Well into the modern age, taste was regarded as something subjective over which housewives and chefs held sway. It was not until about the 1920s that it became the object of rigorous scientific studies. So it should come as no surprise that it is only within the past few decades that we have started to gain a better understanding of its actual physiological basis. This allows us to explain, in detail, how taste is detected in the mouth by certain receptors and converted into nerve impulses that are then forwarded to specific centers in the brain. The neural cells in these centers carry out the final calculation and convey a message about the food—for example, sweet! or salty!
For many centuries, it was customary in the Western world to accept the ancient Greek view, originating with Aristotle, that there were seven basic tastes: sour, sweet, salty, bitter, astringent (causing dryness), pungent (or spicy), and harsh. Over time, people came to the conclusion that there were actually no more than four true basic tastes; namely, the first four on this list. But it was only in the course of the twentieth century that a clear distinction was drawn between sour, sweet, salty, and bitter as genuine tastes and the other three as mechanical or chemical effects caused by substances in the food that damage the cells on the tongue or in the mouth.
In many countries in Asia, however, people have thought that in addition to these four basic tastes, there is a fifth one—pungent or spicy, for example, as in chile peppers. Complicating matters, according to classical Indian philosophy, astringent is also a separate taste. On the other hand, in China and Japan, there is a long-standing tradition, possibly going back more than a thousand years, that there is a particular, identifiable taste associated with food that is especially delicious. In 1909 this taste was given the name umami, a new Japanese word combining the ideas of umai, which means ‘delicious,’ and mi, which means ‘essence,’ ‘essential nature,’ ‘taste,’ or ‘flavor.’ While some Japanese are not overly familiar with this term per se, many others use it not only to denote a mere taste but also as an expression for that which is perfect.
There is no single word in Western languages for this particular taste, nor for a sensation of taste, that is equivalent to how a Japanese person experiences umami. Perhaps this is because the concept of umami is not associated with a universally known and well-defined source in Western cuisines, unlike, for instance, the identification of table salt with saltiness, sugar with sweetness, quinine with bitterness, and vinegar with sourness. In the Japanese kitchen, there is a single ingredient, with a very pure taste, that quintessentially typifies umami—this is the traditional and ubiquitous soup stock dashi, which is used not only in soups but in many other dishes. While there is a great deal of food in the West that is characterized by umami, it is often found in combination with other tastes, for example, in complex mixtures of meat and vegetables, which may also contain considerable quantities of oils and fats. The result is a pleasant, but also more complicated, taste impression. Consequently, if they think about it at all, Westerners tend to view umami as merely a new word for an old, familiar set of taste sensations.
It seems, however, that the Chinese and Japanese have been right all along, as it has now been scientifically established that there are actually five different basic tastes. Of these, the umami, sweet, and bitter ones are the most important in determining how we react to particular foods. Foodstuffs with a sweet or umami taste are generally considered agreeable, while those that are bitter are often rejected.
All of this brings us back to some fundamental questions: What exactly is taste, how do we experience it, and why is it important?
A taste is a sensory impression to which, in principle, we can assign an objective biochemical and physiological perception of a substance; let us just say a molecule, whose chemical nature determines its taste for us as humans. It is not a given that another animal—for example, a mouse—would discern it in the same way.
The experience of taste is much more involved than the physical perception of taste and is often quite particular to an individual. Although it is a function of the same biochemical processes as taste, it is also influenced by the other senses: sight, sound, the feel of the food in the mouth, and especially our sense of smell, which is much more discriminating than that of taste. In addition, the gustatory experience is affected by psychosomatic conditions, social context, cultural background, traditions, degree of familiarity with the food, and, finally, whether we are hungry or already feel full.
There are many types of taste, and a human may possibly be able to distinguish between several thousand different ones. An overall taste is typically made up of a small number of basic tastes. From a scientific perspective, in order for a taste to be considered a true basic one, it must be independent of all other basic tastes and, at the same time, be universally present in a wide variety of foods. In addition, a basic taste must be the result of a physiological phenomenon that, in turn, depends on a chemical recognition of the taste. This recognition takes place with the help of particular proteins, known as taste receptors, which are found in the taste buds on the tongue. It has been known for many years that there are special receptors for the sour, sweet, salty, and bitter tastes. The first receptor for one of the substances that imparts umami, namely, an amino acid (glutamic acid) and its salts (glutamates), was discovered in 2000. As a result, umami could justifiably be elevated to the status of a true basic taste, ‘the fifth taste.’ Subsequent studies have identified additional receptors for umami.
