Measuring Smell

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 

Another complication is that most of our descriptions are of complex mixtures and individual chemical signals are not always additive in a simple predictable arithmetic way. For example, it is known that the odours of the enantiomers of carvone are different. 1-Carvone (13.2) smells of spearmint whilst d-carvone (13.3) smells of caraway. Less well known is the fact that addition of nonanol (13.4), an alcohol with an oily smell reminiscent of unperfumed washing up liquid, to 1-carvone, will create an odour impression very similar to that of d-carvone. This 

Intensity measurements are even more problematical than character assessment. Once again, there are no fixed reference points and, in this case, there are no easy comparisons either. Even at the simplest level, presented with two different concentrations of the same material, how do we decide whether one is twice as strong, two and a half times as strong or a hundred times as strong. Why are we not confused by the statement that A large mouse ran up the trunk of a small elephant The answer is that our scales for mice and elephants are different and we automatically adjust without even thinking about it. The same happens when we try to estimate the intensity of odour stimuli. We unconsciously adjust our scales to the framework of the experiment in hand. 

We also make similar adjustments when thinking about character. Most people will easily distinguish between the enantiomers of carvone and therefore they will be described as different. But how different are they Thirty per cent of people cannot distinguish between them (Leitereg et al., 1971) but would readily distinguish either of them from hydrogen sulfide. 

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The most commonly used method to measure smells is with an organoleptic panel, i.e. a group of, typically, 4 to 8 people who sniff professionally the headspace of an odorant compound. In some cases, the headspace is first passed down a gas chromatography column and then the panel smells the separate compounds as they are slowly eluted Irom the end of the column - this is known as an olfactometer. However, organoleptic panels are expensive to train, take a considerable amount of time and effort to detect the compounds, and are subject to considerable variability -perhaps a factor of 3 or more Ifom panel to panel. Consequently, there has been considerable effort to employ other headspace techniques that are well known in the field of analytical chemistiy, such as ... 

Using a standard vacuum distillation the solvent is distilled off. This shouldn t take too long. The first thing to come over after the solvent was the safrole, which with my vacuum (2mm) started at around 9CfC. The safrole will be a clear liquid, slightly viscous and will smell of liquorice. With the above measurements one can expect a yield of around 85g. No further cleaning up is necessary, and the safrole can be used as is for any further reactions. ... 

Odors are measured by their intensity. The threshold value of one odor to another, however, can vary greatly. Detection threshold is the minimum physical intensity necessary for detection by a subject where the person is not required to identify the stimulus, but just detect the existence of the stimulus. Accordingly, threshold deterrninations are used to evaluate the effectiveness of different treatments and to estabflsh the level of odor control necessary to make a product acceptable (8). Concentration can also produce different odors for the same matenal. For example, indole (qv) in low concentrations has the smell of jasmine and a low threshold of perception. In high concentrations, it has a strong odor of feces and CX-naphthyl amine as well as a considerably higher threshold of perception. 

Cathodic protection with impressed current, aluminum or magnesium anodes does not lead to any promotion of germs in the water. There is also no multiplication of bacteria and fungi in the anode slime [32,33]. Unhygienic contamination of the water only arises if anaerobic conditions develop in the slurry deposits, giving rise to bacterial reduction of sulfate. If this is the case, HjS can be detected by smell in amounts which cannot be detected analytically or by taste. Remedial measures are dealt with in Section 20.4.2. 

Four characteristics of odor are subject to measurement by sensory techniques intensity, detectability, character (quality), and hedonic tone (pleasantness-unpleasantness) (16). Odor intensity is the magnitude of the perceived sensation and is classified by a descriptive scale, e.g., faint-moderate-strong, or a 1-10 numerical scale. The detectability of an odor or threshold limit is not an absolute level but depends on how the odorant is present, e.g., alone or in a mixture. Odor character or qualit) is the characteristic which permits its description or classification by comparison to other odors, i.e., sweet or sour, or like that of a skunk. The last characteristic is the hedonic type, which refers to the acceptability of an odorant. For the infrequent visitor, the smell of a large commercial bread bakery may be of high intensity but pleasant. For the nearby resident, the smell may be less acceptable. 

Limits on emissions are both subjective and objective. Subjective limits are based on the visual appearance or smell of an emission. Objective limits are based on physical or chemical measurement of the emission. The most common form of subjective limit is that which regulates the optical density of a stack plume, measured by comparison with a Ringelmann chart (Fig. 25-1). This form of chart has been in use for over 90 years and is widely accepted for grading the blackness of black or gray smoke emissions. Within the past four decades, it has been used as the basis for "equivalent opacity" regulations for grading the optical density of emissions of colors other than black or gray. 

There are two types of physical properties qualitative properties and quantitative properties. Qualitative properties are those that caimot be measured, such as smell or taste. Quantitative properties, on the other hand, can be given precise mathematical values, for example, the weight of a certain volume of a substance (density), the temperature at which the substance boils (boiling point), or electrical conductivity. 

