Sensor Used in E-nose - The Electronic Nose Artificial Olfaction Technology - page 142

(molecular shape). A trained human can distinguish several classes of compounds,
i.e., ethereal (wine), aromatic (camphor, menthol), balsamic (violet, vanilla),
alliaceous (garlic, iodine), ambrosial (amber, musk), empyreumatic (coffee), which
classify an acrid odor constricted by an organic matter when subjected to the
action of a strong heat, sweat, repulsive (pyridine, opium), or nauseous (feces,
putrefaction) [ 1 ].
Molecules of various chemicals in the air are outcome of smell. Substance
which contains chemicals and volatile in nature has smell. Human nasal cavity
contains millions of neural receptors of 350 different types. Response to different
chemicals depends on type and quantity of different sets of neural receptors and
they are activated and then after brain decodes each pattern and assign meaning to
it. Sense of smell varies among persons and also can decrease with age. Odors
blend when mixed, making individual difficult to recognize.
Hundreds of molecules from the cake, sandwich, and juice are mixed together
in the air, but the person just perceives ''Cake,'' ''Sandwich,'' and ''Juice.'' This
perception of three odors from hundreds of intermixed molecules is a feat of
perceptual organization (Fig. 6.1 ).
There are two traditional methods for odor or SMELL analysis used in various
industries (Sensory evaluation and Instrumental evaluation: discussed in Chap. 4 ).
One method is to use advanced analytical instruments in the laboratories, i.e.,
instrumental evaluation. Instrumental evaluation techniques can give especially
detailed information about the specific molecules contents of the smell. These
classical analytical techniques involve GC-MS, LC-MS, HPLC, HPTLC, etc., that
can separate, identify, and quantify individual chemicals. Odors are usually
composed of a complex mixture of different volatiles; such techniques are too
painful for practical in everyday applications and costly to set up. Also many
volatile chemicals are in very small quantities and beyond their detection limits.
Additionally, the relationship between the physical and chemical properties of the
odorant molecules and their sensory impact is still undergoing research.
The other traditional method is the use of human test panels. Human sensory
evaluation is a powerful method. The human nose has been playing an important
role

in assessing the quality of many products for a long period of time.

Fig. 6.1 Human perception of odor