What is interesting about pure glutamate in the form of monosodium glutamate (MSG, sometimes called the third spice), is that it cannot really be said to be tasty on its own. Rather, one might say that MSG has no taste or, even worse, that it tastes like a mixture of something salty, bitter, and maybe soapy. It is only in combination with other taste substances that it calls forth that sublime taste sensation that is worthy of the splendid name umami. For this reason, MSG is often characterized as a taste enhancer. It interacts strongly with other common taste substances, especially table salt, NaCl. What is distinctive about MSG is the nonlinear synergy between it and other substances that also impart umami—a very small quantity of these other substances, known as 5’-ribonucleotides, has a notable multiplier effect on the action of the MSG. As a result, there are many as-yet-unimagined possibilities for playing with umami by combining a range of different raw ingredients.
So even though unique words for umami are lacking in the vocabularies of Western languages, this taste has, of course, not been absent in our kitchens. When examined more closely, traditional European cuisines are seen to strive as much to incorporate umami into their dishes as do the Asian cuisines. Soups based on meat and vegetables, cured hard cheeses, air-dried hams, fermented fish, oysters, and ripe tomatoes are all evidence that we crave after, and savor, foods that are rich in umami tastes.
The science underlying food is complex. Our sensory apparatus for tasting and enjoying food is equally complex and, in many ways, poorly understood. In fact, the sense of taste is the least well understood of the human senses. It is not a given that all taste impressions can be described using only five elementary types of basic tastes. It is conceivable that there might be more than five. Some researchers have recently published studies indicating that they have found a fat receptor in the taste buds on the tongue, suggesting that fattiness might be a basic taste.
In a modern society where there is an abundance of food, we probably think of taste as something that primarily adds sensual pleasure and delight to the enjoyment of a meal. Some might even think of an appetizing taste as something that induces people to be bothered to eat at all. The majority of us, who are not engaged in hard physical work, are not really hungry when we eat. To be convinced of this, just reflect on how much a hiker looks forward to digging into a simple bag lunch during a rest stop in the middle of a strenuous mountain trek.
It is likely that taste allows an animal species to identify those foods that help to ensure its survival, as well as those that might be harmful. This could confer certain evolutionary advantages, although it is, admittedly, difficult to prove this hypothesis. It is evident, however, that the evolutionary basis for taste is probably not sensual pleasure, but rather a fulfillment of a fundamental need and the will to survive and reproduce. To this end, the individual needs food that is very nutritious (proteins), food that provides energy (calories from fats and carbohydrates), and food that contains salts and minerals. In addition, taste has to indicate whether or not the food is poisonous. In all likelihood, the basic tastes have, since time immemorial, been signals that show us how to meet these fundamental nutritional requirements.
What do the various basic tastes tell us?
• Sweetness tells us that the food contains sugars and the metabolic by-products of carbohydrate breakdown, which provide energy and calories.
• Saltiness indicates the presence of minerals and salts, such as those from sodium and potassium that are vital for preserving a proper electrolyte balance in our cells and organs to ensure their proper functions.
• Bitterness sends a strong message that the food may contain poisonous substances—for example, alkaloids—that we should avoid.
• It is less obvious why we taste sourness. Acidity might steer us toward substances that regulate the pH balance in our bodies while at the same time sharpening the appetite and improving digestion. At any rate, sourness helps us to stay away from foods, such as unripe fruit or rancid fats, that contain so much acid that they can be unpleasant to eat or even poisonous.
• If it should prove to be correct that there are also specific receptors for fattiness, it would presumably be a sign that the food contains a significant energy supply.
• In all likelihood, we can taste savoriness or umami because it tells us that the food contains readily accessible nutrition in the form of amino acids and proteins. And furthermore, the intensity of the umami taste gives us an indication of how ripe and full of nutrition a particular food might be. It is quite possible that we are genetically programmed to enjoy umami.
