Sensors and sensory systems for an electronic nose

312 Pages·1992·29.123 MB·English

by  J W Gardner, Philip N Bartlett

ODOURS - TilE STIMULUS FOR AN ELECTRONIC NOSE
G. H. DODD,t P. N. BARTLETT*t & J. W. GARDNER*
Departments of Chemistryt and Engineering*
University of Warwick
Coventry CV4 7 AL
UK
ABSTRACT. Odorants form a large collection of organic molecules with a
molecular mass in the range 18-300 Daltons. There is typically a single polar
group in an odorant molecule although there are some notable exceptions to this
rule. In laboratory investigations on both the human sense of smell and in
electronic noses, it is convenient to use pure single odorants as the odour
source. In many cases, the odour profile of these odorants is modified by levels
of impurities which would be of no consequence in areas of chemistry other than
osmochemistry. General trends in the relationship between odour type and
molecular structure are known, but the underlying quantitative structure-odour
relationships are poorly understood. In addition to the odour note, the other key
properties are the threshold value and the intensity. Both of these properties
are used in the design of functional smells, such as the perfuming of washing
products, and both are expected to be important for the behaviour of electronic
noses. Natural smells, especially perfumes and flavours, are extremely complex
mixtures of odorants. The interactions between the responses to the individual
odorants which give rise to the complexity of the subjective sensation are not
well understood. However, in some notable cases, key impact flavour molecules
play the dominant role in the generation of flavours. In addition to the problem
of complexity, natural smells are frequently labile. Many key odorants undergo
oxidative degradation to produce a variety of off-odours. Finally, this chapter
discusses the olfactory system and the industrial need for an instrument that is
capable of detecting odours.
1. The Stimulus
In humans the sensation of flavour has contributions from at least three
independent sensory systems: taste (gustation) [ 1] , olfaction, and the trigeminal
sense. These three senses are located in distinct tissues, respectively the tongue,
the olfactory epithelium, and in the case of the trigeminal sense the receptor
cells are found in all mucous membranes and also in the skin. Of these three
senses, olfaction plays the dominant role in the flavour sensation, and is the
t Now at School of Chemistry, University of Bath, Bath, BA2 7AL, UK.
J. W. Gardner and P. N. Bartlett (eds.), Sensors and Sensory Systems for an Electronic Nose, 1-11.
© 1992 Kluwer Academic Publishers.
2
THRESHOLD IN
ODOUR TYPE WATER/ ppb
~CHO Green Leaves 316
trn,oH
Rose 290
~OH
I
Thyme 86
2
Lemons 10
(~'
Green Peppers 2 X J0-3
~SH
Grapefruit 2 X J0-5
FIGURE 1. The properties and structures of some typical odorants.
most influential for the rejection of a beverage or foodstuff in the presence of a
volatile off-flavour chemical. Under normal conditions, only volatile chemicals
can reach the olfactory epithelium and the sense of taste is used to detect
non-volatile chemicals. The trigeminal sense responds to many volatile
chemicals and is thought to be especially important in the detection of irritating
or chemically reactive vapours.
3
In the lives of mammals olfaction plays a key part. The sense of smell is
used to locate and to evaluate food, its is the primary sense used to locate and
evaluate potential sexual partners, and it may play a significant role in social
communication between individuals [2].
1.1 ODORANT MOLECULES
The sensation of smell arises from the interaction of volatile molecules from the
material being smelt with the receptor cells in the olfactory epithelium. These
odorant molecules are typically small hydrophobic molecules with molecular
masses in the range 18 to 300 Daltons. Typically they contain a single polar
group. Although there are some notable exceptions, molecules with more than
one polar group are generally involatile and thus unable to reach the olfactory
epithelium. A majority of odorants contain oxygen, typically in the polar group
of the molecule. Nitrogen and sulfur componds are less frequent, although the
nitrogen containing pyrazines and other heterocycles are an important class of
odorants in roasted products, and sulfur occurs in many important natural
odorants from animals. Some typical structures are shown in Figure I.
