Foul In, Fragrant Out: Transforming Stench Into Scents

Of the five senses, the sense of smell gets the worst rap.

Consider: Clear sight is connected to wisdom. Hearing with attention. Touch with sensitivity. Taste with all that is refined. But smell?

As soon as the sense of smell is mentioned, the first thought that rushes to peoples’ minds is bad smell. Stink. They make awful faces, or bad jokes. Yet with a moment of reflection, most people will recall good smells and the pleasant memories they evoke.

Smell is an emotional topic — and there may be good reason for that. Of all our senses, the sense of smell offers the most direct connection between the world and our brain:

When we touch something, skin comes between our nerve endings and those hot or cold, rough or smooth, wet or dry sensations that find their way from our fingers to our brains.

	Sight is a cascade of chemical changes. These changes begin when light triggers a nanosecond-fast transformation of a tiny hydrocarbon that eventually triggers a nerve impulse to the brain.
	When we hear, our nerves translate pressurized waves bumping against our eardrums into something our brains recognize as sound.
	Taste is a chemical reaction between the receptor cells on our tongues and the molecules in our mouth, but the link is not direct. Eating would be a much more intense experience if the 10 square centimeters of our tongue were made of exposed nerve cells waiting for the next sip of hot coffee!

Smell is different. In a small, protected area of your nose, some very specialized nerves are exposed directly to the world...which may be why a single smell can bring back a whole flood of emotions. This direct nerve link also makes the sense of smell as much as 10,000 times more sensitive than taste.

Your sense of smell is extremely complicated — perhaps on the order of the wiring diagram for the space shuttle. Scientists who study smell have identified many specialized sensors in the human nose that respond only to a specific shape of molecule. These molecular “key and lock” pairs combine with other molecules that dissolve in mucus and trigger a variety of responses. What we experience as a simple aroma is actually a whole parade of different molecules in various combinations and concentrations.

Just as it’s harder to write beautiful music than to make a racket banging pots and pans together, good smells are a much bigger challenge to orchestrate than simply raising a stink. A good smell is subtle. It makes itself known, but doesn’t overload our senses. “Smelling good” means attracting attention among all the other smells in the same room, but not overwhelming them.

Bad smells are just the opposite. Skunk, rotting fish, rotten eggs, overripe fruit, animal waste, diesel smoke, and similar foul smells ring alarm bells in your head. “Don’t eat!” “Don’t touch!” and “Run as fast as you can and don’t look back!” are the messages that bad smells carry. When creating a bad smell, the main challenge is getting a high enough concentration of the right chemicals to deliver the intended message. Bad smells don’t have to work with the background aromas. They overpower them.

One of the lovely powers of chemistry is transformation, and nowhere is this more evident than in the chemistry of smell. Chemists may not be able to turn lead into gold as the alchemists wished, but the transformations possible in today’s labs and plants would be beyond wondrous to all who lived in the era when alchemists were the cutting edge of chemistry.

The modern world is replete with examples, but one jewel of purposeful engineering — and of green chemistry, long before it was cool — is a flavors and fragrances plant in Jacksonville, FL, currently operated by Lyondell Corp.

Built in 1910 — when chemical engineering was still striving for professional recognition, and just two years after the founding of AIChE — this plant uses crude sulfate turpentine (CST) as its main raw material to create a bouquet of delightful tastes and aromas. Pine oil, spearmint, licorice, jasmine and many other flavors and fragrances are transformed from the CST that arrives in rail cars from Alabama and Georgia paper mills.

Transformations like these are the essence of conservation and the definition of green chemistry. Garbage in, elegance out, is an everyday occurrence in the world of chemistry. In this year of celebrating the centennial of AIChE and chemical engineering, it is worth remembering that the power of chemistry to turn waste into wonders is part of our everyday life.

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This article was originally published under the title "We're History" in the June 2008 edition of Chemical Engineering Progress magazine. This article was prepared by Neil Gussman, communications manager for the Chemical Heritage Foundation.