1.3 Taste Categories: Myths & Modern Research

According to the text book tongue map above, the taste buds for "sweet" are on the tip of the tongue; the "salt" taste buds are on either side of the front of the tongue; "sour" taste buds are behind this; and "bitter" taste buds are to the rear.  (Certain wine glasses are said to cater to this arrangement). Unfortunately, the map itself is wrong. In fact, there are five basic tastes officially recognised so far, and the entire tongue can sense all of these tastes more or less equally.

THE HISTORICAL DEVELOPMENT OF THE TONGUE MAP Hanig chose to study the distribution of taste sensitivity around the perimeter of peoples tongues. Apparently he chose the perimeter, because, this is where the taste buds are most densely distributed. He tested from substances, sucrose (sweet), salt, quinine sulphate (bitter), and hydrochloric acid (sour). Hanig's hypothesis was that sensitivity to each taste would vary considerably throughout the perimeter of the tongue. Having plotted 14 points along the tongue, which appear to be more or less evenly spaced, he numbered these points A-O (refer to diagram below), but did not plot sensitivity values at each of these points. Rather, he created impressionistic curves designed to reflect the rate of change from one point to the next. Hanig observed that sensitivity to sweet tastes was maximum at the top and minimum at the base of the tongue. No comparison of real sensitivity values around the perimeter of the tongue could be made. Hanig made a simple conclusion - that sensitivity to the four tastes altered around the tongue. A second scientist, Edwin Boring, accentuated the problem in his re assessment of Hanig's experiment. Boring published a paper in 1942 titled, 'Sensation and Perception in the History of Experimental Psychology.' He took Hanig's raw data and calculated actual sensitivities. However, when it came to displaying the results in graph form, he did a further calculation. He divided the sensitivity at each point by the maximum sensitivity observed for that specific taste. Boring's graph is misleading, as there is no method of measuring how meaningful the variations between different regions of the tongue are. (Refer to graph) If one looks at the bitter curve, the sensitivity at H (the base) is 1.0 whilst at the hip it is just under 0.2. This appears to be a large difference, but they are not actual figures, they simply mean that at the base of the tongue H, the sensitivity is 1/5 the value that it is at A. The graph has been badly misinterpreted as the low points of the graph were interpreted as being of no sensitivity, whilst the high point of the graph was viewed as having the highest sensitivity. In 1974, Virginia Collings re examined the threshold experiment of Hanig's and came to the conclusion that there were in fact variations of sensitivity to salt, sweet, bitter and acid, but that these variations were very small. Furthermore the four taste sensations being examined could be perceived at every point on the tongue where taste receptors were present. Collings did differ from Hanig in one aspect of her research, in that she found that bitter thresholds are actually lower on the front of the tongue than at the back of the tongue. This is something of a curious finding from a survival point of view as many bitter substances are also toxic, and their early detection would be conducive to health. This could in part be compensated by the fact that the lips seem to be very sensitive to bitter / astringent tastes and it is these that are the principle warning signal for what is safe and unsafe to consume.

The Tongue Map Myth

The concept of the classical text book 'tongue map' dates back to last century when a German psychologist at Harvard University, D.P. Hanig was working on a PhD thesis. Hanig's thesis was published in'philosophische studien'in 1901. Hanig's data was misinterpreted for seventy five years (supposedly due to mistranslation), and it was not until 1974 that a paper by Collings re-examined the data and dismissed the theory of the tongue map. Despite this, some quarter of a century later, many textbooks still have not been changed.*

From my own personal observations, I have long being dissatisfied with the very notion of four 'basic tastes'. The model does not account for a host of more complex sensations which the brain seems to interpret as taste.  New research has discovered receptors on the tongue capable of identifying the sensation of water and more famously, the "fifth basic taste", 'umami'(also described as 'savoury').Others, including myself, have advanced still more categories of taste. These are outlined  below.

Water Tastes

The taste of water has only recently been argued as a taste perception.
If water is consumed by humans in a normal manner, that is in the presence of saliva, then the taste of water has been described as flat, bland or tasteless. However, if water is taken up in a medium of citric acid or quinine, then the taste sensation of water has been described as being sweet. On the other hand, if the taste medium of water is either sucrose or sodium chloride (salt) then the reported taste sensations have been described as bitter or sour. Much of the taste perception in humans thus clearly depends upon normal saliva flow, which is the pre-adapting media by which a taste sensation is to be determined. Many people who are undergoing some form of medication will find their ability to taste is altered, due to the fact that some four hundred-odd medications alter saliva flow. The sensations of salt taste and water taste have a dual role, in that they bias the taster to either reject or swallow according to the body's ion / water balance requirements. The body has learnt to reject too much salt just as it has learnt to demand water. These adjustments are initiated by the taste sensory system even before substances are absorbed in the intestine.

