Taste psychophysics

The aroma of fmit, the taste of candy, and the texture of bread are examples of flavor perception. In each case, physical and chemical stmctures ia these foods stimulate receptors ia the nose and mouth. Impulses from these receptors are then processed iato perceptions of flavor by the brain. Attention, emotion, memory, cognition, and other brain functions combine with these perceptions to cause behavior, eg, a sense of pleasure, a memory, an idea, a fantasy, a purchase. These are psychological processes and as such have all the complexities of the human mind. Flavor characterization attempts to define what causes flavor and to determine if human response to flavor can be predicted. The ways ia which simple flavor active substances, flavorants, produce perceptions are described both ia terms of the physiology, ie, transduction, and psychophysics, ie, dose-response relationships, of flavor (1,2). Progress has been made ia understanding how perceptions of simple flavorants are processed iato hedonic behavior, ie, degree of liking, or concept formation, eg, crispy or umami (savory) (3,4). However, it is unclear how complex mixtures of flavorants are perceived or what behavior they cause. Flavor characterization involves the chemical measurement of iadividual flavorants and the use of sensory tests to determine their impact on behavior. 

A persistent idea is that there is a very small number of flavor quaUties or characteristics, called primaries, each detected by a different kind of receptor site in the sensory organ. It is thought that each of these primary sites can be excited independently but that some chemicals can react with more than one site producing the perception of several flavor quaUties simultaneously (12). Sweet, sour, salty, bitter, and umami quaUties are generally accepted as five of the primaries for taste sucrose, hydrochloric acid, sodium chloride, quinine, and glutamate, respectively, are compounds that have these primary tastes. Sucrose is only sweet, quinine is only bitter, etc saccharin, however, is slightly bitter as well as sweet and its Stevens law exponent is 0.8, between that for purely sweet (1.5) and purely bitter (0.6) compounds (34). There is evidence that all compounds with the same primary taste characteristic have the same psychophysical exponent even though they may have different threshold values (24). The flavor of a complex food can be described as a combination of a smaller number of flavor primaries, each with an associated intensity. A flavor may be described as a vector in which the primaries make up the coordinates of the flavor space. 

Much of our present day knowledge of sweetness intensity, both at the threshold level, where taste begins, and above the threshold level, derives from the application of psychophysical techniques. It is now evident that the psychophysical procedure used measure separate aspects of sweetness perception. Hedonic responses cannot be predicted from intensity of discrimination data, and vice versa. The taste-panel evaluation of sweetness is of fundamental importance in the development of worthwhile structure-taste relationships. Therefore, it is vital that the appropriate psychophysical method and experimental procedure be adopted for a particular objective of investigation. Otherwise, false conclusions, or improper inferences, or both, result. This situation results from the failure to recognize that individual tests measure separate parameters of sensory behavior. It is not uncommon that the advocates of a specific method or procedure seldom... 

The second type of study which has contributed to our understanding of the functional properties of oral chemoreceptor systems is human psychophysics, where verbal reports are taken on the taste properties of food and beverages and their chemical constituents. It is often possible for an individual to break a flavor complex down into a variety of distinguisable sensations. These sensations are end products of neural processing that are available to consciousness. Any natural food is of complex chemical composition and thus activates a wide variety of oral and nasal chemoreceptors. These flavor sensations may arise entirely from the oral cavity or require both oral and nasal stimulation. 

Once a purified gymnemic acid became available, much psychophysical work was done to understand the nature of the sweetness inhibition effect. The work of Bartoshuk and co-workers illustrates the course taken (15). The results of a typical experiment are shown in Figure 3. The sweetness of a sucrose solution was almost completely suppressed after holding a gymnemic acid solution in the mouth for a few seconds. Further experiments were carried out to determine the effect of gymnemic acid on the other taste qualities (sour, bitter and salty). No effect of gymnemic acid on these tastes was observed. Early work with GA extracts had produced an apparent inhibition of bitter taste, but this effect was later attributed to cross-adaptation to the taste of the crude leaf extract, which was itself quite bitter. Experiments with refined (and tasteless) extracts showed no bitterness suppression. 

