Character, odour property

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. 

In 1899 Thoms isolated an alcohol from Peru balsam oil, which he termed peruviol. This body was stated to have powerful antiseptic properties, but has not been further investigated until Schimmel Co. took up the subject. The oil after saponification was fractionated, and after benzyl alcohol had distilled over, a light oil with characteristic balsamic odour passed over. It boiled at 125° to 127° at 4 mm., and had a specific gravity 0 8987, optical rotation -1- 12° 22, and refractive index 1-48982. This body appeared to be identical with Hesse s nerolidol, whilst in physical and chemical properties it closely resembles peruviol. The characters of the various preparations were as follows —... 

Phenyl-ethyl alcohol, or benzyl carbinol, has been known for many years, but its powerful rose odour has been entirely overlooked, its discovery having been made by an ordinary research chemist and not a perfumery expert. Its preparation was described in the Berichte (9, 373) in 1876, but the product there noted was evidently impure, as its boiling-point is recorded as 212°. Commercial specimens vary greatly in both their odour and their keeping properties, some samples deteriorating in odour very rapidly. It is, therefore, very important to -obtain it in a state of the highest purity. It has the following characters —... 

It is a colourless crystalline substance, having the characteristic odour of thyme oil, and possessing very powerful antiseptic properties. Its characters are as follows —... 

Enantiomers display a sensory and biological activity differentiation. Their sensory properties are not univocal. Differences in the character of odours of enantiomers eonfirm the supposition that the enzymatic theory perception is based on the activity of enzymes and odour substances. Systemic enzymes being a chiral catalyst of the highest efficiency act in an enantioselective way, and hence the considerable differentiation in biological effects (various odours). 

The two primary aspects of odour are character and intensity. Perfumers are also interested in properties such as tenacity and performance but these are derivative properties combining intensity with physical and chemical properties such as volatility, surface recognition/adhesion, chemical stability in the perfumed medium, Raoult s law deviations and so on. In order to study any phenomenon, it is important to be able to measure it. Unfortunately, both odour character and intensity are very difficult to measure. Odour is a phenomenon that exists only in the higher brain and must therefore be measured using psychological techniques. Moreover, it is highly subjective, even to the point where it would appear that each of us has a unique odour perception of the world around us, as will be explained later. 

Since the 1920s several research workers have investigated the insecticidal effect of various thiocyanates. The action of these compounds is characterised by the knock-down effect and by the contact character of action. They have therefore mostly been used in combination with other insecticides, such as rotenone, pyrethrum and DDT. From the end of the 1940s onwards their use was gradually discontinued, due mainly to their disagreeable odour and their phytotoxic and skin-irritating properties. 

Odour has three properties, viz. character, intensity and tenacity. The character is the property by which we recognise the material and is usually named by association to a natural source with a similar odour, for example, rose, sandalwood, lemon and so on. The intensity is the perceived strength of the smell and is sometimes measured against a standard of defined intensity or in terms of the odour threshold, i.e. the lowest concentration at which it can be detected. The tenacity, also known as persistence, is the length of time for which the smell lasts after it has been applied to a surface or medium. 

Sample purity is always important when measuring properties for SAR work. In the field of olfaction, the organoleptic purity (i.e. odour purity) is even more important than chemical purity, especially when measuring odour character. A small trace of an impurity with a very intense odour is likely to lead to incorrect results in the odour measurement and thus render the data, and hence any resultant SAR, invalid. For example, 1 ppm of thiomethanol will distort the odour of geraniol far more than would 10% of citronellol. Therefore any SAR on odour will be suspect if the data originates from a source which is unfamiliar with measurement of organoleptic purity. 

Characters and Tests.—A greyish-black solid, with a radiated crystalline appearance. Eesembles iodine in odour, and in the property of staining the skin. Soluble in about sixty parts of glycerin insoluble in water, but decomposed when boiled with it. 

The oils were by distillation separated into ten fractions of 10 per cent. each. The characters of these fractions show that one was dealing with a body having the properties of sesquiterpene derivatives, and the odour of the first fractions suggested West Indian oil —... 

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