Odour quality

The use of retention data in combination with odour detection provides a valuable aid to the computer assisted GC-MS analysis of osmogenes. Experience in use of the technique frequently allows quite accurate predictions to be made on sample composition based on retention data and odour quality. 

Odour nuisance is a combination of odour intensity and odour quality. Odour intensity is a function of the number of times odourous air must be diluted with odour-free air (i.e. a dilution factor) for 50% of an odour panel to just detect the odour (8 9). It can also be measured by using a panel of people and a scale of intensity on which the strength of the odour is indicated (16). Large panels of people are needed for these measurements, since differences in sensitivity of individuals are known to be large (10, 11 12). 

Odour quality depends not only on the sensitivity of the human nose but also on the subjectivity of the human language to be able to describe the odour (17). Some chemical characteristics of slurry have been compared to the slurry odour. A relationship between volatile fatty acids (VFA) and odour offensiveness of poultry manure was described by... 

The relationship between odour quality and chemical structure is of considerable practical and theoretical interest. A numt r of methods have been used to determine quantitatively the relationships between the structure of a molecule and its odour quality (7). Though quantitative results were not obtained, a number of interesting theories were present in that the intermolecular interaction in olfaction involved electrostatic attraction, hydrophobic bonding, van der Waals forces, hydrogen bonding, and dipole-dipole interactions. Hydrophobic interactions also appeared to be a major force for substrate binding in olfaction. It had previously been shown that lipophilicity and water solubility were factors diat significandy influenced the odour thresholds of the pyrazines (8),... 

PLS does not appear to have been applied to QSAR of flavours, and although much process has been made in the field of flavour chemistry, a greater insight into odour quality could be derived by the concept of applying many physico-chemical descriptors to the appropriate molecules. 

To test the potential of PLS to predict odour quality, it was used in a QSAR study of volatile phenols. A group of trained sensory panelists used descriptive analysis (28) to provide odour profiles for 17 phenols. The vocabulary consisted of 44 descriptive terms, and a scale fiom 0 (absent) to S (very strong) was used. The panel average sensory scores for the term sweet were extracted and used as the Y-block of data, to be predicted from physico-chemical data. 

The following review focuses on the composition of flavour compounds in spirit drinks, their origin, and their sensory attributes like odour quality and threshold value. Important information on flavour-related aspects of technology, like distillation and ageing, as well as the main categories and brands of spirits to be found on the national and international markets are summarised. Finally, aspects of sustainability in the production of distilled spirits are discussed. 

Table 10.1 gives a summary of the main by-products of fermentation by yeasts and other microbiological activities which can be identified in distilled spirits from different raw materials, like fruits, wine, grain, sugar cane, or other carbohydrate-containing plants. Since the sensory relevance of a flavour compound is related to its odour thresholds and odour quality. Table 10.1 presents also odour qualities and a review of threshold values of the fermentation by-products in ethanol solutions [9-10] and/or water [11-14] (Christoph and Bauer-Christoph 2006, unpublished results). 

Table 10.1 Odour quality, odour threshold value in water and/or ethanol solution, and concentration range of single volatile compounds in distilled spirits produced during alcoholic fermentation from carbohydrates by yeasts and other microorganisms ...

Table 10.2 presents a summary of odour qualities, odour thresholds in water, and concentrations of some selected volatile compounds, which are characteristic flavour impact compounds, owing to their typical flavour quality and their rather low odour thresholds. These compounds are not formed during fermentation but originate from the raw material and contribute significantly to the typical flavour of a fruit. The components summarised in Table 10.2 are important compounds in wine and different fruits and are discussed later. 

Table 10.3 Odour qualities and threshold values of aroma wood impact compounds from toasted oak...

In most cases the concentrations of the compounds detected by GCOH are too small for the identification experiments however, this disadvantage can be overcome when the odorants present in food are first detected in the extract by GC-O and then identified. Some of these odorants are also found by GCOH. As their odour quality, GC properties and chemical structures are known, they are easily identified in the headspace sample. In the case of parsley, a comparison of Fig. 16.2 with Table 16.5 indicates that odorant nos. 4, 6, 9, 11, 12 and 15 (Table 16.5) were known from AEDA. Further applications of GCOH are reviewed in [1]. 

In the identification experiments, the GC and MS data of the analytes have to be compared with those of corresponding authentic samples. However, as mentioned already, odorants are often concealed in the gas chromatogram by major volatile compounds therefore, to avoid misidentification it is necessary to compare by GC-O the odour quality of the analyte with that of the authentic sample at approximately equal levels. The analyte, which has been perceived by GC-O in the volatile fraction, is only correctly identified if there is agreement in the sensorial properties, in addition to GC and MS data. 

Many of the individual reaction products isolated from this reaction have a characteristic meat odour, with thiofuran derivatives the most interesting. The amount of unsaturation and the position of the methyl group play predominant roles for the odour quality as ilustrated in Figure 4. 

Odour is not just a question of intensity, but also of quality. One can try to pick the odour qualities most relevant to Maillard reaction products from Harper s list of44 279... 

The numbers attached are those from Dravniek s scheme of 146 odour qualities, jm which the following can usefully be added ... 

