Sensation and The Senses
SENSATION AND THE SENSES
SENSATION AND THE SENSES. Striving for variety and intensity in sensory experiences may be a fundamental human characteristic, likely reflecting our evolutionary history as both foragers of foods and omnivores. The enduring popularity of high-speed activities such as roller coaster rides or extreme sports, as well as films or computer games with impressive visual and sound effects, points to sensation seeking as still being an important aspect of many people's lives. Yet, in terms of the range of sensations they evoke, these experiences might be considered trivial by comparison with those delivered every day by foods and beverages. Particularly in the chemical senses (the collective term for smell, taste, and pungency), food consumption provides a continuous stream of sensory information to be processed and evaluated. Far from being mundane, even eating an apple provides an enormously varied set of sensations that begins even prior to the first bite.
Appearances and Expectations
When it comes to assigning priority to sensory information, humans are visual animals. Given uncertainty or conflict in the sensory information we receive, we tend to rely most on what we see. This is the basis of ventriloquism illusions. It is perhaps surprising, though, that a visual bias is so important with foods, where we might think that odors and tastes should predominate. Nevertheless, initial judgments of foods and beverages very often rely on appearances. In fact, our ability to identify even common flavors is typically very poor, so vision may sometimes be a more reliable source of information. Thus, foods that are miscolored—for example, a lime drink colored red—are often identified on the basis of the color rather than the flavor.
As well as being appreciated in their own right, visual aspects of foods provide important clues as to other sensory qualities, creating expectations about what we are soon to consume. We learn to associate the appearance of a food with its other sensory qualities, and the impact of these associations is perhaps most obvious in the effects of food colors. Whether an apple is red or green, for example, will lead us to expect a certain degree of ripeness and, often, quite specific levels of sweetness and acidity. Surface textures and color brightness can also provide clues to internal textural properties such as hardness. Because of such associations, colors can also influence what we perceive. Adding red coloring to a sweet solution increases the perceived sweetness, while the addition of any color to a solution containing an odor increases how intense we perceive that odor to be.
Expectations about a food's sensory properties can also be created in advance of consumption by information about a food, for example, on labels. These have the ability to influence a product's acceptability. Serious mismatches between what we expect a product to taste like and its actual sensory properties can be an important determinant of whether the food is ever consumed again. Like color, information can also be powerful enough to influence perceptions. For instance, labeling a product as high in fat has been shown to lead to higher ratings of "fattiness" and lower ratings of flavor intensity, relative to the same product with a low-fat label. The mechanism appears to be that, when mismatches between prior expectations and the reality of the product are relatively minor, "assimilation" takes place. That is, perceptions and preferences can be "brought into line" with product claims. These effects are an excellent reminder that, ultimately, perception is a cognitive process that receives information both from the sensory impact of food ingredients and from other sources of information about the food.
While the odor of a food can also generate expectations for other sensory properties, its most valuable role is to help us to identify foods. Olfaction provides us with more information about what we consume than perhaps any other sense. Most important, whether as part of a flavor in the mouth or as an aroma that we sniff, odors reliably inform us whether we have experienced a food before. To a great extent, this assists us in making a decision to consume or not.
Olfaction has been called a dual sense. Both at a distance from the food, and when the food is in the mouth, our sense of smell acts as a detector of volatile (i.e., gaseous) chemical compounds. The mixture of compounds that we perceive as apple odor reach the olfactory receptors in the patch of tissue known as the olfactory epithelium, a mucus membrane at the top and back of the complex maze of passages found within the nose. The physiological processes that such odors initiate in the receptors are not completely understood. We do know, however, that the small odor molecules bind to receptor proteins present on the cilia, hair-like extensions of olfactory nerve cells that protrude from the epithelium. This binding initiates complex sequences of biochemical changes leading to depolarization, and firing, of the olfactory cell. This electrical signal travels via the olfactory nerve to the olfactory bulb in the brain.
How we recognize such neural signals as specific odors is perhaps the crucial question in the science of olfaction, particularly if we consider just how many odorants humans are capable of detecting. In stark contrast to the relatively small number of different taste qualities (see below), we can certainly perceive thousands, if not tens of thousands, of distinct odor qualities. Moreover, there does not seem to be any underlying organizing principle that we can use to classify these qualities. Historically, various schemes have been proposed for categoriz ing odors. Linnaeus, for example, classified odors as "aromatic," "fragrant," "musky," "garlicky," "goaty," "re pulsive," and "nauseating." This and many other classification systems suffer from the use of categories that are either too general ("fragrant") or clearly related to the acceptability of the odor, rather than its sensory properties ("repulsive," "nauseating").
In practice, odors are often classified into categories that make sense to their users, whether they are winemakers, flavorists, or perfumers, as well as having practical applications. While they often have an internal logic and consistency—fruit odors may be grouped together, for instance—such classifications relate primarily to the object producing the odor, to a lesser extent to the odor chemistry, and not at all to how the brain codes the odor information. The molecular properties of the odorant must be responsible a priori for odor quality, but any laws that could allow us to reliably predict a particular quality from those properties are not as yet apparent. Instances of dissociations between structure and quality make this a challenging task. The compounds D-carvone and L-carvone have the same structure, differing only in that they are optical isomers (effectively, molecular mirror images) of one another, yet one smells of spearmint and the other of caraway.
