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Making Sense of Your Sense of Smell



By Harvey M. Fishman, Consultant



Published August 3, 2010
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The sense of smell is said to be 10,000 times more sensitive than your sense of taste, although temperature and humidity affect the odor’s strength by determining the volatility of the odor from its source. A dog’s sense of smell can be more than one million times more sensitive than a human’s sense of smell. A dog, for example, can identify its master’s odor on a stick that has been grasped for just a few seconds, no matter how many other people have handled it before or after his master handled it.

Yet, with particular odors, a human nose can perform almost as well as a dog’s nose. For example, utility companies add n-butyl mercaptan to odorless natural gas so that people can detect gas leaks. Just one n-butyl mercaptan molecule for every ten billion molecules of methane does the trick. To put this in perspective, it means that if you were standing in front of two Olympic-sized swimming pools where one contains three drops of the mercaptan and the other has none, your nose could tell the difference.

People have various abilities to detect and recognize odors. At one point in my career, I took an odor recognition test with the rest of my colleagues in the laboratory. Most of us did not fare too well, but one of our lab technicians was very successful. I do not know if he went on to better financial rewards by becoming a perfumer.

Follow that Scent!

The sense of smell unleashes a range of emotions.

As a preliminary measurement to establish an olfactory yardstick, Noam Sobel and other scientists in Israel used 32 “human bloodhounds” in an experiment. In an open field, they set down 30 yards of twine that was scented with chocolate. The subjects were blindfolded, wore earmuffs to block out sounds and wore elbow pads, knee pads and work gloves to eliminate any tactile clues. Subjects were then asked to find and follow the chocolate-scented trail using only their noses. They got down on all fours about 10 feet from the start of the scent trail and started to sniff. Unexpectedly, 21 of them were able to follow the scent from start to finish. Whenever they veered off course, they were able to sniff their way back. They were also able to improve their times with extra practice.

Understanding How it Works
Volatile molecules floating in the air latch on to olfactory receptors located on nerve endings in the nostrils. One theory is that only certain molecules have the right shape to connect to certain receptors. A given receptor can latch on to a number of different odor molecules, and a given odor molecule can snag several different receptors. When a receptor grabs a molecule, it causes an electric signal to travel the length of the neuron from the nasal lining to the smell processing regions of the brain. Here, thousands of other neurons are also delivering their own signals. The learning necessary to differentiate odors then, takes place in the brain, not in the nose.

In order to study how the brain distinguishes between odors, Sobel and his staff began to determine the relationship between the structure of a molecule and the way it smells. They built a database of 1,500 odor-producing molecules, and cataloged 1,664 different traits including their size and the strength of the chemical bonds between their atoms. They then looked for traits that consistently varied from one molecule to the next. For instance, the size of a molecule varies along with how tightly its atoms are packed. These patterns were used to give each molecule in his database a single score, like notches on a yardstick. Molecules on one end of the yardstick were unpleasant to people but the other end was delightful to smell. Experiments with mice verified the results with the human subjects.

Sobel claims that an odor is more than the physical properties of a molecule; it is also the emotions conjured up by these properties. We learn to fear certain smells that signal danger just as we make associations with dangerous sights or sounds. Pleasant odors can evoke pleasant memories, such as how smelling a pastry caused Marcel Proust to recall enough information to write“Remembrance of Things Past.”


Harvey Fishman has a consulting firm located at 34 Chicasaw Drive, Oakland, NJ 07436, hrfishman@msn.com, specializing in cosmetic formulations and new product ideas, offering tested finished products. He has more than 30 years of experience and has been director of research at Bonat, Nestlé LeMur and Turner Hall. He welcomes descriptive literature from suppliers and bench chemists and others in the field.


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