Odors and Olfaction
Olfaction, the act of smelling, is as primitive as it is sophisticated. The human sense of smell, although only a fraction as sensitive as most other animals', not only warns us of danger and helps bond babies and mothers, but also allows us to appreciate the finest of wines and perfumes.Chemistry is at the heart of all olfactory perception, a mechanism by which the brain processes a certain type of information.
The chemical theory of olfaction holds that substances emit particles that waft to the olfactory receptors via diffusion and/or convection. Once there, these particles cause chemical reactions that lead to perception of different odors. However, some materials, such as those with high molecular weight, have no odor, while others emit huge quantities of particles that can travel into the sensory organs.
Many organisms have the ability to perceive odor, which requires the presence of certain organs and other apparatus. In terms of the chemistry of smell, the key physical requirements in humans and other higher animals are the olfactory receptor cells, which become stimulated in the presence of odorants; olfactory bulbs, which receive the electrical stimuli from the receptor cells; and olfactive lobes, which distribute the perception to different areas of the brain. In addition, the vomeronasal organ in the nose is a part of a chemosensory system that is independent of the olfactory system. This organ may be associated with the reason that smells can affect human behavior so effectively.
Chemists measure odors according to their intensity, using what they call a "threshold value." On the lowest end of the scale is "detection threshold," which represents the minimum amount of intensity for a person to be able to detect a stimulus. This scale is key to the chemistry of odor, because a single stimulus can be perceived very differently as its intensity rises. For instance, the substance indole, which comes from coal tar and animal feces, actually smells like jasmine at its detection threshold. Although it smells strongly like its sources at higher intensities, its lowest threshold value makes indole a popular ingredient in the fragrance industry.
Understanding and being able to use the chemistry of odor is intrinsically important in numerous industries. Most of the time, manufacturers use this knowledge to make their products smell good--or at least better than the products do on their own. There are two main techniques for improving odor: masking and counteraction.
The process of chemical masking consists of reducing the olfactory perception of a particular odor stimulus by adding another odorant. Masking does not remove or chemically alter the original odor, but merely covers it up with another scent (often one of the aldehydes) so that the brain perceives less or none of the target odor. For this reason, masking is also called "reodorant," as opposed to "deodorant." A potential problem with reodorants is that they raise the overall odor level, which sometimes results in an overwhelming olfactory sensation. Any example of masking is spraying perfume into a diaper-changing room. The perfume will reduce, but not eliminate, the brain's olfactory perception of dirty diapers, but the combination of odors might be even more unpleasant and overwhelming than the original odor alone.
Odor counteraction is a more effective method of changing or eliminating an odor. Similar to the chemical concept of neutralization, odor counteraction consists of mixing two odorous materials at a precise ratio to produce a less-intense odor than those of the separate ingredients. Technically speaking, this process is known as "compensation." Counteraction rarely results in the absence of odor, because there is almost always some residual scent. However, the process of oxidation (i.e., increasing the valence of the odor molecule by removing some of its electrons) can eliminate the offending odor molecule and leave no trace odor at all. An example of counteraction is mixing citral, a pale-yellow liquid used in the perfume and flavoring industries, to neutralize the amines emitted from decaying meat in the dumpster behind a restaurant.
Sometimes, however, industrial chemists are called upon to give a product an unpleasant or sharp odor. This is less common, but an important factor in such areas as oil and gas manufacturing. For example, some of the fuel gases are completely odorless, so there is no indicator (to humans, at any rate) when a leak occurs. Because such an event could be life threatening, inserting an odorant into these gases is crucial to provide a warning of exposure.
There are a whole range of "aroma chemicals," as they are known, that come in a variety of dispersal media, including powder, aerosol, liquid, solid, and electric. These chemicals were originally all obtained from natural sources (e.g., jasmine scent from jasmine flowers, vanilla flavoring from vanilla beans, etc.), but as the amount of agricultural land has declined and the science of chemistry has grown more sophisticated, "synthetics" have become more common and much less expensive than "naturals."
Once chemists began to identify the chemical compounds responsible for a material's characteristic odor, they could make the odor right in the laboratory. Thus, perfume and fragrance makers could simply buy quantities of the synthesized odor they needed rather than search the world over for the exact plant or animal that carried the scent. Thus, perfume makers no longer need natural ambergris, the sperm whale intestinal secretion so good at "carrying" a fragrance, because synthetic versions of the rare and expensive natural substance are now abundant. Likewise, food manufacturers no longer have to use natural grape to flavor a juice or dessert--they need only to add some artificial methyl anthranilate to achieve the same effect at much less expense.
In the ever-expanding science of odor, there are precise categories that help to make odor perception as objective as possible. The main categories in the perfumery industry, for instance, are fruity, citrus, green, floral, and woody. The science has become so precise that chemists have been able to assign characteristic odors to entire functional groups. They know that the ketone group contains aroma chemicals that produce mainly musky scents, for example, while those in the hydrocarbon group produce warm, woody odors. For the increasing number of products that consumers buy based merely on odor, this knowledge represents very big business.
This is the complete article, containing 1,026 words
(approx. 3 pages at 300 words per page).