The volatiles of fresh leaves, buds, flowers and fruits were isolated by solvent extraction and analysed by capillary gas chromatography-mass spectrometry. Their odour quality was characterized by gas chromatography-olfactometry—mass spectrometry (HRGC-O-MS) and aroma extract dilution analysis (AEDA). In fresh bay leaves, 1,8-cineole was the major component, together with a-terpinyl acetate, sabinene, a-pinene, P-pinene, P-elemene, a-terpineol, linalool and eugenol. Besides 1,8-cineole and the pinenes, the main components in the flowers were a-eudesmol, P-elemene and P-caryophyllene, in the fruits (EJ-P-ocimene and biclyclogermacrene, and... 

The significance of a minor component having an important contribution to the odour qualities is illustrated by P-damascenone. Although only present at about 0.14%, it gives 70% of the total odour. 

The three compounds presented in Table 6.34 are the key odorants of butter [63]. A comparison of the odour profiles of five samples of butter (Table 6.35) with the results of quantitative analysis (Table 6.34) show that the concentrations of these three odorants, which were found in samples 1 and 2, produce an intensive butter aroma. In samples 3 and 5, the concentration of 2,3-butanedione is too low and, therefore, the buttery odour quality is weak. In sample 4, the excessively high butyric acid concentration stimulates a rancid off-flavour. 

The key odorants of two olive oils, I and S, with very different odour profiles (Table 6.36) are listed in Table 6.37. The apple-like and green odour qualities, which are characteristic for oil I, are caused by aldehydes nos. 5 to 7 and 15 (Table 6.37). Their concentration is higher in oil I than in S. The very potent odorant no. 10 occurred only in oil S. As a result of its high OAV and its blackcurrant-like odour quality, it is the character impact odorant of this oil [64, 65]. 

An aroma model was very similar to the aroma of the juice when it contained 0.1% fat in addition to the volatiles listed in Table 6.38 [67], The terpene-like odour quality was decreased and the fruity note was enhanced. Omission experiments (Table 6.39) revealed that only the absence of acetaldehyde (no. 1) and (R)-limonene (no. 19) was detectable with high significance (a = 0.1) by either nasal or retronasal evaluation. Retronasally an aroma difference between the complete model and the model, in... 

The odour intensities of volatiles showing similar odour qualities are partially additive [68]. To substantiate such additive effects, three groups of odorants (terpene hydrocarbons, esters or aldehydes) were omitted from the aroma model for orange juice. For all groups, a significant difference from the complete model was observed (Table 6.39). Omission of esters nos. 12,14 and 15 with ethyl butanoate (no. 13) still present was clearly detectable. This indicates that the fruity quality in the odour profile is enhanced by additive effects. In contrast, no difference was perceivable when (R)-a-pinene (no. 17) and myrcene (no. 18) were omitted. The concentration of the odorants in juice differs depending on the variety. Thus, the weaker citrus note of Navel oranges compared with the above discussed variety Valencia late is due to a 70% lower content of (R)-limonene [67]. 

It is extremely difficult to purify this by-product 2-phenylethanol to the odour quality of that produced by either of the above routes. However, most of the companies, such as ARCO in the USA and Sumitomo in Japan, who run the SMPO process can produce 2-phenylethanol of a quality which can be used in perfumery (often in collaboration with a perfumery company). The amount of 2-phenylethanol available from this route is dictated by the demand for styrene and propylene oxide, the market value is dictated largely by material from the other two routes and all three run in economic balance. 

What does this odour smell like The tests used to describe odour quality are known as odour profiling tests. These are the most complex of the sensory tests and, to ensure good quality, accurate and reproducible data are only carried out by highly trained and experienced sensory panellists. 

The traditional method of essential oil analysis is to extract the plant material by steam distillation or with solvent and then fractionally distil the oil or extract and isolate individual components by chromatographic techniques for subsequent identification by spectroscopic methods. At each step the odour of the fractions and isolates is assessed and those with the desired characteristics are investigated further. To answer the enquiry about the key odour components of broom absolute, first a sample of the absolute that is of an acceptable odour quality is obtained. The absolute is the alcoholic extract of the concrete, which is itself the solvent extract of the flowers of Spartium junceum, Spanish broom, often referred to by its French name Genet. The odour of any natural extract can vary according to the geographical origin and quality of the plant material, the time of year it is harvested and the extraction method used. If no sample of adequate quality is commercially available then the fresh flowers would be obtained from the plant and the extraction carried out in the laboratory. 

As odour similarity ratings, where the odour quality of one odour type is rated against that of a standard. For example, if a scale of 0 to 10 is used, a compound which is identical in odour to the standard is given a score of 10 and one which is completely different a score of 0. 

The last was used by Boelens and Punter (1978) to quantify the odour quality of 16 muguet-smelling materials. These data were then used to derive equation (3), which related the odour similarity (OS) to molecular weight (MW) and the Kier connectivity index (A ). The concept of molecular connectivity was introduced by Randic (1975) and further elaborated by Kier and Hall (1976). It involves the calculation of numerical indices which describe the topology of a molecule. The Kier... 

H. R. Moskowitz and C. B. Warren, Odour Quality and Chemical Structure, American Chemical Society, 1979. 

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