The three important properties of odorants are the odour type, the
threshold value and the form of the intensity curve. The properties of the
molecules which determine these characteristics have been discussed elsewhere
[3, 4]. It is known that the shape, size and polar properties of the molecule
determine its odour properties. However the precise rules are poorly understood
and it remains the case that new synthetic odorants are still designed
semi-empirically. There are two significant problems. First, most classifications
of odour type are based on subjective perception and use common names to
signify the odour (e.g. fruity, flowery, musk, etc.). Even a single, highly pure
odorant may require several terms to describe its odour. Furthermore the
classification is imprecise (fuzzy) because it must be based on subjective
associations and because an individual's sense of smell is not invariant with time
or physical health. The second difficulty arises from the large number of
physicochemical properties describing the molecules. This large number of
variables in any analytical model means that the number of odorants studied in
any structure-odour experiments will be prohibitively large [5]. As an example
of the effect of changing size of the molecules, whilst keeing the polar group
fixed, consider the set of simple cyclic ketones shown in Figure 2. The odour
descriptors are taken from Arctander [6] and from Moncrieff [7]. There is a
gradual and regular change in odour as the ring size increases.
The olfactory thresholds of molecules can vary significantly, and molecules
are known for which the thresholds are very low, see Figure I. This creates
problems for those interested in the study of odours since trace levels of
impurities can, in certain circumstances, have very significant effects of the
perceived odour. Odorants with very low thresholds can can also dominate the
4
Odours of Cyclic Ketones
C)=
Cs a Bitter almonds
()=o
c6 Bitter almonds
CJ=o
~ Camphoraceous
. .
j Cw.Cu j Cw·Cu ~
c:J=o
cl2 Camphoraceou~
CJ=o
Cn Cedarwood
wo
C14 Musk
c::J=o
C1s Pine musk
~0
cl7 Civet
FIGURE 2. The odours of a set of simple cyclic ketones.
odour profiles of complex natural flavours and smells - so called key impact
odours, Figure 3. Many off-flavours are due to the presence of small amounts
(sub ppb) of these powerful odorants.
Finally the intensity of the odorant is important. Odorants with a low
intensity, such as dimethylether, are very volatile and hence can reach high
concentrations in air. As a result they can give rise to a strong odour sensation,
but only at high concentration. High intensity odorants, such as the musk
cyclopentadecanone, are much less volatile and so the concentrations in air are
5
THRESHOLD IN
ODOUR TYPE WATER/ppb·
eCHO Off-flavour of
white fish 0.04
0
Off-flavour of
~ butter O.ot
s-s
fS)(J
Off-flavour of
thiamine 2 X J0-6
0 0
FIGURE 3. Examples of compounds responsible for off-flavours.
much lower. Nevertheless molecules of this type frequently have low thresholds
and so can be smelt. This balance between threshold and volatility is a key
factor in commercial perfumery since the less volatile, high intensity odorants
will evaporate more slowly and last longer.
L2 NATURAL SMELLS, PERFUMES AND FLAVOURS
Natural smells, perfumes and flavours are almost always complex mixtures of
chemicals containing at least tens and more often hundereds of constituents.
Differences in the relative amounts of these constituents affect the odour or
flavour. However, the rules relating the composition to the overall perceived
odour sensation are not understood. In some instances the odour of a natural
material is dominated by a single odorant and the other components exert a
secondary, more subtle, effect.
To illustrate the complexity of commercial perfumes and of natural flavours
we will consider two examples: a classical floral chypre perfume and the
constituents of three beverages.
The chypre perfume is a balance of contributions from ten odour "notes" (or
odour classes), Figure 4. Within each odour note a variety of chemical species
is used, made up of both synthetic odorants and natural oils, which are of course
themselves complex chemical mixtures. For example the woody note is
contibuted by patchouli oil, vetivert oil, and sandalwood oil whilst the spicy note
is due to pimento oil, nutmeg oil and isoeugenol. Table 1 list the constituents
and their relative amounts.
