Odour character

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. 

1 Odour Character. Most odour correlation work has been done on character since it is, superficially, the easiest to measure. However, this is a misconception based on the simplistic assumption that, since a subject presented with, for example, samples of rose oil and eucalyptus oil will normally correctly identify the botanical origins of the samples, it can be taken that all odour character descriptions are equally facile. However, this is not the case. For example, one serious issue is that the description of an odour is associative since we have no hard reference points. 

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Among the leafy spices, 45 aroma volatiles of desert parsley have been identified, with the major constituents as myristicin, apiole, /3-phellandrene, p-mentha-1,3,8-triene and 4-isopropenyl-l-methylbenzene (MacLeod eta/., 1985). Among these, apiole in particular has a desirable parsley odour character. The leaf stems of celery show three main constituents of volatiles, e.g. apiole (about 23%), 3-butylphthalide (about 22%) and sedanolide (about 24%). The last two possess a strong characteristic celery aroma (MacLeod et al., 1988). Limonene (40.5%), P-selinene (16.3%), cis-ocimene (12.5%) and P-caryophyllene (10.5%) are some of the volatile oil constituents present in celery leaves from Nigeria (Ehiabhi et al., 2003). 

Each odour is smelled by the panellist, who then scores the perceived intensity of each odour character that she or he can detect (referring to the set of standard odour references for clarification if necessary), which results in a sensory profile for that odour. A minimum of at least 20 profiles is usually collected for each sample and an average profile is then calculated. A set of typical odour profiles is shown in Figure 8.3. These profiles show the differences in perceived intensity of 13 odour characteristics identified in seven perfume materials, and immediately it is possible to see that although all of the materials are floral or muguet in character, one material is far more fruity (cyclamen aldehyde) and another (Mayol ) is far more herbal than the other materials. 

The samples are grouped so that those that are most similar in odour character are closest together on the map (e.g. Lilial and Bourgeo-nal ), and those that are most different are furthest apart (e.g. cyclamen aldehyde and Lyral ). The arrows indicate the direction of increasing perception of the odour characteristics shown. Only the odour characteristics that significantly correlate with the distribution of the samples across the map are shown these are the characteristics that are responsible for the systematic differences between the samples. 

Those fragrances that are closest together on the map are most similar in odour character, whilst those that are furthest apart are most different. So, for instance, fragrances A59 and C33 are relatively similar in odour character, while A59 and D28 are quite different. To interpret the map and describe the nature of the differences between the odours, a correlation analysis is carried out. This analysis enables us to identify the characteristics that are most important in distinguishing between the fragrances, and the direction of increasing perception of each of these characteristics is indicated on the map with an arrow. So we can tell, for instance, that A59 is perceived to be far more fruity than D28. 

We used a similar type of analysis to look for correlations between the odour character of the fragrances (defined by their position on the map) and results obtained from large-scale market research. The market research attributes found to be directly related to the odour perception of the samples are shown on the map in Figure 8.6, with arrows indicating the direction of increasing perception of each attribute. 

Meta and para isomers have similar odour characters and intensity. The ortho isomer has a weaker odour of a different type. 

A range of soaps was profiled by the sensory panel and the results were analysed to give a two-dimensional map, showing the relative similarities in odour character of the perfumes tested (Figure 8.5). 

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. 

At first it might seem that odour character is easy to measure. One smells a rose and defines the odour as rose. However, as any gardener will tell you, different roses have different scents, so how do we set up a scale of rosiness This problem increases when evaluating single chemicals, either natural rose components or novel substances, which have a rose character, or parts of the rose character. For example, how do we rate the rose character of the three major chemicals responsible... 

Determination of odour character is complicated further by the subjectivity of odour. For example, it is easy to demonstrate that, in blind tests, Bangalol (13.1) is perceived as sandalwood by some people, as musk by others and to a small minority, it is completely odourless (Figure 13.3) (Sell, 2000). 

Very intense-smeUing sulfur compounds with low thresholds of perception often act as key aromatic compounds. They often have very similar structures and contain an important odorophore (a group responsible for the odour character), which is sulfur and oxygen in positions 1 and 3 of the molecule (8-120). This structure occurs in mercaptans, thioacetals, mercapto ethers, mer-capto ketones, alkylthio esters and other compounds listed in the following sections. 

Landfill gas is usually slightly lighter than air which promotes its release into the atmosphere. Density of LFG lowers with the increase of CH concentration, which is colourless and combustible gas that burns with blue flame (Itodo et al. 2007). Due to the presence of harmful substances, flammability and odour character, the aim is to reduce LFG emissions to the environment, subjecting it to utilization. The choice of the method of landfill gas utilization is influenced by its quality and quantity (Figure 2.1). When the concentration of CH in LFG is in the range of 35 0% and the output exceeds 30 m h, it is technically... 

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