A milestone in our understanding of the mechanism of odor perception came in 1991, when Linda Buck and Richard Axel of Columbia University identified a family of genes that encode olfactory receptor proteins. It is thought that this family, now believed to include more than 1,000 genes or around 3 percent of our total genome, is able to generate an equally large number of receptors. It is unlikely, though, that we have a unique receptor for each odorant since this would seem to be an inefficient way of coding. The most promising hypothesis regarding olfactory coding suggests that patterns of activity across different receptors, perhaps each expressed by a different member of the gene family, form the basis of odor qualities. Olfactory coding is thought to be also partly mediated by organization of the olfactory epithelium into four different zones, each zone containing receptors expressed by different genes. A given odor will likely activate receptors in more than one zone, creating a spatial, as well as a receptor-specific, code.
Any complete theory of odor perception must explain how what we perceive as apple odor is the product not just of a single apple-like compound, but of the mixture of the compounds 2-methylbutyl acetate, butanol, and hexyl acetate (amongst many others), none of which smells uniquely of apple. The odors of the majority of the foods and beverages that we consume consist of mixtures of the odors of many separate compounds. For example, hundreds of different compounds, each with their own distinct odor, combine to make coffee and chocolate odors. What we perceive, though, is a single unique quality. A major question in our attempts to understand how the brain processes odors is how this information is combined. There does seem to be some sort of limit to the number of individual odors that we can combine and still detect. Beyond a mixture of three or four different odor qualities, we are unable to say which of a set of individual odors the mixture contains, even if we are very familiar with those components. At the same time, however (and seemingly paradoxically), complex odors can contain "notes" in addition to having an overall quality, although these notes cannot be related to the odors of the specific chemical compounds in the mixture. One of the challenges for food scientists in industry is to be able to identify which of the multitude of chemical compounds within a food are essential for producing its characteristic odor and flavor.
Flavor: Sensory Qualities in the Mouth
It is only after we have taken a bite that the characteristic apple flavor, consisting of odors and tastes, emerges. After entering the mouth, the same odor compounds that we detected previously, now released and concentrated by the combined actions of heating and chewing, reach the olfactory receptors via the nasopharyngeal passage at the back of the mouth, a process known as retronasal perception. The reason we commonly refer to characteristic food qualities as tastes—apple taste, coffee taste, and so on—is that we are not conscious of this alternate route for the sense of smell. In fact, these "tastes" are mainly odors.
Odor and taste perceptions are so well integrated in flavors that there are seldom any obvious signs as to where one sense ends and the other begins. This gives rise to the illusion that retronasal olfactory qualities are perceived in the mouth. Our language both reflects and encourages this confusion, in that we use the terms "taste" and "flavor" interchangeably. Simply holding the nose while a food or drink is in the mouth is sufficient, however, to demonstrate just how large a contribution the sense of smell makes to flavor. The complaint of loss of taste during a head cold is also a consequence of this misunderstanding. In fact, taste is largely unaffected, and it is the sense of smell that suffers.
At the same time as the odor volatiles in our apple are released during chewing, acids and sugars stimulate taste receptors in the mouth, producing perceptions of sourness and sweetness. For a sense that is so crucial to both our survival and our enjoyment of life, it is remarkable that taste is so poorly appreciated. Perhaps this is because, after subtracting odor qualities (and other sensations such as pungency and various aspects of "mouthfeel"—see below) from the overall flavor of a food or beverage, we are left with a rather small group of sensory qualities—traditionally, sweetness, saltiness, sourness, and bitterness. This limited set of qualities is clearly inadequate to describe much of the sensory complexity of any cuisine. Compared to the rich, perhaps limitless, inventory of odors that contribute to the flavors in even an average diet, the sense of taste initially seems remarkably unimpressive. Yet, in forming the essence of any flavor, taste supplies information that is crucial to our survival and well-being.
Taste is usually considered to be an analytic sense, composed of a key set of unique, indivisible qualities. In contrast to the synthetic sense of smell in which combinations of odors can produce a new quality entirely distinct from the components, tastes do not combine to form new qualities. Combining salt, sugar, and lemon juice may result in changes in the intensity of the tastes involved (different tastes typically suppress one another in mixtures), but it will only produce a mixture with the qualities of saltiness, sweetness, and sourness. As a result, these taste qualities, together with bitterness, are commonly talked about in terms of a set of basic tastes. It should be noted that this classification system is not without controversy but is adopted here, as it is the premise of most scientific literature on taste.
To be perceived as a taste, a chemical compound or food ingredient has to be soluble (for example, in water or saliva) in order to reach the taste receptors. These are located within 3,000 or so taste buds, which are themselves located within structures on the tongue called papillae (although there are also small numbers of taste buds in other parts of the mouth). The most numerous of these structures, the fungiform papillae, are apparent as tiny bumps on the tongue's front upper surface. The circumvallate papillae, larger structures arranged in a chevron pattern, are located further back on the tongue, while the foliate papillae occupy the tongue's sides. Contrary to popular belief, taste buds are not specialized according to tongue location—we are capable of perceiving all tastes at any tongue location where taste buds are present (although our sensitivity to detecting different tastes does vary somewhat across different locations). The commonly seen tongue map, showing salty and sweet perception at the front of the tongue, sour at the side, and bitter at the back, results from a prolonged misinterpretation of the outcomes of studies published in 1901 by the German psychologist D. P. Hanig. In fact, lack of anatomical specialization for the different qualities is a characteristic of the taste system.