6
'(jdrw, .AlLll>k
.'!f'k't 'W-4
Afddu;k 'f3JCY:P;JW
$~
:}t-al .Al"""'Y'
A~ .Alitw.
FIGURE 4. The odour notes contributing to a classical floral chyphre perfume.
TABLE 1. A formula for a classical floral chyphre perfume
SYNTHETIC ODORANTS NATURAL OILS
Lilia! (Givaudan) 3 Pimento 1
Aurantiol 2 Coriander (Russian) 2
Terpineol extra 3 Nutmeg (East Indian) 1
Isoeugenol 5 Petigrain (Paraguay) 1
Isoamyl salicylate 3 Ylang-Ylang premier 2
Ethyl acetoacetate 0.4 Lavender (English) 4
Isobutyl phenylacetate 4 Bergamot (Sicily) 8
Phenyl acetate 4 Patchouli (lndonisia) 3
Phenylethyl alcohol 3 Vetiver (Haiti) 1
Anisaldehyde 1 Sandalwood (Mysore) 1
Trimethylundecylenic Oakmoss resinoid 4
aldehyde 0.5 Rose abs. (Grasse) 16
Civetone 2 Jasmin abs. (Grasse) 16
Heliotropin 2 Immortelle abs. 0.5
Vanillin 3 Tuberose abs. 1
Galaxolide (IFF) 1 Hyacinth 0.5
Androl (CPL) 1
Pyrolide 0.1
TOTAL 100
For our second example we consider the three beverages, coffee, tea and
cocoa. Each one contains a complex mixture of odorants, which can be divided
up into a number of different chemical classes. For example 670 different
compounds have been characterised and play some role in determining the
flavour of coffee, Table 2. Tea and cocoa contain similar numbers of
components but when the relative ratios of these different classes of chemicals
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TABLE 1. Flavour constituents of coffee
Class Number in class
Hydrocarbons 31
Alcohols 19
Aldehydes 28
Ketones 70
Acids 20
Esters 30
Lact ones 8
Amines 21
Thhiols, Sulfides 13
Phenols 44
Furans 108
Thiophenes 26
Pyrroles 74
Oxazoles 28
Thiazoles 27
Pyridines 13
Pyrazines 79
Miscellaneous 11
TOTAL 670
are compared, Figure 5, some differences between the three beverages are
apparent. Thus coffee contains a greater number of furans, than either tea or
cocoa, while in cocoa esters and acids are more prevalent.
This considerable complexity of natural odours and flavours is further
complicated by the fact that many of the constituent odorant molecules are
labile so that the composition can change with time as oxidative degredation
proceeds. Indeed many of the common off-odours are produced in just this
manner. Consequently it is very difficult to produce standard, complex odours
against which to calibrate electronic noses.
2. The Mammalian System
The sensory tissue that makes up the olfactory system in humans is similar to
that in all higher animals (the biological properties are discussed in some detail
in Chapter 2). The sensory tissue is a patch of thin epitheleum that is located
high up in the nose and contains a large number (-50 million) of olfactory
receptor neurones. Each olfactory neurone has a dendrite that ends in a knob
from which several cilia extend. G-protein binding receptors are located at the
surface of the cilia and act as chemosenory receptors. The exact number of
8
COFFEE
Flavour constituents
FIGURE 5. The relative numbers of odorants in each of 18 chemical classes for
coffee, tea and cocoa.