Two areas of research, psychophysics and sensory evaluation, have made recent contributions to the understanding of oral sensations of heat derived from peppers. Psychophysical studies have characterized observer s responses to heat from spice-derived compounds, focussing on such aspects as time-intensity functions, areas of oral stimulation, correlation with evoked salivary flow, interactions with basic tastes, and effects of sequential stimulation. Sensory evaluation of the heat level of ground red pepper has recently been advanced by the validation of a new method which solves many of the problems inherent in the previous Scoville procedure. The new method is based on anchored graphic rating by panels Who are trained with physical reference standards. The procedure has shown excellent reliability, fine discriminations among samples, and high correlations with instrumental determinations of capsaicinoid content of pepper samples. 

Since K c is proportional to the left-hand side of this relationship, and the right-hand side is related to the work of fracture, a linear relationship between the two sides both confirms the psychophysical approach and provides a new parameter, C, which is inversely proportional to the efficiency of the lower incisors and so can be used to characterize the ease or lack of ease when biting into a food product. This exercise has yet to be performed. It would provide a very effective bridge between the measured mechanical properties of fibrous foods and their properties as measured by a taste panel. 

DeSimone, J.A. and Heck, G.L. Surface active taste modifiers a comparison of the physical and psychophysical properties of gymnemic acid and sodium lauryl sulfate, Chem. Senses, 5, 317, 1980. 

Schiffman, S. S. Changes in taste and smell with age Psychophysical aspects. In J. M. Ordy K. R. Brizzee, Eds., "Sensory Systems and Communication in the Elderly," (Vol. 6). Raven Press New York, 1979 pp. 227-246. 

Man is blessed with the sense of smell, taste, touch, vision, and hearing. Three of these senses (touch, vision, hearing) are referred to as the physical senses and are used for detection of mechanical, thermal, photic, and acoustic energy. The other two, the chemical senses, are used for the detection of volatile and soluble substances. The stimuli that excite the physical senses can be measured by both physical and psychophysical means. The volatile and soluble substances that excite the chemical senses can be defined but the stimuli caused by these substances can only be measured by psychophysical means.l z For all practical purposes these stimuli cannot be expressed as some unit of energy, instead they have to be expressed in the dimensions of quality, intensity, duration, and like and dis-1 ike. 

Human tongue Psychophysical response Taste threshold 0.90 C2-C8 0.49 0.49 w... 

Hunan tongue Psychophysical response Taste threshold -0.97 C2-C10 0.45 4.90 (W)... 

Stevens SS. (1969) Sensory scales of taste intensity. Percept Psychophys, 6 302-308. Borg G, Diamant H, Strom L and Zotterman Y. (1967) The relation between neural and perceptual intensity a comparative study on the neural and psychophysical response to taste stimuli. J Physiol, 192 13-20. 

Duffy VB, Hayes JE, Bartoshuk LM and Snyder DJ. (2009) Taste vertebrate psychophysics. In Squire LR, ed. Encyclopedia of Neuroscience. Oxford Academic Press 881-886. 

Humans detect at least five primary taste stimuli, which include sweet, sour, salty, bitter, and umcmi taste. Representative taste stimuli for the five primary taste qualities consist of polar molecules that are generally presented as aqueous solutions to subjects for psychophysical studies. Medium- and long-chain fatty acids are non-polar molecules that do not readily dissolve in... 

Recent evidence suggests that the oral detection of fatty acids such as stearic acid do occur in the human and rodent oral cavity, and stimulate a fatty acid taste response. Both animal and human studies indicate that stearic acid elicits a small to moderate taste response in the oral cavity. The hedonic appeal of fats may be enhanced especially when consumed with carbohydrates. Future studies will determine whether long-chain fatty acids such as stearic acid or linoleic acid represent a primary taste stimulus in the human oral cavity. Finally, improved dehvery methods for saturated and unsaturated fatty acids for psychophysical studies will stimulate the advancement of this important field of study. 

Lawless, H.T. 1987. Gustatory psychophysics. In Neurobiology of Taste and Smell, eds. T.E. Finger W.L. Silver, pp. 401-422, Wiley Interscience, New York. 

By flavor chemistry is meant the chemistry of the total complement of sensory responses elicited by a food or a food component. In human sensory research, the sensory responses are psychophysical sensations, whereas in neurophysiological studies the responses are measured from neurons. The flavor chemistry of the geniculate ganglion fungiform papillae taste systems would then consist of a description of the neural responses to foods and to the types of compounds present in food. The neural responses of both the dog and the cat have been examined with respect to the excitability of many of the compounds found in vertebrate tissues. The cat has been tested with more compounds than the dog, but the results will also apply in large part to the dog, since the two species are so similar with respect to the majority of the compounds considered. 

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