Intensity, 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. 

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. 

Chloride of Stanethylium. C4H5SnCl. This salt is best prepared by dissolving oxide of stanethylium in dilute hydrochloric acid on evaporation at a gentle heat or over sulphuric acid in vacuo the chloride crystallizes out in long colourless needles, isomorphous with the iodide, which salt it also closely resembles in all its properties it is however more volatile, and therefore emits a more intensely pungent and irritating odour than the iodide. 

Boelens has also used this approach to derive QSAR equations for musk, jasmine, fruit and bitter almond odorants (Boelens, 1976 Boelens and Punter, 1978 Boelens et al., 1983). In the case of bitter almond and musk, he concluded that hydrophobic and steric parameters were important. For the jasmine materials, he found that molecular connectivity indices were useful parameters. Molecular connectivity indices were also used by their inventors, Kier et al. (1977), to analyse anosmia to fatty acids and the odour similarities of ethereal, floral and benzaldehyde-like odorants. Dearden (1994) also developed a QSAR equation relating the odour similarity of bitter almond odorants to two connectivity indices. Greenberg (1979) found that the odour intensity of a series of homologous compounds was correlated to their hydrophobic properties and not to steric or polar properties, while Rossiter (1996b) found that the fruitiness of aliphatic esters was related to steric hindrance of the ester group and either molecular length or log P. 

In general, energy or work is the product of an intensive factor and an extensive factor. Intensive factors are those properties that are independent of the size of the system, such as temperature, pressure, odour and density. An extensive factor on the other hand is one which depends on the size of the system, such as mass, volume and internal energy. 

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. 

The associated analytical instruments separate out some of the chemical compounds in a complex odour according to a physical property, for example, their solubility, mass, mass to charge ratio etc. Reference [1] provides an overview of these different analytical instruments. The disadvantages with these types of instruments for detecting odours are that they are relatively slow (minutes per reading), large, expensive and, more importantly, simply separate out the different constituents of a compound rather than provide a measure of their relative olfactory intensity. Indeed, the smell of a complex odour may well be dominated by a few key flavour compounds which occur below the detection threshold of even these analytical instruments - typically ng/ml. 

Humic matters cause a deterioration of the sensory properties of water, giving rise to a yellow to brown colour of the water and an unpleasant odour. The intensity of colour increases with increasing pH. 

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. 

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