Along the same lines, one might be able to say that the drive to find food that tastes good and that is rich in umami makes Homo sapiens a gourmet ape.
The study of our perception of food, especially of taste, is known as sensory science. Rather than the word taste, we should instead use the word flavor, which denotes the integrated effect of all sensory impressions evoked in the oral cavity. It encompasses both taste and smell, including those derived from aromatic substances in the food, as well as mouthfeel and chemesthesis, which is a sense category that relies on the same receptor mechanisms as those that convey pain, touch, and temperature in the eyes, nose, mouth, and throat.
TASTE OR FLAVOR?
In ordinary speech, the terms taste and flavor are often used interchangeably, but strictly speaking, they are quite different. A taste has to fall into one of the known classifications for which there are distinct taste bud receptors. Flavor, on the other hand, is a perception based on three essential elements: the combination of tastes in the food (think, for example, of real black licorice, which is both salty and sweet), the effect of the aromatic components on the olfactory receptors, and the feelings related to texture, temperature, and so on that are evoked in the mouth.
The words odor, smell, and aroma can all be used to denote that which we perceive through the olfactory system. Although the words in themselves are neutral, odor and smell tend to have a negative connotation. An aroma is also a smell, but the word is used to signify that it is a pleasant one, usually associated with food.
A RECENT ARRIVAL ON THE SENSORY SCENE: KOKUMI
The Japanese expression kokumi(derived from koku, meaning ‘rich’ and mi, meaning ‘taste’) was coined a few years ago by researchers at the Japanese company Ajinomoto. It combines three distinct elements: thickness—a rich, complex interaction among the five basic tastes; continuity—the way in which long-lasting sensory effects grow over time or an increase in aftertaste; and mouthfeel—the reinforcement of a harmonious sensation throughout the whole mouth. It has been shown recently that kokumi is evoked by the stimulus of certain calcium-sensitive channels on the tongue by small tripeptides (for example, glutathione) found in foods such as scallops, fish sauce, garlic, onions, and yeast extract. Whereas glutamate has a significant effect on the umami taste in concentrations of about one part per thousand, substances that produce the most potent kokumi need to be present in concentrations of only two to twenty parts per million.
Because an individual’s experience of flavor results from a very complex combination of several types of sensory perception, it is not always easy to relate a given flavor to the chemical composition of the food.
The sensation of taste presupposes that the taste substances are dissolved in a liquid, primarily in the mouth. As already mentioned, its perception is mediated by the taste receptors, which are located in the taste buds on the tongue.
The sense of smell depends on airborne substances in the form of single molecules, particles, or vapor droplets. These are either released in the oral cavity when the food is chewed and work their way internally to the nasopharynx (retronasal stimulation) or are given off by the food and inhaled through the nostrils (orthonasal stimulation). Smell by the retronasal route appears to be the more important for humans, whereas the opposite is true for dogs. In both cases, the aromatic substances reach the roof of the nasal cavity, where there is an array of specialized neural cells located under a mucus membrane that is covered with tiny cilia. Here sensory cells with olfactory receptors, of which there are thousands of different types, can detect them. As any particular odor generally activates several receptors, humans are able to distinguish among a vast number of different smells. The sense of smell is much more fine-tuned than that of taste, and is now believed to form a sensory image in the brain.
Mouthfeel is a collective term for the sensory perceptions that are neither taste nor aroma but that interact closely with them. It is influenced by the structure, texture, and morphological complexity of a food item and is, to a great extent, responsible for our overall impression of the food. For example, this can involve physical and mechanical impressions such as chewiness, viscosity, mouthcoating, and crunchiness.
The Japanese have a special expression, kokumi, which is rather difficult to convey in other languages. It encompasses thickness, continuity, and mouthfeel, and may overlap somewhat with the taste sensations evoked by umami. Kokumi is not an independent taste, but it does refer to taste enhancement and is associated with food that is truly delicious.
Chemesthesis is a technical term that describes the sensitivity of the skin and mucus membranes to chemical stimuli that cause irritation. It can be thought of as an early warning that these may be harmful. An example of chemesthesis is the painful burning sensation on the tongue that we associate with sharp or spicy tastes caused by a variety of substances such as those in chile peppers (capsaicin), black pepper (piperine), and mustard (isothiocyanate).
