Taste tastants

Here are the bare essentials of taste bud structure. At the top of the taste bud, there is a small pore opening into the bud. Tastants get access to the taste bud molecular machinery through this pore. The body of the taste bud is composed of 30-100 individual neuroepithelial cells, of three types. The key cell type is the sensory cell in whose membrane is embedded the receptors for tastants. The afferent taste nerve... 

Taste bud a structure in the mouth housing receptors for tastants. 

After the saliva has carried the tastants into the taste bud, they interact with the taste receptors on the surface of the cells, or with ion channels, which are pore-like proteins. Salty and sour tastants act through ion channels, and sweet and bitter sensations are mediated by surface receptors. The different taste submodalities rely on specific mechanisms Na+ flux through Na+... 

Researchers have oscillated between emphasizing specificity of neurons ( labeled lines ) and responses to a spectrum of tastants by one cell. More recently, patterns of activation of a number of sensory cells are favored for coding specific taste sensations (Smith and Margolskee, 2001). Neural distinction of different tastes requires simultaneous activation of different cell types. The brain receives a single channel of information, simply bitter for a number of different compounds. 

Prank, R. A. and Byram, J. (1988). Taste-smell interactions are tastant and odorant dependent. Chemical Senses 13,445-455. 

As a direct result of this so-called food-minus trend some of the ingredients have to replaced or reduced. Owing to their role as preference drivers in food consumption, it is important to retain the whole flavour and taste profile of the original product, which can be done in most cases using a mixture of flavours, tastants, taste modifiers and texturants. 

The following chapter (G2) covers the measurement of acid tastants, i.e., chemicals responsible for the acid and sour taste as well as the perception of astringency. unitgu presents methods for potentiometric and colorimetric titration of acid tastants, and unit... 

There are some hundred different 7TM receptors involved in perception of bitter tastants. The bitter taste receptors also couple through the G protein gustducin yielding the activated Gag-GTP which can thence activate C.AMP/C.GMP phosphodiesterase, thus lowering cAMP and cGMP concentration. Bitter tastant receptors can also act via pertussis... 

Salty tastants act directly on Na+ channels in the PM of cells on the tongue surface. Direct passage of Na+ through these channels causes depolarization and thence signalling to the CNS. Much (but not all) salt taste perception is inhibited by the voltage-sensitive Na+ channel inhibitor amiloride (see Chapter 4) and evidently some salt perception also occurs via amiloride-insensitive channels. 

Floriano WB, Hall S, Vaidehi N, Kim U, Drayna D, Goddard WA 3rd. Modeling the human PTC bitter-taste receptor interactions with bitter tastants. J. Mol. Model. 2006 12 931-941. 

The simplest tastant, the hydrogen ion, is perceived as sour. Other simple ions, particularly sodium ion, are perceived as salty. The taste called umami is evoked by the amino acid glutamate, often encountered as the flavor enhancer monosodium glutamate (MSG). In contrast, tastants perceived as bitter or sweet are extremely diverse. Many bitter compounds are alkaloids or other plant products of which many are toxic. However, they do not have any common structural elements or other common properties. Carbohydrates such as glucose and sucrose are perceived as sweet, as are other compounds including some simple peptide derivatives, such as aspartame, and even some proteins. 

Tastants are detected by specialized structures called taste buds, which contain approximately 150 cells, including sensory neurons Figure 32.12). Fingerlike projections called microvilli, which are rich in taste receptors, project from one end of each sensory neuron to the surface of the tongue. Nerve fibers at the opposite end of each neuron carry electrical impulses to the brain in response to stimultation by tastants. Structures called taste papillae contain numerous taste buds. 

Importantly, each taste receptor cell expresses many different members of the T2R family. This pattern of expression stands in sharp contrast to the pattern of one receptor type per cell that characterizes the olfactory system (Figure 32.16). The difference in expression patterns accounts for the much greater specificity of our perceptions of smells compared with tastes. We are able to distinguish among subtly different odors because each odorant stimulates a unique pattern of neurons. In contrast, many tastants stimulate the same neurons. Thus, we perceive only "bitter" without the ability to discriminate cycloheximide from quinine. 

Salty tastants are not detected by 7TM receptors. Rather, they are detected directly by their passage through ion channels expressed on the surface of cells in the tongue. Evidence for the role of these ion channels comes from examining known properties of sodium channels characterized in other biological contexts. One class of channels, characterized first for their role in salt reabsorption, are thought to be important in salt taste detection because they are sensitive to the compound amiloride, which mutes the taste of salt and significantly lowers sensory neuron activation in response to sodium. 

Figure 32.12. A Taste Bud. Each taste bud contains sensory neurons that extend microvilli to the surface of the tongue, where they interact with tastants.

Bottle choice. A widely used method for quantitatively monitoring rodent behavior with regard to taste is the bottle-choice assay. An animal is placed in a cage with two water bottles, one of which contains a potential tastant. After a fixed period of time (24-48 hours), the amount of water remaining in each bottle is measured. Suppose that much less water remains in the bottle with the tastant after 48 hours. Do you suspect the tastant to be sweet or bitter ... 

The sweetness inhibitory activity of plant terpenoids is evaluated by placing 5 mL of 0.5 or 1 mM solution of the compound in the mouth for 2-3 minutes. On expectorating, the mouth is then washed with distilled water. Then, different concentrations of sucrose (0.1-1 mM) are tasted. The maximum concentration of sucrose at which complete supression of sweetness is perceived may then be recorded for each tastant [133]. In practice, antisweet compounds of plant origin have tended to be ranked in terms of sweetness inhibitory potency by comparison with gymnemic acid I (90) [19]. 

At this point, it is important to clarify a few terms that are often used in the context of this chapter. Flavour is usually divided into the subsets taste and smell, which are perceived in the mouth and the nose, respectively [3], The terms aroma and odour are not well defined and often used as synonyms. Qdour is best reserved for the smell of food before it is put into the mouth (nasal perception) and aroma for the retronasal smell of food in the mouth. In this paper, we mainly use the terms aroma and taste , as well as flavour comprising sensory notes imparted by both volatile and nonvolatile compounds (odorants and tastants). In the public domain, however, taste is often used as a synonym for flavour . Proper definitions of these terms are ... 

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