I recall attending my first lecture as a winemaking student in the subject Sensory I proudly being displayed on the OHP boldly showing areas of maximum sensitivity to the basic tastes. I found the concept of the "tongue map" appealing in its simplicity. Everything seemed to tidily fit. There were different types of papillae located, broadly speaking, on different areas of the tongue, and there were four taste primaries. Surely each type of papillae contained different receptors tuned to detect one of the four basic tastes. A corollary to this was that the tongue must therefore contain regions that are sensitive to one of the four taste primaries. Although this was never stated in lecture, this fact was implied, and in any case we all left the lecture thinking that this was the case.

That map was the only diagram given in the entire lecture program that was not sourced to published work. Why should it be? Like the Mona Lisa everyone knew it and had seen it (many in primary school science class). Why clutter the argument with issues such as where it came from! I recall the lecturer stating that it arose from some work "around the turn of the century". Well if it’s been around that long it must be correct I thought. However I did feel ill at ease when I noticed that the tongue map was not included in important and respected sensory works such as the classic "Principles of the Sensory Evaluation of Food" by Amerine, Roessler and the highly respected sensory scientist Rose Marie Pangborn. Nor was it in the then sensory blockbuster and now modern classic "Sensory Evaluation Practices" by Stone and Sidel. But like most students I answered the inevitable question about the tongue map that arose in the exam, stored the facts in memory and moved on.

Many years later I were to read a paper by Virginia Collings (Collings, 1974) describing the variations across the tongue and soft palate in both detection threshold and in the ability to discriminate between realistic taste intensities. Collings found that there were variations in detection threshold around the perimeter of the tongue for sweet, sour and salty but these variations were small and of no practical significance. The bitter stimuli studied were more easily detected, not on the back of the tongue, but on the soft palate which resides on the roof of the mouth above the back of the tongue (yes there are taste buds there). She also reported that differences in suprathreshold concentrations of the bitter substances studied were more easily resolved on the back of the tongue. The suprathreshold sensitivity to the other basic tastes, like absolute sensitivity, varied little across the other parts of the tongue.

So why the contradiction with the earlier work? Well as it turns out the contradiction was only an apparent one. The "work from the turn of the century"turns out to be a PhD thesis written in German by Hanig and published in Philosophische Studien in 1901. This paper includes a diagram (Fig 5) and reproduced here in Figure 1. The y axis represents the reciprocal of detection threshold, a measure of sensitivity. Note however that the diagram is purely qualitative in that the y axis is scale-less. The figure was purely impressionistic. Later in 1942, Boring took Hanigs threshold data and summarised it as a percentage of maximum sensitivity (reproduced in Figure 2). A cursory glance at both these figures would lead you to believe that there is substantial sensitivity variation across the tongue. However neither diagram clearly shows the actual absolute differences in threshold. In fact the differences observed by Hanig were quite small and of no practical significance. The interested reader can consult Bartoshuk(1993) for a more detailed account of the origins of the taste map.

Electrophysical studies, whereby electrical activity of taste receptors is measured in the presence of taste stimuli, also support these interpretations. They show that the vast majority of taste receptors fire electrical signals, and hence elicit a taste sensation, in the presence of all the basic tastes. So much for receptor specificity, the cornerstone concept underling the taste map.

As a wine and general sensory educator I believe that this issue gets far more attention that it deserves. If one steps back and considers how the taste map (whether correct or not) helps us to assess wines, the only conclusion that can be drawn is that it is of little practical use. Firstly, the ability to detect tastes at threshold has been shown to be poorly correlated with suprathreshold sensitivity. As the majority of the compounds resulting in the basic tastes in wines are found in concentrations well above threshold, the practical worth of knowing variation in detection thresholds is questionable. Secondly, we can only apply the taste map if we can effectively localise tastes on the tongue. But can we do this? If you take a cotton wool bud and soak it with a strong salt solution and run it from the tip of your tongue, where receptors are plentiful, to the middle where they are very scarce, you will notice that the taste intensity does not diminish as you might expect. As this simple illustration of a common taste illusion is analogous to wine moving across the tongue whilst tasting, it is unlikely that we as humans can easily localise tastes in realistic tasting situations.

Perhaps as wine educators we should tell our students about things that really affect our ability as wine tasters. Examples are, the enormous variability between individuals (many hundred fold) in the number and distribution of taste receptors, which has been shown to directly affect how strongly we perceive tastes. Secondly individuals differ markedly in the amount and rate of saliva they produce, and this in turn has enormous implications for our perception of bitterness, sweetness, saltiness, astringency and particularly sourness. Lastly and more fundamentally, our perception of a complex product such as wine is determined by the interaction of tastes, aromas and tactile sensations produced by the various wine components. Knowledge of the nature of these interactions is where I believe the real focus of wine tasting education should lie.

REFERENCES

1. Bartoshuk, L. M. 1993. The biological basis of food perception and acceptance. Food Qual. Pref. 4:21-32

2. Boring, E. 1942. Sensation and perception in the history of experimental psychology. New York: Academic Press

3. Collings, V. B. 1974. Human taste response as a function of location of stimulation on the tongue and soft palate. Percep. Psychophys. 16:169-74

4. Hanig, D. P. 1901. Zur psychophysik des geschmacksinnes. Philosophische Studien 17:576-623