How a dissolved chemical compound becomes converted into a perception of, for example, sweetness or sourness, is increasingly understood. A large number of taste cells occupy every taste bud, extending their finger-like microvilli towards the pore through which the tastant compound will enter. For each taste quality, the microvilli of a cell contain different receptor mechanisms, which are responsive to the chemical structure of the tastant. Sodium and some other ions (potassium, calcium, and lithium) interact directly with channels on the membrane of the microvilli, entering the cell and producing biochemical changes that result in a nerve response interpreted by the brain as saltiness; the hydrogen ion (H+) in acids is similarly responsible for sourness. Bitterness and sweetness are thought to bind to specific receptor proteins on the surface of the microvillar membrane, and it is this binding that initiates the biochemical changes within the cell.
Unlike sourness and saltiness, sweetness and bitterness can be produced by substances belonging to a wide variety of chemical classes: not only sugars, but some proteins and amino acids are sweet. Other amino acids are bitter, as are alkaloids and some salts. This diversity appears to be reflected in multiple receptors for each of these qualities, although whether we perceive different types of sweetness and bitterness is currently under debate. Once the neural signals reach the brainstem, carried by the VIIth (chorda tympani), IXth (glossopharyngeal) and, to a lesser extent, the Xth (vagus) cranial nerves, there is still no direct relationship between quality and anatomy. Taste cells in the brain respond best to some qualities ("salt best" or "sweet best," for example) but will in general respond somewhat to each of the basic qualities. This has led to the view that taste is coded (identified) in the brain through a pattern of responses across many taste cells.
In addition to the four well-known qualities, there is now considerable evidence for the existence of another basic taste, known as umami (pronounced oo-ma-me). This Japanese word, translated approximately as "savory deliciousness," refers to the quality of foods containing significant amounts of naturally occurring glutamate (a derivative of glutamic acid, an amino acid), its sodium salt, monosodium glutamate (MSG), or 5′-ribonucleotides. Umami quality is perhaps most evident in the taste associated with rich sauce or soup bases made from stocks, mushrooms, or tomatoes. Adding Parmesan cheese to pasta provides another means of increasing the umami quality of the dish, as do many of the manufactured sauces throughout the world, for example, soy sauce. The status of umami as a unique taste derives not just from this quality being perceived as distinct from the other four basic tastes, but also from evidence for the existence of both glutamate receptors within the tongue's taste cells, and cells within the brain which respond preferentially to the umami taste. Very recent evidence has also pointed to the existence of taste receptors that respond broadly to many of the other amino acids (twenty in total) that make up proteins.
The Hedonic Properties of Tastes
A limited set of basic tastes suggests that each of the qualities must be significant in some way. Why have we evolved to be sensitive to these qualities specifically? The answer lies in our hedonic responses to tastes. If our imprecise use of the word "taste" reflects our confusion about the different qualities that make up flavors, then perhaps in compensation, our language also provides clues as to the role that taste plays. In addition to describing food qualities, we also talk about tastes as a way of indicating good or bad aesthetic judgment, and we say that someone is bitter, or sweet, or has a sour face. Our use of taste qualities to describe such positive or negative emotions or qualities unwittingly reflects the underlying structure of the taste experience itself. In providing a hedonic basis to food and beverage flavors, taste functions as a built-in arbiter of what is good and bad in those things that we consume. As Brillat-Savarin noted in his gastronomical meditations (The Physiology of Taste, 1825), taste can be reduced "in the last analysis, in the two expressions, agreeable or disagreeable."
In contrast to our preferences for odors, which are molded by exposure and reinforcement from an early age, hedonic responses to pure tastes are remarkably fixed. Distinct hedonic responses to sweetness and bitterness are present at birth, and are essentially the same as those we experience as adults. Both in terms of amounts ingested, and also in terms of their ability to elicit characteristic facial expressions, sweetness is highly liked and bitterness rejected in human neonates. Sourness also seems to be disliked. A preference for saltiness, on the other hand, develops in the first few post-natal months. While MSG in solution is not well accepted by neonates (or, indeed, adults), soups to which it is added are preferred. These hedonic responses to pure tastes also seem to be relatively independent of culture or diet. Comparisons across cultures whose diets are very different, for example, Japan, Taiwan, and Australia, have found highly similar patterns of likes and dislikes for pure tastes.
The significance of taste hedonics. Whether as a genetic predisposition, or as a result of in utero influences, the origin of relatively fixed hedonic responses to pure tastes appears to lie in an adaptive capacity to respond appropriately to the nutritional implications of these qualities. Taste palatability, and ultimately the palatability of foods, seems to reflect either provision of energy, an individual nutritional need, or a warning of the presence of a potential toxin.
Sweetness is thought to signal the presence of energy in the form of calories provided by sugars and other carbohydrates, which are crucial to survival. Sweetness is thus a positive quality, reflected in its universally high palatability. This palatability is mediated by opioid (morphine-like) biochemical receptor systems in the brain, which are thought to be the biochemical basis for reward. This explains why sweetness can sometimes act like an analgesic, reducing crying in infants, for example.
Saltiness acts as a survival cue, by signaling the presence of the sodium ion (Na), necessary for maintaining the body's fluid balance. A liking for salt, while present at all times, grows substantially if we are deprived of it below what is physiologically necessary. Although there are many claims that cravings for various foods and nutrients exist, that for salt is the only one that is well-documented in humans.
