2
C o r r e l a t i o n of O d o r Intensities a n d V a p o r Pressures with
Structural Properties of O d o r a n t s
ANDREW DRAVNIEKS
Downloaded by COLUMBIA UNIV on June 27, 2013 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0051.ch002
Odor Sciences Center, IIT Research Institute, Chicago, Ill. 60616
The relationship of perceived odor intensity to the concentration of odorants in headspace samples of various materials tends to follow a similar mathematical function but with different coefficients for different odorants. The task of obtaining experimental data for all compounds of interest would be overwhelming. It is desirable, therefore, to evolve procedures which would enable generalization and estimation of these coefficients from some molecular properties. (1,2) The most readily accessible properties are those derivable from the structural chemical formulas of the compounds. Attempts in this direction achieved some success (3) but were obtained for a relatively narrow selection of compounds. The present study expands odor data to include a more diversified selection of odorants. Furthermore, the properties derivable from the molecular structures were selected in such a way that they could be inventoried directly from inspection of the one-line Wiswesser notation formulas of compounds. Such formulas u t i l i z e specific symbols for certain structural charact e r i s t i c s of molecules (e.g., U for double bond, R for benzene ring, Q for hydroxyl group), and in principle permit their enumeration by computerized methods. Odor Intensity Measurements Concentration of odorants in headspace vapors of a flavorpossessing sample can be measured by appropriate analytical means. The intensity of the odor sensation experienced when smelling the sample can be derived from such data i f the dose-response r e l a tionship is known; dose is described by the types and concentrations of the odorants in the headspace, and response by the intensity of the resulting odor sensation. Three principal methods exist to describe how strong an odor is: 11
In Flavor Quality: Objective Measurement; Scanlan, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
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FLAVOR QUALITY:
OBJECTIVE
MEASUREMENT
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(1)
Magnitude e s t i m a t i o n : "Smell the odor and give a number or some other type of response ( f i n g e r span, length of a l i n e ) i n p r o p o r t i o n to the experienced odor i n t e n s i t y " . The choice of the numbers i s e i t h e r l e f t to the best judgement of the subject or may be "pegged", p r e s c r i b i n g that the i n t e n s i t y of another reference stimulus i s given some d e f i n i t e value, f r e q u e n t l y 10 (ten). The r e s u l t i n g numbers are p r o p o r t i o n a l to the perceived odor i n t e n s i t i e s . Figure 1 i l l u s t r a t e s r e s u l t s of the magnitude e s t i m a t i o n of two odorants at s e v e r a l a i r d i l u t i o n l e v e l s . (2) Category s c a l i n g : "Judging the i n t e n s i t y on a s c a l e of zero to f i v e (or some other f i x e d numerical s c a l e ) " . The r e s u l t ing numbers are not p r o p o r t i o n a l to the perceived odor i n t e n s i t y . Typicaïly7~an ô d ô " ô n e u n i t higher than another on 0-5 s c a l e i s a c t u a l l y perceived to be stronger by a f a c t o r of 3 or 4. (3) Reference s c a l e : "Compare the odor i n t e n s i t y of a sample to a reference s c a l e c o n s i s t i n g of a s e r i e s of known concentrat i o n s of a s e l e c t e d odorant and i n d i c a t e the best i n t e n s i t y match". The magnitude estimate method gives the t r u e s t r e p r e s e n t a t i o n of the perceived i n t e n s i t y . The reference s c a l e method permits the e a s i e s t documentation and t r a n s f e r of information on the odor intensity. The f o l l o w i n g f u n c t i o n s approximately r e l a t e the magnitude of the perceived odor i n t e n s i t y S, category s c a l e number Ν and the odorant c o n c e n t r a t i o n C Ν = m log S
1.
s=
2.
The c o e f f i c i e n t s m and k depend on the choice of u n i t s f o r S; m, a l s o on the choice of u n i t s f o r the category s c a l e . In a d d i t i o n , k and η depend on the type of odorants. Equation 2, known as the psychophysical power f u n c t i o n (sometimes r e f e r r e d to as Stevens law) b e t t e r d e s c r i b e s the r e l a t i o n between odor i n t e n s i t y and odorant c o n c e n t r a t i o n f o r the great m a j o r i t y of odorants over a broad range of concentrations than the t r a d i t i o n a l WeberFechners law. For a few odorants, Equation 2 seems to hold true only over a c e r t a i n range or ranges of concentrations. Recently, a 1-butanol (n-butanol) reference s c a l e (4) has been e s t a b l i s h e d as a recommended p r a c t i c e , ASTM E544, f o r r e f e r encing odor i n t e n s i t y ("sample smells as strong as X ppm v o l / v o l of 1-butanol i n a i r " ) . F i g u r e 2 i l l u s t r a t e s one p h y s i c a l form of t h i s s c a l e . For n-butanol,
,0.66
3.
S = kC log
S = l o g k + 0.66
log C
In Flavor Quality: Objective Measurement; Scanlan, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
DRAVNiEKs
Structural
Properties
of
Odorants
Downloaded by COLUMBIA UNIV on June 27, 2013 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0051.ch002
ο
RELATIVE VAPOR PRESSURE OF ODORANT ( SATURATED VAPOR=l )
Figure 1. Effect of dilution on odor intensity; obtained by magnitude estimation method
Figure 2. A form of vapor dilution olfactometer to serve as an ASTM 544 odor suprathresnold. Intensity referencing scale based on 1-butanol.
In Flavor Quality: Objective Measurement; Scanlan, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
14
FLAVOR QUALITY:
OBJECTIVE
MEASUREMENT
Moskowitz, et a l (5) have proposed to d e f i n e the odor i n t e n s i t y of 250 ppm v o l / v o l of n-butanol i n a i r as 10. With C taken i n ppm ( v o l / v o l ) u n i t s , Equation 3 becomes:
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S = 0.261
C
0 , 6 6
4.
I t permits t r a n s l a t i o n of 1-butanol referenced values of i n t e n s i t y to a open-ended numerical s c a l e (both ends open) i n which the numbers are approximately p r o p o r t i o n a l to the perceived odor intensities. In such a s c a l e , odor i n t e n s i t i e s
