Charles J. Wysocki, a neuroscientist at the Monell Chemical Senses Center in Philadelphia who studies individual variation in olfaction and chemical irritation, provides the following explanation.
As people age they often complain about decreases--or even losses--in their abilities to taste a superb meal or appreciate a fine beverage. When people eat a meal, however, they often confuse or combine information from the tongue and mouth (the sense of taste, which uses three nerves to send information to the brain) with what is happening in the nose (the sense of smell, which utilizes a different nerve input).
It¿s easy to demonstrate this confusion. Take a handful of jellybeans of different flavors into your hand and move them about while your eyes are closed. With your other free hand, pinch your nose closed. Now pop one of the jellybeans into your mouth and chew, without letting go of your nose. Can you tell what flavor went into your mouth? Probably not, but you most likely experienced the sweetness of the jellybean. Now let go of your nose. Voila--the flavor makes its obvious appearance.
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This phenomenon occurs because smell provides the information about the flavor. Chemicals from the jellybean are inhaled through the mouth and exhaled through the nose, where they interact with special receptor cells providing information about smell. It¿s the reverse process that one experiences downwind from a pig farm or chocolate factory. From these sources, chemicals (termed odorants) are carried on the wind, perhaps for tens of miles, and enter the nose during an inhalation. These odorants then interact with the receptor cells and initiate a series of events that are interpreted by the brain as a smell.
Estimates for different types of odorant molecules vary, but there are most likely more than tens of thousands of them. Taste, on the other hand, is limited to sweet, sour, bitter, salty and umami (the taste of monosodium glutamate (MSG)). With advancing age there is an associated diminution in sensitivity of the sense of smell--much more so than a decrease in sensitivity to tastes. This perception may result from an accumulated loss of sensory cells in the nose (perhaps as much as two thirds of the original population of 10 million).
A different nerve provides information about spiciness. Here, too, with advancing age there is a decrease in sensitivity to the sting in hot peppers or the bite in the nose from a dollop of horseradish. As a result, the elderly are in general less sensitive than young people to the overall perception of the food they eat and may complain that the "taste" isn¿t the same as it used to be. There are exceptions, however: some 90-year-olds may be more sensitive to smells than some 20-year-olds.
Answer originally posted February 25, 2002.