The strong dislike that we naturally have to bitterness is thought to be a protective mechanism. Many plants manufacture toxins as a defense against predators, and very many of these toxins are bitter. Not surprisingly, then, we tend to be extremely sensitive to bitterness. However, as a result, we often reject levels of bitterness that are not in fact toxic to humans—witness the common fate of the poor brussels sprout. The significance of our dislike for high levels of sourness is not as clear-cut. It may be a signal for unripeness/spoilage in foods, or the fact that concentrated, and thus extremely sour, acids can cause tissue damage.
Because glutamic acid is an amino acid present in proteins that we consume, it has been suggested that the umami taste of glutamate acts as a signal for the presence of protein, thus promoting consumption. However, preference for umami actually seems to be strongest when protein intake is within normal limits. Alternatively, since dietary glutamate is involved in crucial metabolic processes and may possibly be used as an energy source within the gut, it may be that our preference for additional glutamate in foods reflects the importance of these functions.
These seemingly distinct adaptive processes reflect an underlying principle on which the hedonic properties of tastes are based. Animal studies have suggested that the palatability of any taste compound, and the responsiveness of taste cells in the brainstem, is strongly related to its toxicity. At one end of the spectrum are highly toxic compounds, which are rejected as unpalatable by both humans and many animals primarily due to bitterness; at the other end are highly nutritive substances with low toxicity that are well accepted, mainly because they are sweet. This neural and behavioral organization has led to the hypothesis that preferences for tastes are the method by which our bodies maintain their own physiological well-being. This makes considerable sense if our gustatory system is viewed as being at the interface between the external and internal environments. In this regard, we can view taste as being a "gatekeeper" whose function is to ensure that ingested substances maximize our survival. At least in this functional way, it is appropriate to talk about tastes as a continuum and to view sweetness and bitterness as opposites.
Consistent with the "gatekeeper" idea, regions of the brain responsible for processing information about tastes also receive neural information from the gut, and there is ample evidence of mutual interactions between taste perceptions and internal metabolic processes. Thus, metabolic states can modulate the palatability of tastes. The craving for salt when deprived of it has already been mentioned—but how does the body let us know it wants more salt? Studies in rodents have shown that "salt-best" taste cells in the brain actually decrease their responsiveness to salt following salt deprivation. Interestingly, however, this is accompanied by increases in the responsiveness of "sweet-best" cells to salt. This suggests that salt has become more pleasant, which would act to promote salt consumption to restore normal salt levels. In humans, too, the preferred level of salt in foods increases following salt depletion, and the rated desirability of salty foods goes up. This effect has also been shown for amino acids in cases of malnourished children and the elderly, in studies in which the addition of an otherwise unpalatable amino acid mixture increased the consumption of a soup. Similarly, animals fed a diet deficient in just one essential amino acid such as lysine will recognize its presence in foods, consuming them preferentially. Just the reverse seems to happen as a response to repletion. Sweetness becomes less pleasant following consumption of glucose, a phenomenon that is accompanied by decreased activity in "sweet-best" taste cells in the brain. Conversely, tastes are also able to influence metabolic processes, even prior to the nutrients' absorption by the gut. These so-called cephalic phase responses include increases in salivation and secretion of gastric enzymes and insulin, changes that prepare one for receiving the nutrients and energy provided by foods.
Chemesthesis: The Perception of Pungency
If the acidity in our apple is high enough, we might also perceive a degree of "bite" or "sharpness" due to activation of the free nerve endings of the Vth (trigeminal) cranial nerve. This nerve, which sends branches into the eyes, the nose, the mouth, and especially the tongue, transmits information regarding a wide range of tactile sensations, plus warmth, cooling, and even pain (think very hot curry). These sensations, often called pungency in the context of foods, are important to our appreciation of flavor in many foods and beverages. A cola drink without the fizz; the glass of wine without its sharpness; and onion, mustard, and horseradish without their bite or ability to induce tears have lost much of their defining quality. Whenever we talk about tactile sensations—stinging, biting, burning, numbing, tingling, or cooling—we are referring to pungent or chemesthetic qualities.
Trigeminally mediated sensations also very often contribute to the odors that we perceive in our environment. The eye-opening qualities of ammonia and many solvents derive from their ability to stimulate the trigeminal nerve as well as olfactory receptors. Not all such sensations are unpleasant. The cooling effects of peppermint, producing the pleasant illusion that our nasal passages have suddenly opened, are similarly mediated.
The trigeminal nerve also transmits information about temperature. It is not surprising then that chemically mediated chemesthetic qualities are modified by heating and cooling. The most obvious example of this is the ability of cool water to instantly eliminate chili-induced burning; conversely, this same burning is greater if the food is also (temperature) hot.
While pungent qualities are important sensory components of foods throughout the world, pungency is often associated primarily with cultures such as those of Korea, Vietnam, Thailand, and Mexico, whose cuisines use a lot of chili. These cuisines provide much greater flavor impact than typical Western diets, and their recent popularity in Western countries may reflect not just increased availability, but a striving for new, intense culinary sensations. Even so, many people have reservations about hot (spicy) food. This is not surprising given that pain, oral or otherwise, is a clear signal to warn that damage has occurred or is imminent. The main heat-producing compound in chilies is capsaicin, a powerful irritant. However, despite common anxieties, there is no evidence that it damages otherwise healthy stomach linings or kills taste buds.
