After years of sniffing likely suspects, scientists reported yesterday they have identified the main ingredient in underarm odor. The discovery, while not a cure, could lead to better deodorants.
The researchers, who presented their findings yesterday at the annual meeting of the American Chemical Society here, brought with them vials of a synthetic compound that closely mimics the natural counterpart produced by the body. It did not take long before the conference room was filled with a most human essence.
"It smells like my shirts at the end of the day," said Wendell Roelofs, an expert in insect pheromones from Cornell University, as a group of scientists gathered around the vials to take a whiff.
The active ingredient in underarm odor was discovered by a team of researchers led by George Preti of the Monell Chemical Senses Center in Philadephia, a nonprofit research institute dedicated to the study of taste and smell. Preti, a leading expert in the field of human scents, explained the rigors of working in a field at which many researchers would turn up their noses.
To begin, the scientists collected samples of underarm sweat from male volunteers who wore absorbent pads under their arms. The researchers made extracts of the secretions, which they then submitted to chemical analysis, including a device called a "smell chromatographer," which sorted out the individual compounds produced by the armpit and presented them one-by-one to the researchers, who then sorted the compounds by smell.
Previous studies had implicated a group of volatile steroid compounds as the main cause of underarm odor. But Preti and his colleagues were suspicous. Only about half of the population can even detect and identify the smell of the pure volatile steroids, which give off an unpleasant urine-like smell.
Instead, the scientists focused on a group of acids. The most prevalent and characteristically malodorous of these is 3-methyl-2-hexenoic acid, which Preti and his colleagues believe is the chief underarm odor. The researchers say that about 95 percent of the population can detect hexenoic acid. The other 5 percent, while lucky enough, probably detect other smells that they associate with underarm odor.
The production of 3-methyl-2-hexenoic acid takes several steps. The underarm environment harbors at least three kinds of glands associated with hair follicles. The most important is the apocrine gland, known to science as "the human scent gland." Apocrine glands can be found in many locations around the body, especially the breast and gential regions. But Preti said the gland is "most numerous and largest" in the underarm.
The scientists found the apocrine gland produces a precursor chemical that is partially digested by bacteria that thrive in the moist, warm habitat found in the armpit. The bacteria, in particular microbes known as the lipophilic diphtheroids, feed on the glandular secretions and turn them into 3-methyl-2-hexenoic acid.
Interestingly enough, about 90 percent of all men support a healthy population of lipophilic diphtheroids. Only 67 percent of women do. While bathing and the use of deodorants can knock down the microbial population, one can never kill them all off.
"Despite the fact that hundreds of millions of dollars are spent on deodorants, very little basic research has been done on human odors," Preti said. The scientist suspects deodorant and antiperspirant manufacturers will pounce on the finding and attempt to come up with better-working creams, sprays and roll-ons to mask the smell that many in certain cultures -- Americans and Japanese, in particular -- find so unpleasant.
Deodorants contain antibacterial agents to kill the microbes that produce the malodorous compounds. Antiperspirants add metallic salts to inhibit perspiration. Both may contain scents mixed by perfumers to mask body odors.
"Our work should help," Preti said.