9
receptor proteins is not known but recent evidence suggests that there may be
several hundred or more. The specificifity of the sensing system is believed to
be derived from a combination of receptor cells with partially overlapping
sensitivities. The cilia also act as transducers as they contain enzymes to
amplify the olfactory signal and generate secondary messengers. The secondary
messengers probably control ion-channel modulation as the olfactory signal is
propagated down axons from the olfactory receptor neurones to several
thousand glomerula which are in turn connected to about 10 0 thousand
secondary olfactory neurones. This neural architecture provides an enormous
amount of computing power through the high degree of convergence at the
primary level, and hence a good amount of fault-tolerance. There are some
similarities between chemoreception, photoreception and mechanoreception
which have been reported recently [8]. Table 3 shows the similarities and
differences between olfaction and the other senses at various levels.
The olfactory bulb is the bulbous tissue in the brain that contains glomeruli
and is where the higher level processing takes place. Two dimensional activity
patterns have recently been observed on the surface of the olfactory bulb [9].
There appears to be no spatial correltaion of area to smell, i.e. there is not an
area for minty and another area for pungent odours, rather the processing
appears to be more in terms of a spatial fourier transform. Thus, damaging one
part of the olfactory bulb restricts the precision of the human sense of smell
rather than its range. Such a biological design is more fault-tolerant and has
several advantages in signal processing. These olfactory bulb activity patterns
have a striking resemblance to the artificial olfactory images that are discussed
in Chapter 18. However, it will be some time before the full details of the
olfactory code are determined. Readers are directed towards an earlier book
that discusses the biological aspects of chemosensory reception [9].
TABLE 3. Comparison of the human senses. Adapted from [8].
Olfaction Taste Vision Audition
Quality coding +-----Ligand specificity - Absorption Vibrational
spectra resonance
Detection +------Protein receptors-------+ Organelle
movement
Transduction +-----Enzymes, carriers - Filters,lenses Outer hair
cell feedback
Sensitivity +----G-proteins, secondary messengers ----+ Direct link
(amplification) to channels
Cellular response +------Ion channel modulation -------+
10
3. The Need for an Electronic Nose
The human sense of smell is still the primary instrument used by various
industries for evaluating the quality of a wide range of products. These include:
- Foodstuffs: Fish, cheeses, biscuits etc.
- Beverages: Beers, lagers, spirits, coffees, whiskies, wines etc.
-Perfumes: Extrait perfumes, deodorants, soaps etc.
- Others: Tobaccos, washing powders, air quality etc.
At present the sensory qualities of these products are evaluated by
oraganoleptic tests, i.e. trained panels of people who smell the products on a
routine basis to maintain product and process control. This is an expensive
process - not-with-standing the problems associated with the variation of the
human sense of smell with age, health and diet! In addition the use of
conventional analytical instrumentation, such as gas liquid chromatography or
mass spectrometry, to determine the odour or flavour of the product, has proved
to be both costly and of limited value. Thus, there exists a great need in
industry for an instrument that is capable of mimicking the human sense of
smell. Progress towards such an instrument has already been reported by us
[10] but recent progress is reviewed in Chapters 4 and 11.
The following chapters in this book review the research that has been
carried out so far into the development of an electronic nose. In fact, they
discuss the development of electronic noses suitable for application in several of
these areas, for example, fish freshness (Chapter 16}, beers, lagers, coffees,
tobaccos (Chapter 11 ), whiskies (Chapter 14 }, air freshness (Chapter 15) as well
as other odorous or environmental gases (Chapters 13 and 17).
References
1. S. C. Kinnamon, Taste transduction: a diversity of mechanisms, T. I. N. S.,
11 (1988) 491-496.
2. H. Shorey, Animal Communication by Pheromones, Academic Press, New
York, 1976.
3. M. G. J. Beets, Structure-activity Relationships in Human Chemoreception,
Applied Science Publishers, London, 1978.
4. D. G. Moulton, A. Turk, and J. W. Johnston, Methods in Olfactory Research,
Academic Press, New York, 1975.
5. G. H. Dodd, Ligand-binding phenomena in chemoreception, in G. Benz
(ed.), Structure-activity Relationships in Chemoreception, IRL Press, Oxford,
1976, pp. 55-61.