Even regular hot-food eaters complain, though, that if a food is too hot, appreciation of other aspects of flavor are spoiled or overwhelmed. On the face of it, it seems evident that such a strong sensation should overwhelm a weaker one. We are used to suppression of flavor and taste qualities by other flavors and tastes—for example, reducing the sourness of lemon juice by adding sweetness. Yet, the research evidence for this occurring with "heat" is fairly weak. When capsaicin (even at levels equivalent to a very spicy meal) is combined with tastes or odors in mixtures, only sweetness is reliably reduced.
The failure to find stronger effects of burning sensations is certainly contrary to popular belief and, perhaps, experience. Wine commentators, grappling with the question of what to drink with spicy food, commonly invoke the idea that hot foods overwhelm "subtle" wine flavors. However, it may be that burning sensations are simply a more prominent or memorable sensory experience since they persist long after the tastes and flavors have disappeared. Evaluating the intensity of something, we typically make comparisons ("sweet compared to what?"). Especially if the level of pungency is higher than an individual finds pleasant, this phenomenon might simply reflect the fact that the burn is intense and the flavor is not a "real" reduction of flavor intensity.
Texture and Mouthfeel
Problems with texture are a common reason for rejecting foods. It is unlikely that we will finish an apple if it is either mushy or rock-hard. Our perception of many texture qualities relies on information from mechanoreceptors in the tongue, gums, and palate (also part of the trigeminal nerve) that detect the shape of food particles, together with pressure sensors in the jaw and gums. These sensors give feedback on how much force to exert in chewing, information that forms the basis for hardness perception. Compared to some other senses, hearing is perhaps of lesser importance in food appreciation. Nevertheless, the sound of the crunch when we bite into an apple allows us to fully appreciate its freshness and ripeness, and forms an integral part of our perception of its texture.
Sometimes in the past considered a taste, the property of astringency is now generally accepted as a set of mouthfeel sensations. Characteristic of foods and beverages containing tannins, including some fruits, nuts, tea, and cranberry juice, astringency consists of sensations of drying, puckering, and roughness felt on the mucous lining of the mouth. These sensations result when the tannins cause the lubricating proteins in saliva to precipitate out. Although it sounds largely unpleasant, astringency is a good example of how responses to sensory qualities are often highly dependent on context. While one of the reasons that we might not eat a green banana is that it tends to leave the mouth feeling like a sandpit, drinkers of red wine value these sensations, at least to some extent. When a wine is described as "dry," the sensation is due to a significant extent to astringency.
Other textural sensations are produced by the water (for example, juiciness) or fat content of foods. Fat, in particular, is a key component of many of those foods that are highly liked, including red meat, desserts, cakes, chocolate, and dairy foods. Fat not only produces a sought-after texture, it is also an important carrier for flavor. Fat-reduced foods have often failed to be accepted because they were generally low in both flavor and textural properties such as creaminess. In addition to the desirable properties that fats create, liking for fat may actually be innate, because of its ability to provide energy (in a similar way to sweetness). This possibility is supported by recent evidence to suggest that fatty acids may have their own receptor—in other words, our bodies adapt to be able to detect fat. Given these considerations, it is not surprising that many people report that it is the sensory properties of low-fat foods that are responsible for poor compliance to dietary advice.
Sensory Integration: What Are We Really Perceiving?
Despite the contribution from all of these senses, what we ultimately perceive—as opposed to the sensations we've experienced—is an apple. In his seminal The Senses Considered as Perceptual Systems (1966), the psychologist J. J. Gibson proposed that the purpose of perception was to seek out objects in our environment, particularly those objects that are biologically important. Nowhere is this more evident than in our perception of food qualities. Although we know implicitly that, while eating, a variety of signals are impinging on our gustatory, olfactory, tactile, visual, and auditory systems, this type of analysis does not come naturally. Thankfully, what we perceive when we sit down to dinner are "objects"—a steak and a glass of red wine—rather than a collection of distinct sensory signals. Moreover, likes and dislikes naturally spring from this synthetic mode of perception since we are responding to objects that we have learned to recognize as foods and that are therefore important to survival. Initial, "gut" responses to foods are almost always hedonic, and this naturally precedes accepting or rejecting the food.
The perception of food qualities reflects the integration of information from multiple sources. This is seen in the convergence of inputs from different sensory modalities in the brain. Edmund Rolls of Oxford University has described convergence of taste and odor information in what may be the physiological basis of flavor perception. He identified neurons in the olfactory area of the monkey cortex that responded specifically to qualities that occur together in flavors, for example, the sweetness of glucose and fruit odors. However, these neurons did not respond to incongruous combinations, such as saltiness and these same odors. Such neurons may actually start off responding only to odors and learn to respond to sensory combinations during repeated pairing of particular tastes and odors when they occur together as a flavor. Multimodal neurons in other sensory systems are thought to enhance the detection of, and reduce ambiguity associated with, external stimuli. In the case of odors and tastes, these neurons could help to resolve any ambiguity regarding the wisdom of consuming particular foods. Perceiving whole flavors, rather than distinct sensory signals, can be seen, therefore, as a survival strategy.
Interactions within Flavors
The brain's integration of food qualities makes it difficult to discuss the different sensory systems in isolation from one another since these systems tend to interact. The way in which sensory properties like color can influence odors and tastes was mentioned earlier. Because sensory properties are perceived as aspects of the same "object," we repeatedly associate the occurrence of one property with that of another.
