Odor similarities in structurally related odorants

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Abstract
Similarity assessments against one or several references were used to estimate “earthiness” in 7 pure cyclic alcohols. The “earthy” odor quality of 3 isomers (I, II, IV) of the mold metabolite “geosmin” (1, 10dimethyl-9-decalol C12H22O) was found to be present in 2 other alcohols possessing a partial geosmin structure: exo-2-ethyl-fenchol (C11H22O) (V) and cis-cis 2,6-di-methyl-cyclohexanol (C8H16O) (VI). All have in common an axial hydroxy group attached to a 5 or 6 carbon ring with alpha methyls on both sides. The trans-cis and trans-trans isomers of 2,6 dimethyl cyclohexanol (VII, VIII) both with equatorial hydroxy groups, were ranked as different from the above group. Instead their odor resembles more that of their aromatic precursor — 2,4 dimethyl phenol (IX) — than that of their axial isomer (VI). The two trans-ring geosmin isomers (I,II) and 2-ethylfenchol (V) were still judged earthy at up to 1000× the lowest concentration tested. However, the cis-ring geosmin isomer (IV) and the axial cyclohexanol (VI) were perceived by most subjects to change in quality: earthy at lower concentrations but increasingly different at higher ones. Discrimination between earthy odorants decreased with decreasing concentration. Geosmins I, IV and 2 ethyl-fenchol (V) were particularly confused. However, discrimination did not appear to be affected by prior adaptation to geosmin IV. Subjects appeared to be able to focus on the earthy odor quality component in single odorants IV, V, VI in which several other quality components could be discerned as well.

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