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, researchers have revealed.

These compounds are produced by the actions of bacteria, which break down the tissues in the body into gases and salts.

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

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 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

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.

WHAT HAPPENS TO YOUR BODY AFTER YOU DIE? Far from being 'dead', a rotting corpse is teeming with life. Decomposition begins several minutes after death with a process called autolysis, or self-digestion. Soon after the heart stops beating, cells become deprived of oxygen, and their acidity increases as the toxic by-products of chemical reactions begin to accumulate inside them. Enzymes start to digest cell membranes and then leak out as the cells break down. This usually begins in the liver, which is rich in enzymes, and in the brain, which has a high water content. Eventually, though, all other tissues and organs begin to break down in this way. Damaged blood cells begin to spill out of broken vessels and, aided by gravity, settle in the capillaries and small veins, discolouring the skin. Body temperature also begins to drop, until it has acclimatised to its surroundings. Then, rigor mortis – 'the stiffness of death' – sets in, starting in the eyelids, jaw and neck muscles, before working its way into the trunk and then the limbs. In life, muscle cells contract and relax due to the actions of two filamentous proteins (actin and myosin), which slide along each other. After death, the cells are depleted of their energy source and the protein filaments become locked in place. This causes the muscles to become rigid and locks the joints. During these early stages, the cadaveric ecosystem consists mostly of the bacteria that live in and on the living human body. Advertisement

Different species colonise a cadaver in successive waves and at different stages of decomposition, using their exquisitely sensitive olfactory systems.

This has been 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.