Smell physiology

Human perception creates difficulty ia the characterization of flavor people often, if not always, perceive flavors differently due to both psychological and physiological factors. For example, certain aryl thiocarbamates, eg, phenylthiocarbamide, taste exceedingly bitter to some people and are almost tasteless to others (5). This difference is genetically determined, and the frequency of its occurrence differs from one population to another 40% of U.S. Caucasians are nontasters, whereas only 3% of the Korean population caimot perceive the strong bitter taste of the aryl thiocarbamates (6). Similar differences were found ia the sense of smell for compounds such as menthol, carvone, and ethyl butyrate (7). 

Whatever the physiology of odor perception may be, the sense of smell is keener than that of taste (22). If flavors are classed into odors and tastes as is common practice in science, it can be calculated that there are probably more than 10 possible sensations of odor and only a few, perhaps five, sensations of taste (13,21,35—37). Just as a hereditary or genetic factor may cause taste variations between individuals toward phenylthiourea, a similar factor may be in operation with odor. The odor of the steroid androsterone, found in many foods and human sweat, may eflcit different responses from different individuals. Some are very sensitive to it and find it unpleasant. To others, who are less sensitive to it, it has a musk or sandalwood-like smell. Approximately 50% of the adults tested cannot detect any odor even at extremely high concentrations. It is befleved that this abiUty is genetically determined (38). 

Senses includes any personal physiological reactions to incident proximity, such eis smell, odd noise, dizziness, etc. 

It is the almost universal practice to record the nature and strength of an odour at the particular temperature which may obtain at the time of examination. Substances at their temperatures of boiling have a common vapour pressure equal to that of the atmosphere, but it is clearly impossible to smell a substance in such a condition, whereas, if we could go to the other extreme, the absolute zero, it is probable that no vapour would exist as such and for this reason alone, apart from any physiological one, no odour would be discernible. 

The "smelly shoe" of the elements. The oxidation product S02 has an acrid, burning smell, the reduction product H2S stinks like rotten eggs and is very toxic. Sulfur is, nevertheless, a most useful element. It occurs in elemental form and has therefore been known for a long time is mentioned in the Old Testament. Its main application today is in the production of fertilizers. Considerable amounts of sulfur are used in tire production for vulcanization. Sulfur is also a component of gunpowder. Physiologically indispensable as thioacetic acid and especially the S-S bridges that fix proteins in their shapes (e.g. insulin, but also in perms). A 70-kg human being contains 140 g of sulfur. 

Olfactory sensitivity for one individual varies about factor three due to climatological, physiological, environmental reaons etc. The sensory sensitivity also varies from odorant to odorant. So it is difficult to select a panel with a sensitivity distribution similar to that of the population. The preferred method in the United Kingdom for screening panelists uses the actual odor to be tested as a key component. In France selection is carried out on the basis of the threshold for five standard odorants. In Germany a normal sense of smell is requested of persons between the age of 18 and 50 years, in the Netherlands no exact specifications are given. Anyway, an extreme clustering around the mean or towards the extremes has to be avoided. 

Pheromones are volatile chemicals that allow communication between individuals via air or water, over a distance which can be quite long. That is, one individual animal produces and emits a chemical that changes the biochemistry and physiology of another member of the same species. This can result in changes in social or sexual behaviour. The pheromones are known more generally as smells or scents. Communication via pheromones is well... 

The perception of flavor is a fine balance between the sensory input of both desirable and undesirable flavors. It involves a complex series of biochemical and physiological reactions that occur at the cellular and subcellular level (see Chapters 1-3). Final sensory perception or response to the food is regulated by the action and interaction of flavor compounds and their products on two neur networks, the olfactory and gustatory systems or the smell and taste systems, respectively (Figure 1). The major food flavor components involved in the initiation and transduction of the flavor response are the food s lipids, carbohydrates, and proteins, as well as their reaction products. Since proteins and peptides of meat constitute the major chemical components of muscle foods, they will be the major focus of discussion in this chapter. 

With the aim of maintaining and restoring the physiological acidic conditions that can be compromised for example in postmenopausal age, with irritation of the mucosa, itching, and unpleasant smell, a mucoadhesive formulation capable of sustained release of lactic acid, based on a chitosan lactate gel has also been proposed [59,60]. 

Optical isomerism occurs in molecules with an asymmetric carbon atom - that is one that is attached to four different atoms or functional groups. The two different optical isomers are called optical isomers or enantiomers and rotate polarized light in opposite directions. The cZ-isomer rotates it clockwise and is called dextrorotatory the Z-isomer rotates it anticlockwise and is called Zaevorotatory. Despite their structural similarity, optical isomers exhibit significantly different physiological properties. In the case of essential oils their smells can be very dissimilar. When a compound is made up of equal amounts of the d- and Z-isomers it will be optically inactive and is called a racemic mixture, or racemate. 

Flavor chemists typically subdivide the perception of food into three types of sensations taste, smell and flavor. This latter category almost invariably consists of sensations during consumption. The flavor sensations are considered largely to arise from the stimulation of smell receptors, although research has not demonstrated this to be so. From a biological and physiological point of view, these flavor sensations have little reality. 

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