Umami / Savoury Tastes

As research has advanced, a taste sensation called "Umami" (pronounced"oo-marmi") has been accepted. It was first identified by Professor Kikunae Ikeda[pictured below]at the Imperial University of Tokyo in 1909. He had observed that "There is a taste which is common to asparagus, tomatoes, cheese and meat but which is not one of the four well-known tastes of sweet, sour, bitter and salty."

It was in 1907 that Ikeda started his experiments to identify the source of this distinctive taste. He knew that it was present in the "broth" made from kombu (a type of seaweed) found in traditional Japanese cuisine. Starting with a tremendous quantity of kombu broth, he succeeded in extracting crystals of glutamic acid (or glutamate, an amino acid and a building block of protein). Ikeda found that glutamate had its own distinctive taste and he named it "umami"  - a Japanese noun (literally meaning "delicious flavour").
Umami is now used to designate a pleasant gustatory sensation that is different from sweet, sour, salt or bitter. It is the taste sensation that is most influential in determining how delicious a food is. Sometimes described as "savoury" or "meaty", the same taste is referred to as "xianwei" in Chinese cooking and is considered a fundamental taste in Japanese and Chinese cuisine, but is little discussed in the west. Umami has also been described as a salty, seaweed-like characteristic, especially well defined in certain Scotch Whiskies, particularly those from the island of Islay off the west coast of Scotland. Indeed, the glutamate taste sensation is most intense in combination with sodium. This is one reason why tomatoes exhibit a stronger taste after adding salt.

The active ingredients of umami were initially identified as L glutamate(found in human and chimpanzee milk),other amino acids and small peptides. After this discovery, research investigating the connection between amino acids - a structural element of protein - and the taste of food continued, and it was eventually discovered that each of the twenty kinds of amino acids possesses its own unique taste. The combination of these various tastes is an important element in determining the flavour of foods. Apart from the amino acid glutamate, the umami taste is also given by the nucleotides inosinate, which can be found in meat and fish, and guanylate, which is found in mushrooms. The synergistic effect of these different types of umami has been scientifically proven - that is, that by combining these different kinds of umami, the umami taste is significantly magnified. Examples include onion (glutamate) with veal (inosinate); Chinese cabbage / leak with chicken bones; and Japanese Konbu / kelp with Bonito flakes. 

Glutamate is naturally present in most foods, such as meat, poultry, seafood and vegetables as are the two kinds of nucleotides that contribute most to the Umami taste, inosinate and guanylate. Inosinate is found primarily in meat, whereas guanylate is more abundant in plants. Another nucleotide, adenylate, is abundant in fish and shellfish. The foods listed below contain high quantities of Umami elements:

It may be conjectured that having a critical need for protein, with amino acids being the building blocks for protein, our taste for umami drives our appetite for high protein foods like meat. Bacon, for example, incites a strong umami sensation because it is a rich source of amino acids. On the tongue, it appears that a subset of savoury taste buds responds specifically to glutamate in the same way that sweet ones respond to sugar.(For more information regarding umami, including recipes and the latest research updates there is now an official website dedicated to the fifth taste:www.umamiinfo.com)


Trigeminal Flavours or "Mouthfeel" Sensations

A further addition to the emerging new model of taste is a more generalized chemical sensitivity in the mouth, which in fact exists over the whole body (even mucus membranes in the anus are sensitive giving rise to the old Hungarian saying that "good paprika burns twice"). In the mouth, this general chemical and tactile irritability is primarily mediated by the trigeminal nerves in the tongue.(Also called the 'fifth' nerve, the trigeminal nerve is one of the twelve paired nerves that originate in the brain stem. It is both the main sensory nerve of the face and also has certain motor functions including biting, chewing and swallowing).

One isn't required to be an experienced wine taster in order to appreciate the significance of trigeminal stimulation or 'mouthfeel': Everyday experiences include the fizzy tingle of CO2 in carbonated drinks, the burn from hot peppers and spices such as ginger and cumin,  the tastes of chemical compounds, alcohol, tannins and alkaloids, fats and oils, as well as tactile sensations of pain, texture, temperature, electricity. All of these sensations can be experienced in various parts of the mouth, contributing to the overall sensation of 'flavour'. Taste buds appear organized to provide trigeminal access to the oral milieu with trigeminal fibers ascending around the taste bud forming a chalice-like structure. The trigeminal endings seem to use the specialized structure of the taste bud to find a channel to the external environment. This speculation is consistent with the observation of high responsiveness to pepper chemicals in areas such as the top of the tongue that are rich in fungiform papillae.

