Taste bud

A taste bud is an organ that contains taste receptor cells that allow a person to perceive different tastes. Most taste buds are located on the tongue, but some are in the throat, in the upper part of the esophagus, or on the roof of the mouth. The average person has about 10,000 taste buds at birth, which decrease as the person ages. Scientists generally agree that taste buds can detect five tastes: sweet, sour, salty, bitter, and umami (savory). Taste is different from flavor. Thousands of flavors exist. The taste buds work together with olfactory receptors in the nose (i.e., the sense of smell) and the brain to identify distinct flavors.

Background

Many people believe that the raised bumps they see when they stick out their tongues are taste buds, but taste buds are microscopic. The bumps people can see are actually tiny structures called papillae, and each papilla contains from three to more than one hundred taste buds.

The tongue has four types of papillae, each of which is found in a particular region. Fungiform papillae, which contain three to five taste buds each, look like tiny round bumps and dot the outer edges of the tongue. Filiform papillae, which contain no taste buds, are longer and thinner and are concentrated near the center of the tongue. Foliate papillae, which may contain several hundred taste buds each, resemble ridges and sit on either side of the tongue near the throat. Circumvallate papillae, which also may contain more than one hundred taste buds each, are large and round and located on top of the tongue near the throat. Most people have about twenty foliate papillae and seven to twelve circumvallate papillae. Fungiform papillae are the most numerous.

Each taste bud contains between thirty and one hundred taste receptor cells called gustatory cells. These cells have tiny hairlike projections called microvilli that project toward a microscopic opening called a taste pore. When a person eats a piece of food, gustatory cells sense food particles through the taste pore. The sensation sends a signal from the cell to the brain via cranial nerve fibers. The brain processes the signal, which allows a person to determine the taste of their food.

Taste buds last  about 10 to 14 days before they are replaced and renewed continuously. The thousands of taste buds in a person’s mouth are replaced about every two weeks. As people get older, this replacement process slows down. As a result, people may lose some of their ability to taste as they age, just as they might lose their ability to see or hear.

Overview

For centuries, scientists believed taste buds could distinguish four tastes: sweet, sour, salty, and bitter. In the late nineteenth century, a Japanese chemist named Kikunae Ikeda identified a fifth taste associated with savory foods such as soy sauce, broth, ripe tomatoes, and aged cheese. He called the taste, which comes from the amino acid glutamate, umami (a Japanese word for “delicious”). More than a hundred years later, scientists confirmed that taste buds are indeed sensitive to glutamate, and in 2002, umami (savory) became accepted as the fifth taste. Researchers in the twenty-first century have proposed that taste buds can detect several other tastes, such as fat and calcium, but the scientific community has not yet come to an agreement on these.

A common misconception is that taste buds are grouped into certain areas of the tongue, and that each area can detect only one taste. A diagram found in many elementary school textbooks shows the tip of the tongue able to detect sweetness, the sides able to detect sourness and saltiness, and the back able to detect bitterness. (Umami is left out altogether.) In truth, each taste bud, regardless of its location on the tongue, can detect any of the five tastes. For example, a person who touches a potato chip to the tip of their tongue will be able to tell that it is salty. The so-called “tongue map” was developed in the 1940s by psychology professor Edwin G. Boring. Boring based his map on early twentieth-century research by German scientist David P. Hänig, who determined that specific areas of the tongue are more sensitive to certain tastes than others are. His research, however, did not indicate that taste buds in certain regions were specific to particular tastes.

In terms of sensitivity, taste buds at the back of the tongue generally seem more sensitive to bitter and sour tastes. This trait is present from birth and likely helped early ancestors of humans survive. Poisonous plants, for example, often have a bitter taste as a defense mechanism to protect them from being eaten. Spoiled foods often taste sour. If an early human tried to eat a poisonous plant or spoiled food, the bitter, or sour taste would likely cause them to spit out the toxic food before swallowing it.

Many people believe that taste and flavor are one and the same, but they are different. Taste buds can detect taste, they cannot detect flavor on their own. Flavor depends on other senses, particularly the sense of smell. Olfactory cells in the nose work similarly to taste buds on the tongue, but instead of sensing five tastes, they sense thousands of odors. The nose senses many odors as they enter the nasal cavity from the front through the nostrils. However, the nose can also sense odor molecules that enter the nasal cavity from behind as a person chews and swallows food. These odor molecules trigger olfactory receptors in the nose that send messages to the brain. Combined with the messages it receives from the taste buds, the brain is able to distinguish flavor. For example, if a person holds their nose and eats two green jelly beans, one at a time, they will likely be able to tell that both jelly beans are sweet because the taste buds recognize sweetness. However, they would not likely be able to distinguish flavor. If the same person eats the same two green jelly beans, one at a time, without holding their nose, they can tell that one is green apple and one is lime because the olfactory cells are engaged. This is because the olfactory cells work with the taste buds and the brain to recognize flavor.

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