The perception of taste has its physiological origin in the taste receptor cells. These are found in the taste buds, which are embedded in tiny protrusions (papillae) located primarily on the top of the tongue but also distributed over the soft palate, pharynx, epiglottis, and the entrance to the esophagus. There are approximately 9,000 taste buds on the human tongue, clustered together in groups of or 50 or so. Each taste bud is made up of 50–150 taste receptor cells. Like other cells, they are encapsulated in a cell membrane, and it is this membrane that holds the secret of taste perception.
The taste receptor cells are bundled together in onion-shaped taste buds in an arrangement that fits together much like the segments of an orange, forming a small pore at the surface of each papilla. Small hair-like structures (microvilli) extend from the cell membranes at the top of these taste receptor cells. Taste substances must pass through the pores in order to be detected, and they can do so only if they are dissolved in water or saliva.
The receptors that can identify the taste substances biochemically are located in the membranes of the taste cells. A receptor is a large protein that functions as a sort of antenna that normally can detect and identify only one type of taste. But in order to be detected by the receptor, the taste molecules generally have to be small. The majority of pure carbohydrates and proteins have absolutely no taste because their molecules are just too large.
In the past few years, major advances in the field of molecular biology have provided us with a much more detailed insight into how these receptors look at the molecular level and how they function.
Schematic illustration of a taste bud. The taste bud is covered by some epithelial cells that form a little pore, through which the taste substance can enter the taste bud. The taste bud consists of a bundle of taste receptor cells, which are individually responsive to one basic taste: sour, sweet, salty, bitter, or umami. Cells that detect the same tastes transmit the information through a single nerve fiber that forwards the collective signal to the brain, which then registers the taste sensation.
A number of models have been proposed to explain how the various taste receptor cells transmit their signals from the taste buds. There now appears to be a consensus around a model in which each taste receptor cell is primarily adapted to detect one particular taste. The various cells that are sensitive to the same taste collect the signals in a single nerve fiber that sends the total signal onward through the cranial nerves to the taste center in the brain. The picture is complicated by the presence in the taste buds of another type of cells, known as the presynaptic cells, which have no taste receptors but still participate in the transmission of the nerve signals. In contrast to the taste receptor cells, the presynaptic cells can respond to several different types of taste, given that they receive signals from a number of taste receptor cells.
There are two principal types of taste receptors. One type, called the G-protein-coupled receptors, which are sensitive to sweet, bitter, and umami tastes, traverse the membrane of the sensory cell. When such a receptor on the surface of the cell has identified a taste molecule for which it is adapted and has bound it, a signal is passed through the protein that a certain other protein (known as the G-protein), located on the other side of the membrane, is also to be bound. This binding sets in motion a cascade of biochemical processes that eventually cause particular sodium channels in the cell membrane to open. Sodium ions flow through the channels, resulting in a drop in the electrical potential across the membrane. This generates an electrical signal that passes through the nerve and ends up in the brain.
The other type of taste receptors is sensitive to sour and salty tastes; that is to say, especially hydrogen ions (H+) and sodium ions (Na+), but also potassium ions (K+). These receptors are ion channels that cut across the cell membrane. A change in the concentration of these ions is registered by the taste receptors, which leads to a change in the electrical potential across the membrane, and an electrical impulse can be sent to the brain.
In contrast to umami, which in its pure form is brought out by only a small number of substances, an incredible range of very diverse substances brings out sweetness and bitterness. For a long time, it was thought that the receptors for sweet and bitter tastes were very closely related. This is due to the fact that many sweet substances, such as the artificial sweetener saccharine, can have a bitter aftertaste. In addition, only very minor chemical modifications can change a molecule from sweet tasting to bitter tasting. Mirror images of molecules can actually have sweet and bitter tastes, respectively. For example, the artificial sweetener aspartame, which is made up of two amino acids, tastes sweet if the amino acids are left turning. But if the same molecule contains right-turning amino acids instead, it will have a bitter taste.