However, with mouthfeel, the distinction between a chemical sense and a tactile sense becomes somewhat blurred for the reason that the nerves in question mediate tactile, thermalandpain sensations. This blurring is perhaps best illustrated by sensations of astringency, a familiar characteristic of some red wines. Tannins in wine (and food) are chemical stimuli and yet the astringent sensations they produce seem largely tactile. They make the mouth feel rough and dry, and cause a drawing, puckery or tightening sensation in the cheeks and muscles of the face. Although scientific analysis would categorise astringency as a group of chemically-induced oral tactile sensations, most wine tasters would say that astringency is an important component of wine "flavour." This highlights, once again, the integrative nature of flavour in combining inputs from multiple sources.
The mechanisms giving rise to these sensations are not completely understood, but one long-standing popular theory has it that tannins bind to salivary proteins and mucopolysaccharides (the slippery components of saliva), causing them to aggregate or precipitate, thus robbing saliva of its ability to coat and lubricate oral tissues. One feels this result as rough and dry sensation on oral tissues, even when there is fluid in the mouth. Note that "roughness" and "dryness" are difficult to perceive unless a person moves the tongue against other oral tissues (which we do all the time when eating).  **


Taste Categories

If we observe our own sensations of wine and food carefully we will note that tongue and mouth receptors are capable of determining many more sensations than those already outlined (salt, sweet, acid, bitter, water & umami). I have attempted to classify these in the following paragraphs as either "taste" or "mouthfeel". For example, I have included the sensation of pressure. Whilst pressure arguably contributes nothing to taste, it cannot be disputed that pressure is a part of mouthfeel. For a wine taster, pressure is a 'taste' sensation because of its influence in creating the sensation of 'gas release' in the mouth, which differentiates the various Champagne qualities and styles. An experienced Champagne taster can pick up pressure in the mouth and recognise the difference between a carbonated, tank fermented and bottle fermented sparkling wine. This recognition is based upon the size of the bubbles and their impact in the mouth. On the other hand, an experienced taster of sparkling wine can also tell when a wine has sufficient pressure, too much pressure or too little pressure. So for these reasons, pressure is incorporated as a category. No doubt my classification is controversial. There are many fine points for the academics and scientists to debate.

Tastes can be defined within primary, secondary or tertiary categories.

Primary Taste Sensations
These belong to the unprepared, or unaltered grapes, fruits, vegetables etc, and indicate the taste sensation that one would experience if one was to taste the produce in its unprepared state. For example, winemakers not only test for chemical ripeness of grapes, i.e.- sugar levels, acidity, pH etc, but they taste the grapes to experience the flavour development, even though the sweetness of a grape may have quite a different taste sensation from the sweetness of the finished wine.

Secondary Taste Sensations
These are the taste sensations that most people experience and are the modified taste sensations which have resulted from winemaking practice, food preparation or cooking. For example, the sugar from a grape can be entirely converted into alcohol, thus creating a dry beverage. Alternatively, fermentation can be arrested creating a sugary sweet wine. Semillon and Riesling are two classic grape varieties which can produce grapes and wine with a variety of sugar levels. Acidity will also be different, which will reflect different degrees of ripeness.

Tertiary Taste Sensations
Tertiary taste sensations, like tertiary aromas, develop or are modified over time. A young wine might start out with very high levels of acidity but over the years, this acidity could drop off to the point where the wine tastes 'flat' or 'dull'. The taste sensations of sweetness can also be modified overtime. A Sauternes might have a very high degree of sweetness as a young wine whilst as an old wine it will become significantly drier.



Major Taste Sensations

Major Mouthfeel Sensations (Trigeminal Flavours)

As mentioned above, these are technically not 'tastes' but belong to the sense of touch and augment the overall perception of flavour in the mouth. Nevertheless, their significance should not be underestimated. Considering CO2 alone as a trigeminal flavour, the carbonated beverage industry which includes soda, beer and sparkling wine production, amounts to billions of dollars in annual sales of  'trigeminal flavour' worldwide. Similarly, the pepper business and so-called ethnic foods have experienced rapid growth in the West due to a continuing influx of immigrants from cultures with hot, spicy cuisines, and a growing trend toward more adventurous dining on the part of many Westerners. (Salsa was outselling ketchup in the U.S. as early as 1992). 

Researchers at Canada's Brock University have made a study (2008) to define wine taste by a range of textures rather than aromas and flavours, thus accentuating the importance and understanding of the mouthfeel. They have devised a vocabulary based on familiar liquids, foods and household materials they feel best describe the wide range of textures experienced in wine. The Australian Wine Research Institute has also created a flavour wheel based upon red wine mouthfeel.

Conclusion
While the number of accepted taste sensations will undoubtedly continue to be added to, the tongues sensitivity is meager compared with that of the nose.  Our olfactory system has at its disposal fifty million plus chemo-receptors. The number of odours recognisable by humans remains a matter of debate; the combinations at least, are likely to be infinite. Olfaction remains the last sense to be explained by science and its ongoing study forms the subject of the remaining chapters in this section.