Flavors and Fragrances
What are odors? Chemicals, of course, but what chemicals lead to which smells? Taste is the exact same thing, with the addition of sweet, salty, bitter, sour and the extra bonus of texture. When we say that something smells like something else tastes, we are referring in both cases to the sense of smell. The mouth heats the food to body temperature, causing the smaller, smell-able molecules to vaporize more readily, passing through the rear of the mouth and up into the nose (from the back side), allowing us to smell the food more intimately while eating. The simple act of chewing forces air repeatedly with each chew from the mouth through the nose so that our smell sensors can 'analyze' the food. If you hold your nose while eating, the air passage is blocked so you cannot 'taste' the food, and the restricted airflow makes chewing more difficult. We can smell a disgusting item and simply say its disgusting. If we smell the same thing while eating mashed potatoes, we might lose our appetite; if the smell occurs as a burst of smell at each chew, we might vomit!
The sense of smell is not connected directly into the brain. Nerve cells that contain the sensory apparatus connect to the olfactory bulb, a grouping of nerve tissue within the brain cavity, positioned immediately above the roof of the nasal cavity. In the olfactory bulb, smell signals are modified to suppress 'background' odors and combine the signals from various sensor signals to make new combinations. The signals are finally sent to those parts of the brain responsible for emotion and involuntary actions. When we smell something, it is as though we have to sort through our 'feelings' to identify the smell!
It is remarkable how many different molecules we can smell. Each chemoreceptor in the nose responds to one aspect of a molecule's shape and electronic configuration. Analysis of human DNA shows some 350 different genes for smell receptors, but it seems our ability to differentiate different smells far exceeds this limit, as combinations of excitation at the 350 receptor sites can create a huge range of sensations. A single molecular shape can excite more than one receptor; perhaps one end of the molecule at one receptor, the other end at a different receptor. Slight modifications to molecules cause changes in their overall smell, and often a small change can produce a very different organoleptic response.
From a chemist's point of view, the nose is a powerful instrument, allowing instant recognition of various molecules, with no need to perform further analysis. To the synthetic chemist working on new smell molecules, his initial tests are conducted with his built-in instrumentation, in an instant. To better understand the chemistry of smell, you should start with The ORGANIC CHEMISTRY PRIMER.
Perhaps the best compilation of molecules and their corresponding odors can be found at The Good Scents Company. This resource contains molecules in the JMOL viewer (as this site does), with odor descriptions.
Modern life is filled with chemical odorants. Most are completely nature-identical (synthetic but also found in nature), some completely new (synthetic), and many are extracted from natural sources (natural). The distinction between these three categories is in certain respects a legal one, as completely natural ingredients have different requirements from the synthetic materials, (depending on the country) for inclusion in food. An often debated issue is that of natural vs. synthetic, with the idea that the 'organic' sources are somehow better than the synthetic ones. This may be, or not be true, depending on your viewpoint, and such viewpoints can be either rational or political in origin. Note the distinction; political viewpoints are often irrational!
If you understand the chemistry involved and the abundance of toxins in our environment (both natural and synthetic), then your viewpoint is most likely a rational one. If you avoid such knowledge, then you are relying on 'feelings' that most likely correspond with a parallel set of feelings which determine your political worldview.
In any case, the study of molecular shape, charge, and even resonant frequency, as they relate to the sense of smell is fascinating. For further reading on the subject, which just might launch you into a new career, check out " The Emperor of Scent" which is a passionate story about a curious gentleman named Luca Turin. Luca suggests that molecular vibrations are responsible for our sense of smell, but has not yet proven his theory. One day, he may receive the Nobel prize for his genius insights.download