Sensory responses with taste

When using the term flavor , a certain inherent understanding of the term is evident. However, its use in the technical discussion of food requires a more imprecise definition. A common technical definition of the word flavor is the sum total of the sensory responses of taste and aroma combined with the general tactile and temperature responses to substances placed in the mouth. Flavor can also mean any individual substance or combination of substances used for the principal purpose of eliciting the latter responses. This latter usage will be the way in which the term is used in this chapter. 

As humans, we have developed five perceptive senses smell, taste, touch, sight, and hearing. Sensory response is a good example of adaptation at the molecular level and for this we shall examine the potential of the first two. Odor, like taste, is related to a direct contact of molecules with the olfactive (or gustative) epithelium. These membranes or surfaces contain chemo-receptors which upon excitation by a stimulant give characteristic organoleptic sensations. The specific interactions between small molecules and receptors involve an equilibrium between absorption and desorption and can be studied by chemists using models. 

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. 

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. 

Results from sensory evaluation of mixed solution are seen in Table IV. The data list the theoretical response for both the independent and competitive receptor hypothesis as well as the actual sensory score. The actual sensory scores were found to agree fairly well with the competitive model. The minor dissimilarity between the actual and theoretical is due to the inability of individual to taste bitterness in solutions that are extremely sweet, i.e., there is some masking of overall sensory perception which is concentration dependent. The data, therefore, clearly indicate that sweetness and bitterness act in a competitive manner and should be considered to compete for the binding sites at the same receptor. 

Taste of amino acids was studied using the taste sensor [23]. Taste of amino acids has had the large attention so far because each of them elicits complicated mixed taste itself, e.g., L-valine produces sweet and bitter tastes at the same time. Thus, there exist detailed data on taste intensity and taste quality of various amino acids by sensory panel tests [26]. The response of the sensor to amino acids was compared with the results of the panel tests, and response potentials from the eight membranes were transformed into five basic tastes by multiple linear regression. This expression of five basic tastes reproduced human taste sensation very well. 

Flavors and fragrances are sensory stimuli. Of the two, flavors are more complex because they act on the olfactory bulb via their volatile components and on the taste buds which are stimulated by both volatile and non-volatile components. The overall response to a flavor is a synthesis of the effects of both types of components. The response to fragrances, on the other hand, results only from the action of volatile components. Because flavors and fragrances function via a common mechanism, many volatile materials are used for both purposes. This is nicely illustrated by the perfumers vocabulary for fragrance materials. A collection of some 160 words published by a famous perfumer, Ernest Shiftan (1) included 75 words usually associated with flavors such as almond, bacon, coconut, honey, lime, raspberry, spicy and vanilla. 

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. 

Crude efforts were made to assess the effect of the removal of the maxillary palps and maxillae, both accessory mouth parts with many receptors assumed to be connected with the senses of taste or smell. The results were inconclusive, since insects so operated on refused to feed on either treated or untreated leaves, although this could also be interpreted to indicate that 24,055 had the same effect as removal of the sensory receptors—that is, it inhibited the response of the receptors so that the insect failed to recognize the treated material as food. 

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