Toward Sensor to Reproduce Human Sense - The Electronic Nose Artificial Olfaction Technology - page 119

Taste sensor or E-tongue is a logical tool including an array of non-specific, low
selective chemical sensors with cross-sensitivity to different components in solu-
tion accompanied by an appropriate method of PARC and/or multivariate cali-
bration for the data-processing. The stability of sensor behavior and enhanced
cross-sensitivity is a critical criterion, which is understood as reproducible
response of a sensor to as many species in solution as possible. If properly con-
figured and trained the E-tongue has the potential to determine quantitative
composition and to recognize complex liquids of different nature. The sense of
taste may have two meanings. One aspect devotes to the five basic tastes of the
tongue; sour, salt, bitter, sweet, and 'umami'. These tastes are sensed from dif-
ferent, discrete regions on the tongue including specific receptors known papillae.
The other aspect denotes the perception obtained when food enters the mouth [ 6 ].
When chemicals are detected by specific cells on the tongue resulting in nerve
impulses which are sent to the brain for analysis than taste occurs. Taste reception
happens at the apical tip of taste cells that form taste buds. Each onion shaped taste
bud is composed of 50-100 taste cells that have microvilli. Each single taste bud
contains 50-100 taste cells signifying all 5 taste sensations. Implanted in the cell
membranes of these taste cells are receptor proteins. Each of the taste receptors are
transmembrane proteins which role either by physically binding to a flavor
ingredient (sweet, bitter and umami) or by acting as a channel to allow ions to flow
directly into a taste cell (salty and sour). This interaction triggers a signaling
cascade that culminates with signals to the brain through a network of taste nerve
fibers.
Gustation, the sense of taste, is important for the location and identification of
food and can be a very highly developed sense. e.g., some Cyprinids have sen-
sitivity to sugars and salts that are 512 and 184 times higher than humans,
respectively. Different types of olfactory receptors, taste receptors can be found in
more than one location on the body. Besides being found in the mouth, pharynx,
gill arches and skin, some fish, such as the Siluri formes have well developed
barbells that have dense concentrations of taste receptor cells. Figure 5.2 shows
the taste bud cell.
The performance of an artificial sense such as the E-tongue can be considerably
enhanced by the combination of sensors based on different technologies. The
reason is, of course, that for each new measurement principal added, also a new
dimension of information is added. A natural extension of this fundamental con-
cept is the combination of different artificial senses. This is especially important
when estimating the quality of food, since the guide is the impression of the human
being using all five senses [ 6 ].
A taste sensor system is used to categorize the different basic taste sensations
(sour, salt, bitter, sweet and ''umami''), and the results are compared with human
test panels. An E-tongue classifies a quality of one or another kind in food, drinks,
water, process fluids etc., and the results are not necessarily compared with human
sensations, but with other quality properties of the sample [ 1 ]. Figure 5.3 repre-
sents the schematic analysis of the E-tongue system.