In addition to setting up expectations for other sensory qualities to follow, learned associations between different qualities in foods can actually determine the qualities that we perceive. One of the most interesting examples is the ability of odors to rapidly form associations with other sensory qualities, especially tastes. In what appears to be the perceptual equivalent of the neurons described by Rolls, novel odors that are repeatedly experienced combined with a sour taste start to smell "sour"; those combined with sweetness start to smell "sweet." These effects are borne out in everyday experience. When asked to describe the odor of vanilla or caramel or raspberry, we will commonly use the word "sweet." This seems to be more than simply the fact that the odor recalls a food that was sweet since such odors can influence the intensity of that taste. Strawberry odor when placed together with tastes in solution can both enhance the sweetness of sucrose and reduce the sourness of citric acid, just as the addition of "real" sugar would.
Thus, our final perceptions receive input not just from a variety of sensory systems, but also from our memory of past associations. In practical terms, it means that our perception of a quality like sweetness within foods will often include a contribution from a sweet odor and a sweet taste. Such phenomena also help us to appreciate that the sensory properties that foods possess derive not just from perception of their chemical constituents, but also from complex cognitive processes. Understanding these processes—even for the humble apple—requires inputs from a variety of scientific disciplines, most notably psychology, food science, neurophysiology, and molecular biology.
See also Brillat-Savarin, Anthelme ; Eating: Anatomy and Physiology of Eating ; Wine .
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How Many Tastes?
The conventional wisdom in both the scientific community and among the general public is that taste is composed of a set of discrete qualities, almost like separate senses. Sweet, sour, salty, and bitter have formed the core of our understanding of taste since Aristotle. While other tastes have been proposed from time to time—astringent, pungent, fatty, insipid, alkaline, to name some—these four qualities have almost always been recognized as fundamental.
Outside of the laboratory, however, we seldom experience so-called pure tastes. Fruits are often simultaneously sweet, bitter, and sour, and savory dishes may be salty, sweet, sour, and high in umami. Similarly, many chemicals that produce tastes are not "pure" examples. Potassium chloride is both bitter and salty. To what extent do we perceive these mixtures as sets of basic qualities, and to what extent as unique tastes themselves? Are such compounds steps along a continuum of tastes (as orange exists along a continuum of colors from yellow to red), rather than having discrete and separate qualities? These questions challenge the orthodoxy of a set of discrete basic tastes.
The recent evidence that the taste of glutamate appears to constitute a fifth basic taste, umami, is somewhat embarrassing for those who strongly argued for four primary qualities. If five, then why not six, or seven? In fact, there is evidence to support the notion of a taste associated with fatty acids and of the quality metallic as an independent taste. A case can be made for the survival value of such tastes: fats are important sources of energy, while the salts of metals such as iron, copper, and zinc are both metabolically necessary and toxic at high levels. Arguments have also been mounted for recognition of the tastes of other amino acids and starch as basic qualities.
Beyond such challenges to the concept of four (five) basic tastes, it has been argued that the whole notion of basic tastes is flawed. Robert Erickson and Susan Schiffman, both of Duke University, have proposed that the paradigm of a limited number of basic tastes has impeded our understanding of this sensory system. In the early part of the twentieth century, Hans Henning proposed that the four basic tastes could be represented at the points of a tetrahedron. Did Henning mean to convey that there were no intervening qualities on the tetrahedron's surface or were the basic tastes merely distinct points on continua? This influential attempt to classify tastes has been interpreted both ways, but Erickson and Schiffman argue that the evidence favors the latter interpretation. They point out that there are no strong physiological reasons to accept only four basic tastes. There appear to be more than four transduction (receptor) mechanisms for taste; not only are there not four distinct types of taste neurons in the brain, but taste cells are broadly sensitive to many qualities.
What do we really perceive?
Erickson and Schiffman have suggested that the acceptance of the idea of a limited number of basic qualities prejudices our understanding of taste. In other words, if you accept that sweet, sour, salty, and bitter are the only tastes we can detect, then all taste sensations will be a priori classified into one of these categories. In one study, they asked subjects to indicate which of a variety of taste compounds, some of them mixtures of basic tastes, were singular tastes or mixtures of tastes. Many of the mixtures were viewed as a single taste, sometimes one that was distinct from the four basic tastes. Spatial mapping of the quality of a selection of tastants, based on ratings of the similarity of each pair (more similar tastants are mapped closer together, and so on), also did not conform to a pattern that could be represented by four basic qualities.
Similarly, Michael O'Mahony and colleagues at the University of California, Davis, asked subjects to sort a number of tastants into groups based on taste quality, but without using the traditional category names. Their subjects not only came up with more than four categories, but also used more than four self-generated labels to describe the different qualities. When asked to use only the four traditional qualities, these subjects were forced to label different groups using the same word, suggesting that restricting labels to the four terms was inadequate to describe the range of taste experiences.
Despite all these potential problems with the doctrine of basic tastes, there is widespread acceptance of sweet, sour, salty, bitter and, increasingly, umami as fundamental taste qualities, although perhaps not the only qualities. Certainly, for the foreseeable future, the existence of distinct, basic tastes will continue to be the key assumption behind research aimed at elucidating the fundamental mechanisms of taste perception.
Taste and Smell Disorders
The loss of our ability to smell or taste is perhaps not as immediately debilitating as the loss of the senses of vision or hearing, but such disorders can nevertheless have a profound impact on people's lives. The enjoyment of foods, beverages, perfumes, and nature, and our ability to avoid spoiled foods and environmental toxins, depend upon the proper functioning of these systems. Chemosensory disorders are also relatively common. As a conservative estimate, up to one percent of the population have some degree of smell or taste loss or disturbance. Given that alterations in the flavor of foods are the most immediate consequence of smell loss, it is not surprising that problems with "taste" are the most common chemosensory complaint.