In the foregoing we loosely talked about the intensity of a sensory attribute for a given sample, as if the assessors perceive a single (scalar) response. In reality, perception is a dynamic process, and a very complex one. For example, when a food product is taken in the mouth, the product disintegrates, emulsions are broken, flavours are released and transported from the mouth to the olfactory (smell) receptors in the nose. The measurement of these processes, analyzing and interpreting the results and, eventually, their control is of importance to the food... 

Natural gas a clean, reliable, and energy-efficient fuel an odorless, colorless, combustible fuel. These attributes accurately characterize the form of energy known as natural gas. Nevertheless, we are constantly advised to contact the gas service company whenever we detect the smell of gas as a precautionary measure to locate leaks. An examination of the chemical composition of natural gas can explain this paradox. 

In humans, breathing acrylonitrile at a concentration of 16 parts of acrylonitrile per million parts of air (ppm) causes headaches, nausea, and disorientation (Table 1-1). This concentration is close to that at which acrylonitrile can be smelled in air (about 21 ppm). Breathing acrylonitrile in air for long periods of time and at high concentrations can cause death. The actual concentrations of acrylonitrile and breathing times which cause death have not been measured. There is no information on human health effects from eating or drinking acrylonitrile. Acrylonitrile can be smelled at a concentration of 19 ppm when dissolved in water. 

The second possibility to measure poor air quality is the detection of volatile organic compounds (VOC), but the number of different VOCs in room air is high, and often their impact on human health or comfort is not known. Additionally, the inconvenience caused by VOC s and smell is difficult to measure because it depends on human rating. 

It is easily forgotten that aspirin degrades to acetic as well as salicylic acid. And, indeed, any smell aspirin might have is due to acetic acid. However, the volatility of acetic acid does not make the determination of acetic acid a reliable tool to measure stability or degradation. 

As noted above for the AEGL-1, chronic occupational exposure of adult males to >10 ppm produced symptoms of headache, weakness, changes in taste and smell, irritation of the throat, vomiting, and effort dyspnea (El Ghawabi et al. 1975 NIOSH 1976 Blanc et al. 1985). For a few individuals, chronic exposures occasionally produced more serious adverse effects, such as fainting and psychotic episodes. There was no evidence that these symptoms occurred after one exposure. A concentration of >25 ppm for 1 h resulted in numbness, weakness, vertigo, nausea, rapid pulse, and flushing of the face (Parmenter 1926). Only one individual was involved, and neither the exposure duration nor the concentration were measured. 

The results from total odour strength measurements of different chemical scrubbers, show odour reduction efficiencies between 95 per cent and 98 per cent. ED50 of the cleaned air has been found to be between 50 and 100, and the air has been characterized as free from sewage odours, but it smells like chemicals . It seems as if a chemical sembber always gives this scrubber odour . 

There is, too, an important aspect which adds to the problems we face. Human reaction is often to transient smells, perhaps lasting less than a minute. Indeed, variation may often create more complaint than a steady level of smell. But it is difficult to collect enough samples over such a short time to allow measurement by a panel or even by chemical means. And it is equally difficult to be ready to sample when the problems of smell are being experienced. 

Furthermore, the olfactometer should be able to deliver the above mentioned ranges of stimuli over a wide range of dilution ratios (from 1 to 10 6) in order to cover the variability of human sensitivity to different substances. Such a range may still be too small in some cases. If so, provisions for predilution may be required. Finally, it will be clear that both the dilution air and the ambient air in which the measurement takes place should be odour free. Although this may seem quite obvious, in many cases this requirement is not fulfilled. Either the dilution air has an intrinsic smell which is insufficiently filtered out or parts of the olfactometer give off an odour. Thus, contact of the dilution air to be delivered to the nose with pumps, ventilators, valves etc. should be avoided as much as possible. 

As part of a study to test the use of population panels as a method for assessing odour annoyance in a direct way, a comparison of the odour annoyance experienced by such population panels and the odour immission concentrations expressed in odour units/m3 of air samples taken simultaneously, was made. It was found that in a city where odour annoyance occurs regularly, no relationship could be found between the amount of odour annoyance experienced and the immission concentrations. Furthermore, it was shown that the odour concentrations of pleasant smells (meadows forest) in an unpolluted area may be as high as 24 odour units. It is concluded that the amount of annoyance caused by odours can not be deduced from concentration measurements, but should be assessed in a direct way. Population panels provide a good means of obtaining such data. They are reliable and can give indications about the important sources of annoying odours in complex industrial areas. 

Odour measurement by chemical analysis of the odorous compounds presents also a number of limitations. As will be shown further on, it is a difficult analysis, which is essentially due to the great sensitivity and specificity of the human sense of smell. But... 

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