The nose can sometimes even beat the eyes in the race for setting up the tasting expectations. An aroma can carry from one room to another, beyond the line of sight. Of the five senses, smell is the most acute, approximately 1,000 times more sensitive than the sense of taste. As a result, what is termed flavor is influenced by roughly 75% smell (olfaction) and 25% taste (gustation) in healthy individuals.
Smell and taste are the chemical senses because their receptors are stimulated by chemical molecules. As little as one molecule in a million may be detected by the nose, but it takes a minimum of one part per thousand to stimulate the tongue. As sensitive and accurate as this organ is, relatively few people ever realize its potential for sensory enjoyment by learning how it works and the language of smells.
For a substance to be smelled, it must have a certain degree of volatility (be readily evaporating) and it molecules must be hydrophobic (able to dissolve in oil, but not water). Odor molecules are typically larger than those that stimulate taste.
The odor vapor must contact receptors which cover the organs of smell, a pair of olfactory membranes. Located deep in each uppermost nasal cavity, they are brownish-yellow, roughly the size of a postage stamp, about two centimeters thick and covered in a thin layer of mucous, which the molecules must penetrate. There are 200 distinct kinds of nasal receptors. They function using 50 million olfactory neurons, each with cilia that extend into and through the mucous. On the cilia are the receptors that capture the scent molecules, signaling the neurons to send the scent message to the brain for interpretation.
The sense of smell is ancient and primal, one of the earliest senses evolved, for locating food, warning of danger, and regulating sexual behavior. Unique among the senses, the scent message passes directly through the limbic system, the emotional center of the brain, on its way to conscious identification in the cortex. Reaction to certain smells may be instinctive; identification of those smells requires a certain amount of experience and training.
While smell is the most easily stimulated of the human senses, it is also the most fragile. Most of us have experienced detecting the aroma of cooking, maybe even from outside the house. In pursuit, we trace it to the kitchen where it becomes stronger. After standing there for a few minutes, however, the cooking odors may no longer be noticeable. This fatigue of the sense of smell is part of sensory adaptation: the self-adjustment to a constant level of stimulus in an environment, so that the individual retains sensitivity to changes. This adaptation also occurs for the sense of sight in a darkened theater or hearing in a noisy city.
There is a great variation between individuals in the elements to which they are sensitive. A person’s absolute threshold is the smallest amount of stimulus required to produce a sensation. Once that threshold is reached, unless trained, the individual can only recognize and unconsciously catalog the smell as either "familiar" or "new". Scientists have proven that the nose can detect and distinguish between thousands of different smells, depending upon individual aptitude and training.
Scientists have cataloged over 17,000 different smells. About 10,000 can be distinguished by humans, although no one knows just how this ability works.
The chemical make-up of wine includes many trace elements that contribute to the combination of smells. Some of these same elements are also found, frequently in higher concentrations, in other familiar foods, spices, flowers, etc. Consenquently, wine smells may often bring to mind these other familiar things, albeit with more subtlety and much less obvious or instant recognizability. With training, concentration, and practice, nearly anyone can learn to dissect and describe these elements of complexity.
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