Ruby would talk in terms of dust to dust, but the process is much more complicated than that.
Muscle, comprising 40 to 50 percent of the weight of a human body, is composed of protein, which is composed of amino acids. At death, the fermentation of fat and protein yields volatile fatty acids, or VFAs, through bacterial action. Inside the gut, other microbes do their part. As putrefaction advances, liquids ooze from the body, carrying with them the VFAs. Death investigators call the mixture soup.
Laslo's research focused at the microbial level, analyzing organic components contained in the dirt under and around a body. Years of work had demonstrated a correlation between the decay process and VFA production.
I watched him filter soil through a stainless-steel sieve.
“Exactly what do you look for in the dirt?”
“I don't use soil, I use soil solution.”
I must have looked blank.
“The liquid component between soil particles. But first I have to clean it.”
He weighed the sample.
“As body fluids flow through, the organic matter becomes bound to the soil. I can't use chemical extractants for separation, because that would partially dissolve the volatile fatty acids from the decomposing body.”
“And alter their measurements.”
“Exactly.”
He placed the soil in a centrifuge tube and added water.
“I use deionized water in a ratio of two to one.”
The tube went onto a vortex for one minute to mix the solution. Then he transferred it to a centrifuge and closed the lid.
“The temperature inside is held at five degrees. I'll centrifuge for forty minutes, then filter the sample to remove any remaining microorganisms. After that it's simple. I'll check the pH, acidify with a formic acid solution, and pop the thing into the gas chromatograph.”
“How about a crash course.”
Laslo finished adjusting settings, then gestured to a desk and we both sat.
“O.K. As you know, I'm looking at the products of muscle and fat breakdown called volatile fatty acids. Are you familiar with the four stages of decomposition?”
Anthropologists and death investigators think of corpses as being in one of four broad stages: fresh, bloated, decayed, or skeletal.
I nodded.
“There's little change in VFAs in a fresh corpse. In the second stage, a body bloats due to anaerobic fermentation, primarily in the gut. This causes skin breakage and the leakage of fermentation byproducts rich in butyric acids.”
“Butyric acids?”
“Volatile fatty acids include forty-one different organic compounds, of which butyric acid is one. Butyric, formic, acetic, propionic, valeric, caproic, and heptanoic are detectable in soil solution because they're soluble in water. Two of them, formic and acetic, are too abundant in nature to be of much use.”
“Formic is the one that causes pain from ant bites, right?”
“That's the one. Caproic and heptanoic are only found in significant amounts during the colder months. Propionic, butyric, and valeric are my boys. They're released from a decomposing corpse and deposited in soil solutions in specific ratios.”
I felt like I was back in Biochem 101.
“Since butyric and propionic acids are formed by anaerobic bacteria in the gut, the levels are high during the bloat stage.”
I nodded.
“Later, during decay, aerobic bacteria join the act.”
“So at stage three there's a surge in all VFA formation.”
“Yes. Then there's a rapid fall-off at the onset of stage four.”
“No flesh, no bacteria.”
“The soup kitchen closes.”
Behind us the centrifuge hummed softly.
“I've also found that all fatty acid values are highest just after maggot migration.”
“When larvae abandon the corpse to pupate.”
“Yes. Until that point the presence of the insects tends to restrict the flow of body fluids into the soil.”
“Doesn't pupation occur at approximately four hundred ADD?” ADD stands for “accumulated degree days,” a figure calculated by summing average daily temperatures.
“With some variation. Which brings up a good point. VFA production is temperature dependent. That's why it can be used to determine time since death.”
“Because a corpse will produce the same ratios of propionic, butyric, and valeric acids for any given accumulated degree days.”
“Exactly. So the volatile fatty acid profile can provide an estimate of TSD.” TSD is the investigator's shorthand for “time since death.”
“Did you get the National Weather Service data?”
He went to a set of shelves and returned with a printout.
“It was amazingly fast. Normally it takes much longer. But we do have a slight problem. For a really accurate TSD estimate I need three things. First, the specific fatty acid ratios.”
He pointed at a computer screen linked to the gas chromatograph.
“We'll have those shortly. Second, the National Weather Service data at the location where the corpse was found.”
He held up the printout.
“Third, information on the weight and condition of the corpse. And you ain't got no body.” He sang the last.
“Everyone's a comedian.”
“Two variables are important: the amount of moisture in the soil, and the weight of the body prior to decomposition. Because everyone has a different ratio of fat and muscle tissue, if I don't have a body, I use a standard of one hundred fifty pounds, then apply a correction factor. I think we're safe in assuming your deceased weighed between one hundred and three hundred pounds?”
“Yes. But in doing this, our range broadens, right?”
“Unfortunately. Did you try a rule-of-thumb estimate?”
Since volatile fatty acid liberation ceases at accumulated degree days 1,285 plus or minus 110, it is possible to obtain a rough estimate of time since death by dividing the average daily temperature on the day a corpse is found into 1,285. I'd done this for Lucy Crowe. Yesterday's average temperature in Bryson City was 18°C (64°F), yielding a maximum time since death of seventy-one days.
“That would be the date on which full skeletonization had taken place, and no more VFAs would be detectable.”
Laslo looked at the wall clock.
“Let's see how accurate you were.”
He rose, filtered and vortexed the soil solution sample, tested its acidity, then placed the tube into the gas chromatograph. After closing and sealing the chamber, and adjusting the settings, he turned back to me.
“Let's give this a few minutes. Coffee?”
When we returned the screen showed a series of peaks in varying colors, and a list of components and their concentrations.
“Each curve shows the concentration of a volatile fatty acid per gram of dry weight of soil. First I'll correct for dilution and soil moisture.”
He hit a few keys.
“Now I can calculate an ADD for each VFA.”
He started with butyric acid.
“Seven hundred accumulated degree days.”
He performed more calculations, using each acid. With one exception the ADDs fell within the 675 to 775 range.
“Now I'll use the National Weather Service data to determine the number of days needed to obtain 675 to 775 accumulated degree days. We may have to adjust later if the readings at your body site differ from the officially recorded temperatures. Normally, I like to know that in advance, but it's not a major problem.”
A few more keystrokes. I held my breath.
“Forty-one to forty-eight days. That's your range. According to your calculation, full skeletonization would have taken place in seventy-one days.”
“So death occurred six to seven weeks ago.”
He nodded. “But keep in mind that this time frame is based on an estimated, not an actual, predeath weight.”
“And at the time the stain was produced, the body was fleshed and actively decomposing.”
He nodded.
“But I ain't got no body.”
“And nobody cares for me.”