Thermal perception of warmth and temperature in the mouth is related to chemesthesis. It is based on the chemical activation of six different temperature-sensitive ion channels located in the membranes of the sensory cells. This sense is so finely tuned that we are able to detect temperature fluctuations to within 1 degree. If the temperature of a substance is less than 15˚C or more than 43˚C, we experience it as pain. Some chemical substances can fool this sensory system and activate the ion channels directly, leading us to think that a taste experience is warm or cold, even though the temperature is actually unchanged. This is referred to as a false perception of heat or cold. For example, we experience capsaicin from chile peppers as hot and menthol, peppermint, and camphor as cool.
A more mechanical sensory impression is that of astringeonucry, which we know from the taste, for example, of tea or red wine, both of which are rich in tannins. It is caused when certain chemical substances interact with proteins found in the mucus on the surface of the tongue and in saliva. It is described as causing feelings of sharpness, dryness, and friction.
Since the early 1900s, it has been commonly believed that the threshold for detection of the different basic tastes varies across the tongue and that the experience of each of the tastes is exclusively localized to a distinct area on it. This concept, which turns out to have been mistaken, is derived from subjective impressions that we taste sweetness at the tip of the tongue, saltiness at the sides toward the front, sourness also at the sides but further back, and bitterness at the root of the tongue. Seemingly, there is an area in the middle of the tongue where we feel that there is a decreased sense of taste.
More recent scientific research has shown that this so-called taste map is incorrect. The different regions of the tongue are sensitive to all the basic tastes, although they may perceive them to varying degrees.
Controlled experiments to determine precisely which areas of the tongue are most sensitive to umami have identified the part around its root as the area of greatest sensitivity. Nevertheless, when research subjects are asked where they taste umami, they generally answer that they taste it everywhere on the tongue. This indicates that the subjective taste sensation of umami is not always in accord with the physical distribution of the specific receptors for different tastes. In all likelihood, this explains why umami is often perceived as a wall-to-wall taste experience that completely fills the mouth with delicious sensations.
Sour
Sweet
Salty
Bitter
Umami
The taste map. Schematic illustration of the areas on the tongue, indicating the location of the greatest number of taste buds and taste receptors. The five basic tastes are all detected in each of the areas.
This particular way in which we experience umami may be one of the reasons why people in the West have been so slow to accept it as a true basic taste. Some chefs think that Westerners have a sort of serial experience of taste, in which the different taste sensations and nuances are perceived by way of contrasts and complementarities in a linear, stream-like fashion, whereas Asians take them in all at once and process them in parallel. As a result, umami can possibly be regarded as a parallel or complete taste.
Taste and, to a much greater extent, the sensation of taste, both of which relate directly to palatability, have a subjective and psychological component that puts all of our senses into play. Palatability is central to our choice of food, as well as to how it is processed and digested in our body. Our experience of palatability typically is a combination of many factors. The brain carries out the final assessment of these and tells us whether or not a particular food tastes good.
How these many complex impressions combine and affect one another is of special importance for our understanding of the relationship between palatability and umami. Knowing something about these interactions will help us to understand the nature of umami, and, in addition, enable us to work out distinct ways of enhancing this taste in our own cooking.
A particular aspect of what makes umami delicious is aftertaste. Umami develops over a different time frame than do saltiness and sourness, which disappear quite quickly. Experiments have shown that the intensity of those substances in the food that bring out umami actually increases for a short period of time after the research subject has spit out or swallowed the food. Umami persists for longer than all the other basic tastes.
This lingering aftertaste is probably one of the reasons why we associate umami with deliciousness and something pleasant. It is a taste sensation with fullness and roundness that completely permeates the oral cavity and then dissipates very slowly.
Air-dried hams are rich in glutamate, which brings out an abundance of umami.
It is probably easiest to describe what umami is by talking about its absence, which leaves us with food that we characterize as boring, flat, and uninteresting. In this book, we will describe how the richest and most delicious umami tastes arise when certain substances are present in particular combinations. We have only to take the trouble to develop expertise in combining different ingredients and handling them in the right way to be able to tease out their inherent taste substances.
