Here at Death/Scent we tend to play with pleasant fragrances that are used to create an atmosphere of sacred space during death rituals. I am not, however, adverse to talking about the darker side of scent. Especially when new research on the fetid haze of human decomposition may lead to improved forensic recovery. So let’s take a look at what’s new in scent chemistry, forensics, and their interesting similarities to practices in the perfume industry.
The foul odour of decomposition may simply read as putrid to humans, but to our animal friends, a wealth of information can be determined by the scent of a corpse. The process of decomposition starts 4 minutes after death, but it usually takes several hours before it becomes noticeable to humans. Research conducted in forensic entomology (the study of insects and other arthropods for criminal matters) determined that scent, or the detection of volatile organic compounds (VOCs), is a significant factor in drawing insects, like blowflies, to a corpse. They can detect the scent of death in the air in a matter of minutes. In 2014, research done by Dr Bekka Brodie concluded that female blowflies (who need carrion to lay their eggs) can detect remains by picking up precisely on dimethyl trisulfide in the air in minuscule quantities, 1 part per trillion, in open-air, and from up to 1.6 kilometres away.
If we could train blowflies to find lost bodies, there would never be another missing person, but they are surprisingly stubborn with the whole leash training thing. Instead, we have cadaver dogs. Hounds and retrievers have been used to search for human remains for centuries, but little science or specific training was applied. Often they were hunting dogs repurposed for search and rescue. Cadaver-sniffing dogs that are specially trained just to find bodies are relatively new; with the earliest schools going back to the 70s. While anecdotal evidence shows dogs finding bodies buried under meters of soil, concrete, and even under water, there is still the problem of false positives. Sometimes dogs indicate the scent of human remains when none are there, or they send the search party off track due to the scent of a dead rabbit in the woods. False positives have the potential to lead investigations astray or incriminate an innocent person. Drug or explosive-detecting dogs are trained to key in on a certain combination of volatile compounds that are relatively consistent and not usually found in nature.
Decomposition, however, changes with time, weather, and the state of the body upon death. Unlike drugs or explosives, we are asking cadaver dogs to find one particular decomposing thing in a big old world full of decomposing things. When cadaver dogs were first scientifically evaluated in the 90s their success rate in one study was only 57%, that is just a hair above chance. Improvements in training helped cut down on handler-induced false positives, where the dog gave a positive reading because its handler unconsciously gave the dog cues that they thought the location was right. Likewise, trainers have tried improving accuracy by using human analogues like chicken carcases or isolates of cadaverine and putrescine (you might remember them from our indole post). No one asked, however, does rotting chicken smell like human decomposition? Any animal tissue produces cadaverine and putrescine, so do tools like this really help? Elien Rosier and her team at the University of Leuven in Belgium, are fine-tuning the instrument used to train cadaver dogs, which might one day replace them altogether.
Perfumery and Forensic Science
In their research, Rosier’s team first analysed the full chemical cocktail of human decomposition. Rosier used a technique that commercial perfumers will know very well. She collected headspace samples and processed the headspace through a thermal desorber, gas chromatograph and a mass spectrometer. In plain English, they captured the volatile organic compounds present in the air just on the outside of a sample of decomposing flesh. They then ran the sample through several machines that would separate and identify all the components of the stink cocktail. They found 452 volatile organic compounds in human decomposition.
This forensic methodology is also an industry standard in perfumery for scoping out the competition. Want to know what is in the new Atelier Cologne? Just buy a bottle, run it through a GC-MS and it will tell you precisely. This practice has become such a problem that perfumers in the commercial sector have started putting their signature in their formulae by adding trace amounts of ingredients too small for the human nose to detect, but that will show in GC reading. The intention of this invisible signature is to throw off those that would try to dupe their work.
The dead, however, are more generous with their perfume. Once they gave up their formula to Eau d’ Cadaver, it could be compared to similar samples of different animal decomps. What they found was that the closest animal analogue to human decomposition was the pig, not chicken. 8 compounds (ethyl propionate, propyl propionate, propyl butyrate, ethyl pentanoate, pyridine, diethyl disulfide, methyl(methylthio)ethyl disulfide and 3-methylthio-1-propanol) were distinctive among human and pig remains and not present in other animal remains. Likewise, they were able to analyse what VOCs were unique to humans. Five esters were present in humans that were not in pigs. These chemical components are a little surprising to a perfumer as most of them have commercial usages.
- 3-methylbutyl pentanoate, used as a flavouring, smells like bananas and pears
- 3-methylbutyl 3-methylbutyrate, used in perfumery as a sweet, minty, herbaceous note
- 3-methylbutyl 2-methylbutyrate, used mainly as a food additive to give an apricot taste
- butyl pentanoate, used in perfumery, smells like fruit punch, or apples and raspberry
- propyl hexanoate, also called propyl caproate. Smell like blackberries, pineapple, or red wine in small doses.
These esters which can both be isolated naturally from fruit or synthetically produced, are common ingredients in fragrance and flavourings. At first glance, these five sound like a fabulous sangria, but remember your Latin classes kids, sola dosis facit venenum, the dose makes the poison. A microdose in a controlled environment might smell like a fruit salad’s dream. In high doses as part of the heady brew of death, they are vile. I wonder if these contribute to the sour lemon note some people report when smelling human decomp?
Further research is needed to verify that these five esters are truly unique to humans, and they will need extensive outdoor trials. However, the possibilities of this study, in the short term, is that cadaver dogs may have better scent training that will clue them in specifically to the scent of human decomposition. That has the potential to dramatically reduce false positives. There is also the possibility of retiring the cadaver dogs in the next decade.
New sensing equipment is being developed to detect human remains buried in soil by testing for ninhydrin-reactive nitrogen which accumulates in the headspace of grave soil. It is not detectable by humans, and it’s unclear if dogs can smell it in small doses. It is not impossible that within a few generations of this new device, research may have advanced to the point that equipment may be able to detect early chemical signals of human decomposition from the air, like the blowfly does, and be used in the field.
You can download the complete peer-reviewed article on how Rosier developed her methodology here, and her 2015 article with her findings is here. It is great to see more journals and scholars opting for open source for their research exposing their work to a wider audience.