Odour properties

Oliver (2003) reported pure (S)-(+)-linalool from oil of coriander by formation and recrystallization of its 3,5-dinitroben-zoate ester, followed by regeneration and distillation. Fathima et al. (2001) noted that microwave drying affected the colour, appearance and odour properties of coriander. 

Other fused heterocycles have very attractive flavour and odour properties. Pyrazines, in general, are important in many strong food flavours a fused pyrazine with a ring junction nitrogen atom is one of the most important components in the smell of roast meat. You can read about the simple pyrazine that provides green peppers with their flavour in the Box on the next page. 

Finally, although KIP is found to be as active as HIPK in film matrix [89,91], its better compatibility and low-odour properties make the polymeric system superior. 

Odour has three properties, viz. character, intensity and persistence. All three are subjective and can only be measured in sensory terms. Most correlation work has been done on character since it is, superficially, the easiest to measure. However, the description of an odour is associative, since we have no hard reference points. Difficulties in finding correlations can arise from the use of odour classification systems. For instance, in our fragrance work, we found that the use of the term fruity to describe an odour family led to confusion, since the criteria for a molecule to possess an apple odour are not the same as those for pear. We classify the two together because of the similarity of the botanical sources, but this is not necessarily related to the odour properties. To study structure-odour correlations, we must therefore ensure that we are using meaningful parameters. 

Once a compound with the desired odour properties has been discovered it is normal practice to prepare a range of analogues. The reasons for this are two-fold firstly, materials with even better odour characteristics than the parent compound may be identified (i.e. lead optimization) and, secondly, information about the structural requirements for the desired odour characteristics are acquired. The latter is of particular importance when filing a patent. 

Carveol is one of the minor components responsible for the odour of spearmint. It is easily prepared by reduction of carvone. /.vopulegol is prepared from citronellal, as discussed in the section on menthol, and is a precursor to other materials in the group. The phenols carvacrol and thymol are important in some herbal odour types, but the major use for thymol is as a precursor for menthol q.v. Piperitone and pulegone are strong minty odorants. The latter is the major component of pennyroyal oil. 1,8-Cineole is the major component of such eucalyptus oils as Eucalyptus globulus. These oils are inexpensive and so there is no need to prepare cineole synthetically. Menthofuran is a minor component of mint oils and can be prepared from pulegone, though normally, its presence in mint is undesirable because of its odour properties. 

Figure 13.3 Some molecules with interesting odour properties...

It is interesting to compare the odour properties and structures of (3-bisabolol (30) and iso-(3-bisabolol (29). Changing the position of just one double bond converts an odourless molecule into a strong floral... 

Berends and van der Linde (1967) synthesised and organoleptically assessed 24 analogues of cyclamenaldehyde (Figure 14.7). They varied the nature of the alkyl substituent at the para position, altered the position of the alkyl group and replaced one of the a-hydrogen atoms with small alkyl groups. This enabled them to make the following qualitative conclusions about the effect of structure on the odour properties of substituted phenylpropanals. 

Following the exciting discovery of 1-methyl-3-(2-methylpropyl)cyclo-hexanol, we synthesised over 25 analogues to investigate the effect of the alkyl substitution pattern and ring size on the odour properties of... 

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