Odorous impression

Jiroma. The fragrance or odor of food, perceived by the nose by sniffing. In wines, the aroma refers to odors derived from the variety of grape, eg, muscat aroma. It is the overall odor impression as perceived by the nasal cavity. 

Oakmoss. Extracts of oakmoss are extensively used in perfumery to furnisli parts of the notes of the fougnre or chypre type. The first step in the preparation of an oakmoss extract is treatment of the Hchen Evemiaprunastri (L.) Ach., collected from oak trees mainly in southern and central Europe, with a hydrocarbon solvent to obtain a concrete. The concrete is then further processed by solvent extraction or distillation to more usable products, of which absolutes are the most versatile for perfumery use. A definitive analysis of oakmoss volatiles was performed in 1975 (52). The principal constituents of a Yugoslav oakmoss are shown in Table 15 (53). A number of phenoHc compounds are responsible for the total odor impression. Of these, methyl P-orcinol carboxylate is the most characteristic of oakmoss. 

Since muscone (101), by itself, does not reproduce the total odor impression of this musk, IFF chemists (56) as early as 1971 in an analysis of tincture of Tonquin musk, reported a series of macrocycHc ketones (Table 16) which play a key role in creating the characteristic odor of this musk (11). The introduction of a double bond into a macrocycHc ketone (eg, 102) changes the odor from flowery musk to animal musk. 

The relative contribution of a particular compound (its odor or flavor value) to the odor impression of a composition can be expressed as the ratio between the actual concentration of the compound and its threshold concentration [9, 9aj. 

As a rule furan derivatives are considered important aroma constituents from a sensory point of view. They are mainly associated with sweet, fruity, nutty or caramel-like odor-impressions. The furans have no meaty characteristics, but it seems possible that they contribute to the overall odor of broiled and roasted meat. 

One obtains a physically detectable signal but no odor impression at the same retention time. 

When one is fortunate enough to have an idea of the structure of one or several odor producing substances then the next step is to synthesize the substance or substances or reproduce them using another way. Only when the same retention time, the same mass spectrum and the same odor impression are obtained as in the sample material can the identification be considered to be assured. 

An odor can be described by the combination of threshold value and odor quality. The threshold value, the lowest concentration that creates an odor impression, can be considered the intensity factor, whereas the odor quality describes the character of the aroma. As has been mentioned under olfactory theories, attempts at reducing the number of characteristic odor qualities to a small number have not been successful. In many cases, the aroma and flavor of a food can be related to the presence of one or a few compounds that create an impression of a particular food when smelled alone. Such compounds have been named contributory flavor compounds by Jennings and Sevenants (1964). Some such compounds are the pyrazines, which give the odor quality of green bell peppers nootkatone for grapefmit esters for fruits ... 

I n analogy to other- better u nderstood - receptor systems like some hormone and opiate receptors it is generally accepted that the olfacto receptors are proteins, and there are some facts known that support this hypothesis. One of these arguments is that, sometimes, slight modification of the chemical structure of a stimulus molecule can lead to big changes in the odor impression this might be qualitative or quantitative. 

Proteins are chiral, so they should interact differently with the two enantiomeric forms of a chiral molecule, which should eventually translate into a difference of the odor impression of these mirror images of the molecules. A more detailed knowledge of the relations between the chemical structure of a molecule, including its absolute configuration, and its odor properties will contribute to the elucidation of the receptor mechanism. 

These examples that add to the existing list show to what extent modification of the chemical structure of a molecule can alter the perceived odor. The fact that two enantiomeric forms of odorants show distinct differences in their organoleptic properties supports the hypothesis that the initial event, the interaction of the stimulus with the receptor is enantiose-lective, leading to diastereoisomeric stimulus-receptor complexes and these events are transduced to give rise to different odor impressions, the mechanism of which remains to be discovered. 

In situ conversion of essential N A flavor components from one molecular form to another, as a result of ion pairing, is common in food and drug products. Therefore, the inadvertent conversion of flavors between types during drug formulation studies (e.g., effect of pH, salts, and temperature) can present a serious challenge in flavor-quality assessment. The fact that one and the same N A flavor component can deliver several flavor and odor impressions implies that a blend of several flavor compounds would be preferable. Such blends show improved stability. In addition, flavor impressions from N A flavor blends are usually... 

Whether an odorous impression is considered pleasant or unpleasant is largely determined by the functional groups of the chemical compounds (Table 18.1.7). 

Odorous impression Functional group Substance group... 

A series of increasing concentrations of a chemically pure stimulus material in a neutral substrate (water or oil) may be used to determine sensory thresholds to those compounds. The assessors have to indicate the concentration at which stimulus is perceived for two successive concentrations. Increasing concentrations of sucrose, sodium chloride, citric acid, and caffeine are used for the basic tastes sweet, salt, acid, and bitter. Likewise, the olfactory threshold is determined by using some odorants in aqueous or alcoholic solutions (absolute ethanol is often used as a solvent), to measure the ability of the panelists to identify an odor impression and evaluate their odor memory. 