We have still not identified the mechanism that is responsible for this difference. It is possible that it is due to an as-yet-undiscovered interplay between the taste cells and the presynaptic cells.
Schematic illustration of a taste receptor that is embedded in the membrane of a sensory cell. The membrane consists of a double layer of lipid molecules, which form an effective barrier between the inside and the outside of the cell. The receptor is a large protein molecule that traverses the membrane in serpentine fashion with, in this case, seven twists and turns.
Research carried out recently has gradually reduced the number of different types of taste receptors to a relatively small number. In the case of sweet, bitter, and umami, the main focus of attention at the moment is on two classes, labeled T1R and T2R, which can function independently or in particular combinations. The receptors are all G-protein-coupled receptors. Their defining characteristic is a large terminal domain, which protrudes from the outer side of the membrane. This domain is where taste molecules are identified and bound.
When the taste molecule is bound to that part of the receptor that protrudes out of the membrane (pointing upward in the illustration), what is known as a G-protein is bound simultaneously on the end of the receptor that bends around to the inside of the cell. This binding action sets in motion a cascade of signals, which results in the transmission of an electrical signal through the nerve cells to the brain.
The T1R receptors are distantly related to the receptors that are sensitive to the types of amino acids that function as neurotransmitter substances in the brain.
Surprisingly, we know unbelievably little about the G-proteins that are bound to the receptors on the inside of the membrane. This is why we have only very limited knowledge about what happens at this end point inside the cell and the series of signals that causes the ion channels to open and, in so doing, send the resulting electrical sensory signal to the brain. A single G-protein, referred to as gustducin, has been isolated and shown to couple to the T1R and T2R receptors, and hence is involved in the sensing of sweet, bitter, and umami tastes.
Each class of receptors can have a number of different members, such as T1R1, T1R2, and T2R3, which all belong to the T1R family. The different members can be expressed in varying quantities in the individual taste cells. For instance, some cells may have only T1R3 receptors, while others may contain the combination T1R1/T1R2, and still others may contain T1R1/T1R3.
Sweet, bitter, and umami tastes make use of a variety of receptor combinations. This is in contrast to sourness and also, presumably, saltiness, each of which is based on a single receptor. It is therefore probable that the signal pathways for sweet, bitter, and umami tastes are different from those for the sour and the salty.
SWEET
Given that our gustatory sense perceives a large number of really diverse substances as being sweet, one might think that there would be a correspondingly broad range of taste receptors for sweetness. But in recent years, it has finally been established that the combination T1R1/T1R2 is the most important, and possibly the only, receptor for sweetness in mammals. Its perception of sweet tastes covers everything from naturally sweet substances to artificial sweeteners, sweet amino acids, and certain sweet proteins. The reason that the receptor is sensitive to a range of substances that are, chemically speaking, very different is that the various substances bind to different sites on the rather large part of the protein that protrudes from the membrane.
The detailed molecular structure of the receptor can vary significantly from one mammal to another. For example, mice, unlike humans, cannot taste the artificial sweetener aspartame, and cats are missing the gene that allows them to form T1R2 and, therefore, cannot taste sweetness at all.
As we will see later, the receptors for sweetness are closely related to the receptors for umami taste.
BITTER
Unlike sweetness, for which there is only one type of receptor, there are many receptors that are activated by bitterness. Bitter substances stimulate a large family of about thirty different members of the T2R receptor class. It has now been shown that members of this family are both necessary and sufficient to allow the taste cells to respond to a bitter taste. It is possible that the active T2R receptors are arranged in pairs. Each type of T2R can bind several different types of bitter substances. T2R receptors have a much smaller terminal domain that protrudes from the membrane than that of T1R.
Perceptions of bitterness have to be very unambiguous and wide-ranging, as they are an indication that the food might be poisonous. On the other hand, a bitter taste does not have to be especially nuanced in order to play an important evolutionary role. Consequently, the receptors for bitterness are stimulated by many different chemical substances that are not even remotely related. It has also been discovered that most T2R receptors in the taste buds are bundled together in the same cells, where they act as a very broad-spectrum sensor for bitterness.