In fact, complete loss of the ability to taste is rare mainly because, as a sensing apparatus, taste is highly redundant. Unilateral damage to the cranial nerves carrying taste information can eliminate sensitivity to taste on half of the tongue, but appears to produce few noticeable changes in our ability to appreciate tastes. Likewise, taste is able to survive even severe trauma to the tongue. Aguesia (total inability to taste), when it does occur through illness or accident, is reported to have devastating consequences in terms of food acceptability, food intake, and, consequently, physical and mental health. Dysguesias, or distortions of taste sensations, are, however, not uncommon. These, and partial taste losses (hypoguesias ), can result from disorders of oral health, dental procedures, and some commonly prescribed medications (including antibiotics, antidepressants, anti-hypertensives, psychiatric drugs, analgesics, and chemotherapy agents). Neurological disorders such as Alzheimer's disease, renal and liver disease, diabetes, and viral infections have all been reported to be associated with taste losses or distortions.
The list of diseases associated with partial (hyposmia ) or complete (anosmia ) smell loss, or smell distortion (dysosmia ) is even more extensive. Significant losses are reported in renal and liver disease, HIV infections, thyroid illness, epilepsy, Alzheimer's, and Parkinson's diseases. Most commonly, though, smell losses are associated with both acute and chronic diseases of the upper respiratory tract (including colds and influenza), nasal sinus disease, and allergies. Five percent of victims of head trauma also have some degree of smell loss. In cases of severe head trauma, smell loss is often complete and irreversible, resulting from a shearing of the olfactory nerve fibers at the point where they enter the brain through the base of the skull.
Olfactory functioning is also susceptible to environmental toxins, making smell loss an occupational health issue. A variety of compounds used in manufacturing and other work environments, including metal dusts such as those of cadmium and nickel, solvents (acetone), and irritant gases (formaldehyde) have been implicated as causes of smell loss, particularly with long-term exposure. Cigarette smoking is known to produce chronic decreases in olfactory sensitivity, although this tends to recover once smoking is given up.
Diseases that affect smell and taste tend to be more prevalent as we age, as does the taking of medications that can produce deficits. Beyond these causes, however, we can also look forward to a "natural decline" in our ability to detect and identify smell and taste qualities, and a lessening of their impact. Using a 40-item "scratch and sniff" odor identification test, Richard Doty and colleagues at the University of Pennsylvania Medical Center showed that in both men and women, odor identification is reasonably stable until we reach our eighth decade. From this point on, the decline is fairly pronounced, corresponding to some extent to declines in vision and hearing during these years. Odors, and consequently flavors, are also less intense as we age, and our ability to detect subtle changes in intensity is reduced.
The sense of taste tends to survive aging somewhat better than our ability to smell. The threshold level at which tastes are detected increases, and taste intensity decreases, although not substantially, and not equally for all taste qualities—bitterness perception is particularly diminished. The ability to distinguish between different concentrations of tastants is also affected. Such losses, while not dramatic, can still have significant consequences. The levels of sweetness and saltiness that are considered by elderly people to be optimum in foods are generally higher, and food that is not adjusted accordingly may be considered bland.
One notable consequence of both smell and taste losses due to aging is that eating enjoyment is reduced. Particularly in institutionalized or hospitalized elderly people, this may exacerbate problems of anorexia and poor nutrition.
Measuring the Sensory Qualities of Foods
To produce foods that meet consumer needs, food manufacturers need to know the relative contributions of the various sensory qualities—tastes, odors, and textures—to the flavor of foods. Arguably, until this is known, it is difficult if not impossible to understand the consumer's responses to the product. Such information can be used to guide product development and ensure a quality product by allowing measurement of the effects of different production methods, changes in ingredients, and storage.
The process of describing and measuring the sensory qualities of foods and beverages is known as descriptive analysis (DA). To perform DA, small panels of typically ten or twelve individuals receive extensive training, often over a period of many months. During this time, the panelists learn to be consistent in their use of specific labels to describe sensory qualities. Such intensive training is necessary because of our generally poor ability to identify odors and flavors. Even with common food flavors, correct identification can be as low as 50 percent. Despite being able to say that an odor or flavor is highly familiar, we are often at a loss to identify the correct name. This has been labeled the "tip of the nose" phenomenon. In addition, to describe texture qualities, an entire vocabulary must be learned and applied appropriately. Fortunately, our ability to attach names to sensory qualities improves with feedback and practice. Importantly, too, training allows "concept alignment"—essentially an agreement as to the meaning of sensory descriptors and what constitutes examples of the concept. For example, the panel might need to agree that the term "lemon odor" refers to the odor of fresh lemon juice but not that of lemonade.
Providing labels for sensory qualities actually improves our ability to "see" those qualities in the midst of a complex food or beverage. To a novice wine drinker, a glass of sauvignon blanc tastes like white wine; with experience, however, we learn that this wine variety often has odor "notes" reminiscent of asparagus or cut grass. Providing examples of these notes allows panelists to perceive these qualities within the wine. As a result, they are increasingly better able to detect that note each time they encounter this wine variety—in effect, panelists end up perceiving a collection of sensory qualities, whereas before they could only identify the taste as that of white wine.