Proteins and amino acids have a special role to play in this book because they are the main sources of umami. Proteins are composed of amino acids, which are small molecules that can bind chemically to each other with what are known as peptide bonds to form long chains of molecules. Some protein molecules are extremely long, made up of as many as a thousand amino acids. An example of this type of protein is wheat gluten, which, as its name implies, contains a great deal of the amino acid glutamic acid.
Our food contains twenty different naturally occurring amino acids. Nine of them are called essential amino acids because our bodies cannot synthesize them and, therefore, must derive them from what we ingest. Glutamic acid, the source of umami, is not one of these, and our bodies can produce it, even in great quantities.
Amino acids are chiral molecules, meaning that they are found in two versions that are chemically identical but are mirror images of each other, like the right and left hands. One of the properties that distinguishes them from each other is how they rotate a plane of polarized light that is passed through them. Those that rotate it counterclockwise are called levorotatory or left turning (L-amino acids), and those that rotate it clockwise are known as dextrorotatory or right turning (Damino acids). The direction in which the amino acid turns can lead to differences in taste.
Amino acids can form salts with, for example, sodium, potassium, magnesium, calcium, or ammonium. We have already come across the sodium salt of glutamic acid, which is monosodium glutamate (MSG).
Proteins are important in a nutritional context, because they provide some of the building blocks and energy necessary for cellular function. On their own, large protein molecules are rather insipid, whereas they can make a major contribution to how food tastes when they are broken down into small peptides or free amino acids. Knowing how best to break proteins down to free amino acids, usually by cooking, fermenting, curing, drying, marinating, or smoking, is an essential aspect of the culinary arts. Many free amino acids taste bitter, and many are predominantly sweet. (See the tables at the back of the book.) Some that are sweet actually taste bitter in large quantities.
Two amino acids are sources of umami taste: primarily glutamic acid in the form of glutamate and, to a considerably lesser extent, aspartic acid in the form of aspartate. For example, monosodium aspartate (MSA) imparts umami, but the effect is only 8 percent of what can be achieved with glutamate. Only a small portion of the glutamic acid in the protein content of fresh food is found in the form of free amino acids. Furthermore, only the free glutamate ions and aspartate ions, rather than the amino acids themselves, result in umami.
In this book, we will use these three terms in connection with descriptions of umami. Which one we use will depend on the context. Normally, we will discuss the amino acid glutamic acid in connection with its presence in proteins, where it is bound to many other amino acids. In this bound form glutamic acid has no taste. By appropriate processes, it can be liberated from the proteins and act as a free amino acid when dissolved in water. As long as the glutamic acid is in the form of an acid (for example, in a sour solution), it does not give rise to umami. On the other hand, if it forms a salt by combining with another compound (for example, sodium), the glutamic acid takes on the form of glutamate, in this case monosodium glutamate (MSG). In solution, this salt separates into sodium ions and glutamate ions. The glutamate ion stimulates the glutamate receptor and produces the umami taste.
So it is not the actual MSG that results in the umami taste, but only the glutamate ion. For the sake of convenience, we will also refer to the glutamate ion as glutamate, depending on the context in which the glutamate imparts umami. The word glutamate will therefore be used to describe free glutamic acid that has formed a glutamate ion. In this sense, all the glutamate that will be discussed is really free glutamate, which can be perceived by the glutamate receptor.
Nucleotides are molecular groups that can bind together in long chains (polynucleotides) and form nucleic acids, such as RNA or DNA, which are the foundations of our genome. With regard to umami, it is particularly the 5’-ribonucleotides derived from inosinic acid, guanylic acid, and adenylic acid—namely, inosinate (IMP), guanylate (GMP), and adenylate (AMP)—that are important, as they interact synergistically with glutamate to increase umami.
ATP (adenosine-5’-triphosphate), which is the primary biochemical energy source in living cells, is another important polynucleotide. When it is broken down it can form, among other substances, the three 5’-ribonucleotides mentioned above that are linked to the umami taste.
