Moheb Costandi
(neuroscientist and author of '50 Human Brain Ideas You Really Need
to Know )
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 the human body after death ?
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.
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.
Decomposing Human Blood: Canine Detection Odor Signature and Volatile Organic Compounds.
Rendine M1, Fiore C2, Bertozzi G1, De Carlo D1, Filetti V3, Fortarezza P2, Riezzo I1.
J Forensic Sci. 2019 Mar;64(2):587-592. doi: 10.1111/1556-4029.13901. Epub 2018 Sep 3.
Abstract
The
admissibility of human "odor mortis" discrimination in courts depends
on the lack of comprehension of volatile organic compounds (VOCs) during
the human decay process and of the lack in standardized procedures in
training cadaver dogs. Blood was collected from four young people who
died from traffic accidents and analyzed using HS-SPME/GC-MS at
different decompositional stages. Two dogs, professionally trained, were
tested to exactly locate blood samples, for each time point of the
experiment. We found a long list of VOCs which varied from fresh to
decomposed blood samples, showing differences in specific compounds. Dog
performance showed a positive predictive value between 98.96% and 100%
for DOG A, and between 99.47% and 100% for DOG B. Our findings
demonstrated that decomposing human blood is a good source of VOCs and a
good target for canine training.
© 2018 American Academy of Forensic Sciences.