Quantifying the intensity of those qualities that are identified is a key aspect of DA, allowing us to measure differences between products in a scientific manner. Training improves our ability to measure sensory qualities using rating scales. Measurements made with rating scales are always relative—they do not quantify an absolute quantity unlike, for example, measuring the concentration of a chemical compound. But they can nevertheless be used reliably with training. Moreover, there are no alternatives. No instrument yet devised can reflect the complexity of human perception.
When developing these skills, trained panelists become less and less like consumers of the product. In fact, the aim is to have them approach the product in an entirely analytical way, which means ignoring any likes and dislikes and responding as though they were an instrument.
Once a panel is trained for a specific food, they are able to produce a flavor profile for a selection, or sometimes all, of the product's sensory qualities. In effect, this becomes the "sensory recipe" for that product. While flavor profiles say nothing about whether or not a product is liked, knowing the flavor profile of foods that are highly preferred can provide valuable information to guide future product development and predict the effects of variations in the sensory qualities.
Taste Perception: Are We All the Same?
"Now, anatomy teaches that all tongues are not equally provided with these papillae, and that one tongue may possess three times as many as another . . . the empire of taste also has its blind and deaf subjects." —Brillat-Savarin, The Physiology of Taste (1825)
Black coffee and beer are not only amongst the most commonly consumed beverages in many countries, they are also amongst the most commonly rejected by first-time users—primarily due to their bitterness. Clearly, preferences for initially disliked foods and beverages can develop. Repeated consumption itself tends to lead to increased liking. But why do some people more easily develop a liking for beer than others, and why, for some, does it remain unpalatable because of the bitterness? Research has begun to focus on individual differences in taste sensitivity as an explanation. While there have been some previous attempts to classify responses to tastes—for example, into those whose liking for sweetness tends to increase (sweet likers), versus those whose degree of liking flattens out or decreases (non-likers), with increasing concentrations of sweeteners—such classifications are poor predictors of food likes and dislikes. Recently, however, there has been a growing body of research that has investigated individual differences in taste sensitivity, the results of which have raised the possibility that these variations may be important influences on food preferences.
In 1931, A. L. Fox, an industrial chemist with the DuPont Company, reported the discovery that some people appeared to be "blind" to the bitterness of a compound, phenylthiocarbamide, or PTC. Subsequent research confirmed that this difference between individuals had a genetic basis, and it was initially thought that non-tasters might lack a receptor for PTC and other thiourea compounds. For several decades following Fox's discovery, the main focus of research was on this genetic basis, and how this might vary across different population groups.
In the 1970s, however, this phenomenon began to interest taste scientists, in particular, Linda Bartoshuk from Yale University. Since then, our understanding of such genetic variations in taste sensitivity has grown considerably. Using a compound chemically related to PTC, 6-n-propylthiouracil (PROP), Bartoshuk and colleagues have demonstrated that each of us belongs to one of three groups—non-tasters, medium-tasters, or super-tasters—each varying in their response to PROP. Non-tasters, around 20–25 percent of the population (at least in Western cultures), find PROP tasteless, or very weakly bitter. Medium-tasters (approximately 50 percent) find PROP mildly to moderately bitter, while super-tasters (20–25 percent) find this compound almost traumatically bitter.
Such findings would merely be of academic interest if it were not for the fact that, as observed so long ago by Brillat-Savarin, there is also considerable variation between individuals in the number of fungiform papillae on the tongue. In fact, different degrees of sensitivity to PROP are highly correlated with such variations in the density of fungiform papillae. Since individual papillae are not specialized for specific tastes, it is not surprising that those sensitive to PROP also tend to be sensitive to other bitter compounds, including caffeine and quinine, and also to the sweetness of sucrose, the sourness of citric acid, and the saltiness of common salt. The artificial sweetener, saccharin, is perceived as both sweeter and more bitter by PROP tasters than by non-tasters. Bartoshuk has suggested that, effectively, the different taster groups inhabit different taste worlds.
Although these are not taste qualities, PROP taster groups also vary in their perception of the texture of fats and the pungency of alcohol and chili. This is because fibers of the trigeminal nerve in the mouth, responsible for transmitting information on the majority of tactile and irritant qualities that we perceive in foods, tend to be anatomically associated with taste cells. So, the more taste cells that an individual has, the more trigeminal fibers they also possess.
These differences in taste perception influence how much tastes are liked, which ultimately produces different patterns of food preferences. For example, coffee, spicy, and sharp-tasting foods such as some cheeses are liked more by non-tasters than tasters. Liking for cruciferous vegetables such as broccoli, cabbage, and brussels sprouts also appears to be related to variations in perception of their bitterness, and hence they are less likely to be consumed by PROP medium-and super-tasters than by non-tasters.
These relationships, apparent in both young children and adults, may turn out to be crucial in our understanding of food choice as it relates to disease risk. Increased consumption of cruciferous vegetables and reduced consumption of fats have both been linked to reduced risks for certain cancers. While the links between diet and diseases such as cancers and cardiovascular disease are increasingly being demonstrated, the notion that taste sensitivity could also predict risk has only recently been suggested, and may represent an important step in our understanding of susceptibility to dietary-related diseases. Sensory factors have frequently been implicated in the difficulties that we face in switching to healthier foods, e.g., lower-fat foods. These recent findings suggest that how successfully we are able to switch from a food containing a high level of sugar, salt, or fat to a version that is more consistent with health requirements may also be partly determined by genetic variations in taste sensitivity.