factory workers identify spoiled products before they are sent out to be sold.

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