In contrast to proteins, nucleic acids are not in and of themselves nutritionally important, but the free nucleotides formed as by-products of their breakdown can act to increase umami. Furthermore, recent research has, surprisingly, shown that even though our body can synthesize the nucleotides that it needs, the free nucleotide content found in our food intake seemingly plays an important role in building up the immune system, especially in the intestines of newborn babies. This possibly explains why human breast milk contains so many free nucleotides.
A particular type of proteins called enzymes can break other proteins or nucleic acids down into their constituent parts; that is to say, either into free amino acids or free nucleotides. This is where taste comes into the picture, because we can taste them in this form, even though we cannot taste either the proteins or the nucleic acids from which they are derived.
Because glutamic acid is such a vital building block in proteins, it is found in large quantities in many of our foodstuffs, in either bound or free form. It makes up 10–20 percent by weight of animal proteins and as much as 40 percent by weight of plant proteins.
In the animal kingdom, glutamic acid is found in meat, poultry, and fish, while in the plant kingdom, it is abundant in vegetables but occurs only in small quantities in fruits. Vegetables are characterized by a relatively large content of free glutamate; for example, in tomatoes, corn, potatoes, and peas. (See the tables at the back of the book.) From the third major kingdom, the algae, which are not yet eaten very widely in Western countries, we obtain large quantities of free glutamate from, among others, large brown marine algae (seaweeds) such as konbu (Saccharina japonica), which is used to make the Japanese soup stock dashi.
On average, persons living in the Western world ingest about 30 milligrams per kilogram of body weight of free glutamate from their regular daily food intake. This corresponds to about 2 grams daily for an adult. One can also factor in an additional 0.3–1 gram daily sourced from additives. In many countries in the East, such as Korea and Japan, the daily intake of glutamate from additives is up to three times as great.
It is important to be aware that a given foodstuff can have a relatively low free glutamate content compared to another food, but at the same time have a relatively high content of bound glutamic acid, or vice versa. For example, cow’s milk has very little free glutamate but a quite large amount of bound glutamic acid. Consequently, fresh cow’s milk does not have much umami, whereas fermented milk products, such as aged cheeses, are good sources because glutamate was released in the course of the fermentation process.
It is very difficult to carry out objective, quantitative measurements of taste perceptions, and the results of experiments depend to a great extent on the methods that are employed to do so. In this connection, it is important to understand that both the taste threshold and taste intensity come into play. Taste threshold is an expression for the minimum quantity of a substance that is needed in order for us to perceive its taste. Determining a parameter for taste intensity is more problematic, as it is highly subjective.
Experiments have shown that the lower limit for tasting MSG in pure water is 0.01–0.03 percent by weight. (See the tables at the back of the book.) As mentioned, however, this threshold is very dependent on the method used to measure it. The equivalent threshold for table salt in pure water is about twice as high. As we will see later, the taste threshold for umami can be hundreds of times lower if other substances, such as inosinate, that enhance this taste are also present. The taste intensity of glutamate increases logarithmically with the concentration, but it has a tendency to saturate. It should be noted, however, that umami in foodstuffs is normally a mild and subtle taste, not nearly as intense as that which we associate with sweet and sour ones found in honey and lemons, respectively.
In a typical soup, there needs to be about 10 grams of salt per liter for it to taste sufficiently salty, and a reasonably narrow range of 8–12 grams per liter determines whether the soup comes across as insipid or too salty. In the case of MSG, a relatively broader range of 1–5 grams per liter ensures that it tastes good. The optimal salt content in a dish will decrease when MSG is also present, just as nucleotides depress the threshold for the optimal MSG concentration. (See the tables at the back of the book.)
What is probably surprising is that the taste of pure MSG is neither particularly pleasant nor interesting. In fact, it is rather bland and somewhat soapy. Its taste is perceived as delicious only when it is eaten in combination with a variety of foods. Here we are getting closer to what umami is all about—it is not the taste of pure glutamate. It is a much broader concept.
ἢ τῷ μᾶλλον καì ἧττον ἕκαστοί εἰσιν, εἴτε…ὁ μὲν ο
ν λιπαρòς τοȗ γλυκέος ἐστì χυμός, τò 
στηρὸς καì στρυφνὸζ καì ὀξὺζ ἀνὰ μέσον