Other GC-0 applications are also reported in literature using the so-called peak-to-odor impression correlation, the method in which the olfactive quality of an odor-active compound perceived by a panelist is described. The odor-active compounds of the essential oils of black pepper Piper nigrurri) and Ashanti pepper Piper guineense) were assessed applying the aforecited correlation method [78]. The odor pro le of the essential oils of leaves and owers of Hyptis pectinata (L.) Poit. was also investigated by using the peak-to-odor impression correlation [79]. 

Other GC-O applications are also reported in literature using the so-caUedpeak-to-odor impression correlation, the method in which the olfactive quality of an odor-active compound perceived by a panelist is described. The odor-active compounds of the essential oils of black pepper... 

The most crucial precondition for the application of hydroformylation in fragrance production is ensuring a high degree of chemo- and regioselectivity, since even small amounts of byproducts can spoil the desired overall odor impression. In case of asymmetric hydroformylation, the formation of almost perfect enan-tioselectivity is aimed, since most chiral odorants are perceived by humans in a highly stereoselective manner [28]. Noteworthy, in rare cases, also naturally occurring compounds (e.g., some terpenes) may not be completely enantiomerically pure [29]. 

The odor impression of the product mixture was estimated to be slightly green, aldehydic, sweet, floral, spicy, cherry, and almond. 

Aldehydes and alcohols may resemble each other in their odor characteristics. However, in several cases (e.g., Cg-enals in relation to their corresponding alcohols) the odor strength of alcohols is 100-1000 times lower, which might be important for achieving certain odor impression [190]. 

An entirely unsolved problem concerns the iso-selective hydroformylation of tertiary olefins. Thus, it is known that 2-methyl-indanyl-2-carbaldehydes and then-corresponding alcohols can show a distinct odor impression. The subsequently illustrated branched alcohol was found as a new member of floral muguet (Uly-of-the valley)-type odorants (Scheme 6.79) [5]. The compound can be considered as a conformationally restricted analog of Majantol , a trademark of Haarmann Reimer (now Symrise) and Wacker, which is now also manufactured by Shanghai Richem. 

Up to now it was not possible to establish general rules as to correlations between odor quality and chemical structure. Some valuable concepts have, however, been established by approaching the molecular basis of stimulant interaction at the olfactory receptor level (16, 34, 35, 36, 45, 46, 313, 346, 446, 448, 452, 602, 630, 631). The attempts to create a code of odor impressions for flavors are still rudimentary (213, 447, 612). Psychologists interested in sensory measurements try to use physicochemical data of stimuli to establish empirical sensory correlations (131, 238, 324, 365, 392, 393, 396, 477, 533). Several techniques for the characterization and measurement of odor qualities for practical purposes have been described (328, 392, 477). 

A careful organoleptic evaluation of the a,p-unsaturated aldehyde (17) which was detected in various food products such as carrot root oil (72), tomato (74), beef (342) and cranberry (17) has shown that this aldehyde has some remarkable flavor properties. Above the threshold concentration of about 0.1 ppb (72), for example at 0.4 to 2 ppb in water, ( )-2-nonenal (17) possesses a woody character (470). Above 8 ppb the sensory impression turns into a fatty one which becomes unpleasant above 30 ppb. Finally, an aqueous solution of 1,000 ppb of aldehyde (17) has a strong taste of cucumber. The fresh-brew woody note of roasted and ground coffee as well as the woody effect in bell peppers is due to the presence of this compound (470). Addition of (jE)-2-nonenal (17) to cranberry juice at a level of 1 ppb causes a considerable reduction in the normal astringent character of the juice (470) without any change of the original odor impression. An antagonistic effect of (17) is observed on the flavor of (Z)-3-hexenal (5) (375). Above 2 flavor units (E) 2 nonenal (17) causes the stale flavor of spoiled beers (381). 

Together with (H-)-pulegone (78%) [derived from alcohol (49)], two new /7-menthene ketones, viz. (+)-4-hydroxypiperitone (92) 410) and ( —)-8-hydroxy-4-/7-menthen-3-one (93) 411), were isolated from Seto-mint grown in the Tokai area. Both hydroxyketones (92) and (93) develop a strong minty odor impression 443). A detailed report on the natural occurrence, the methods of preparation and the chemistry of piperitone [carbonyl derivative of (48)] and related compounds was published recently 422). 

The smallest monocyclic aroma compounds of the carotenoid series are the C-9 fragments (279) to (281). They occur in various oxidation states in Burley tobacco (111, 502) and tea (397, 491). 2,2,6-Trimethylcyclohex-2-ene-l,4-dione (280), which also occurs in saffron (727), produces an odor impression similar to that of dried leaves or straw... 

Another more elaborate variant of the dilution analysis requires, in addition, that the duration of each odor impression is recorded by a computer and CHARM values are calculated (CHARM acronym for combined hedonic response measurement), which are proportional to aroma values. The result of an AEDA can be represented as a diagram The FD factor is plotted against the retention time in the form of the retention index (RI) and the diagram is called a FD chromatogram. 

Instead of the dilution steps you may use the log of the fiavor dilution (FD) factor. This corresponds to Stevens s law, that sensory impression follows the logarithmic scale of the concentration. The result is a profile that can be read as a sensory profile, and people with a trained memory of the odor impressions of molecules will be able to smell the profile. It is, to my understanding, the first direct objective link between analytical results and a sensory flavor profile, linking the contribution of character impact molecules to the perceived sensory profile. 

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