SOUR
Over time, many mechanisms and receptor models have been proposed to explain sourness. In all cases, these have involved two classes of membrane proteins—namely, ion channels and membrane pumps—to facilitate the transport of sodium ions, potassium ions, and hydrogen ions across the membrane.
At present, the spotlight is on one particular ion channel, known as PKD2L1, as a receptor for sourness. This ion channel is found primarily in the taste cells that are not sensitive to sweet, umami, and bitter tastes. These taste cells, therefore, act as the sensors for sourness.
The taste cells sensitive to sourness have also been found to host a special receptor, called Car4, that is sensitive to carbon dioxide. Hence carbon dioxide, for example as found in carbonated beverages, stimulates the sour-sensing cells and induces a mild sour taste sensation, although the fizzing and tingling are of a mechanical nature.
SALT
It may come as a surprise that the mechanism behind the perception of saltiness is the one about which we know the least. In 2010 in an experiment with mice, an epithelial sodium channel called ENaC, localized in a specific population of the taste cells, was found to mediate the salty taste.
Everyday vocabulary can easily come up short when one is trying to describe a particular taste. Because many food cultures have evolved their own individual ways of characterizing a basic taste with a single word, cultural differences can lead to difficulties in describing taste impressions that are less common or unknown in a particular cuisine. For example, people accustomed to food prepared in the Western world are usually able, without hesitation, to categorize substances as having sour, sweet, salty, and bitter tastes. Few would doubt which word to choose to describe definitively the taste of a food such as a lemon. On the other hand, it would immediately become much trickier for them to describe the taste of MSG.
As shown in a classic psychophysical experiment, carried out by Michael O’Mahony and Rie Ishii from the Department of Food Science and Technology at the University of California, Davis, USA, it boils down to a question of expressibility or codability. These researchers compared the taste-naming strategies of two groups: monolingual Japanese speakers and monolingual American English speakers. For the most part, neither language group had much difficulty in choosing a single word to describe taste samples that were sour, sweet, salty, and bitter. But when it came to MSG, there was a significant difference.
The English speakers were able to differentiate the taste of MSG from that of the other taste samples, but they had no single expression to describe it. In fact, they used expressions that bore little resemblance to each other and that would not be classified as basic tastes. They described it as ‘salty,’ while noting that it was not the same as ordinary saltiness, but they also used descriptors including ‘indefinite,’ ‘fishy,’ ‘beef bouillon,’ and so on. The majority of Japanese, however, linked the taste of MSG to the word umami or expressions closely related to umami, such as dashi (a soup stock), although several of them also said ‘salt-like.’ Many used the word Ajinomoto, which is the trade name for a common MSG taste enhancer. Those Japanese subjects participating in the experiment who were professional tasters tended to employ the more scientific term umami. The researchers concluded that the differences between how the two experimental groups described tastes were not attributable to physiological mechanisms or underlying sensory concepts. Culture, rather than language, was the determining factor in the number of basic tastes that were clearly associated with a single word.
▶ Examples of raw products that most people would, without any hesitation, associate with one of the four classic basic tastes. Sour: red currants with crème fraîche. Sweet: melon with honey. Salty: oysters and sea asparagus (sea beans). Bitter: radicchio and walnuts.
There is still another quality, which is quite distinct from all these [sweet, sour, bitter, briny] and which must be considered primary, because it cannot be produced by any combination of other qualities.... It is usually so faint and overshadowed by other stronger tastes that it is often difficult to recognize it unless the attention is specially directed toward it... For this taste quality the name ‘glutamic taste’ [umami] is proposed.
これらの味とは全く別の、そして他の味を如何に組合わせても作ることができないことから、本源的とみなければならない別の味がある。その味は通常非常に弱く、他の強い味によってボカされるので特に注意をそれに向けないと識別することがむずかしい。私はこの味に「グルタミン酸の味 という名称をつけようと思う。
Kikunae Ikeda (1864–1936)
