Odor unit

In the CEN procedure, the European odor unit per cubic meter, ou m-3 is used. The odor concentration at the detection threshold is defined as equal to 1 ou m-3. 

Furthermore, as an extract of a natural product is concentrated, the number of odorants detected increases indefinitely. Clearly, most of the odorants in a natural product are below their odor threshold, and it is only the most potent compounds that are involved in generating the flavor response. An odorant can be very potent at extremely low concentrations if it has an extremely low odor threshold, (unit go). In practice, early GC/O analysts attempted to concentrate the sample as far as possible to identify as many potential odorants as possible. Compositional studies combined with threshold studies were then used to sort out the important odorants from the ones that did not contribute to the flavor experience. Rothe s odor units (OU = concentration in sample/threshold in sample) were an early attempt to rank odorants by potency. The process of determining OU values for a food required a lot of chemical and psychophysical analysis. Dilution analysis was developed to produce an OU-like value directly from GC/O without the need to know the identity of the odorant. In fact, the real value of dilution analysis is that it can tell the analyst which compounds to identify. 

The dynamic range of OSME and GC-SNIFF data is generally less than a factor of ten, whereas dilution analysis frequently yields data that cover three or four powers of ten. It has been determined, however, that compressive transforms (log, root 0.5, and so on) of dilution analysis data are needed to produce statistics with normally distributed error (Acree and Barnard, 1994). Odor Spectrum Values (OSVs) were designed to transform dilution analysis data, odor units, or any potency data into normalized values that are comparable from study to study and are appropriate for normal statistics. The OSV is determined from the equation ... 

The composition of the volatile fraction of bread depends on the bread ingredients, the conditions of dough fermentation and the baking process. This fraction contributes significantly to the desirable flavors of the crust and the crumb. For this reason, the volatile fraction of different bread types has been studied by several authors. Within the more than 280 compounds that have been identified in the volatile fraction of wheat bread, only a relative small number are responsible for the different notes in the aroma profiles of the crust and the crumb. These compounds can be considered as character impact compounds. Approaches to find out the relevant aroma compounds in bread flavors using model systems and the odor unit concept are emphasized in this review. A new technique denominated "aroma extract dilution analysis" was developed based on the odor unit concept and GC-effluent sniffing. It allows the assessment of the relative importance of the aroma compounds of an extract. The application of this technique to extracts of the crust of both wheat and rye breads and to the crumb of wheat bread is discussed. 

A more general approach to estimate the importance of a flavor compound in a particular food is the calculation of the ratio of its concentration to its flavor (odor and taste) threshold (9) or to its odor threshold (1 , 11) The result is denoted "aroma value" 19), "odor unit" (10) or "odor value" (11) the higher the value or unit, the more intensely this component contributes to the flavor or odor of the food. 

Acree et al. (28, 29) used a video-terminal in addition to the gas chromatograph. They calculated CHARM-values on the basis of the duration of the sensory responses which were maintained during the GC-effluent sniffing of three-fold dilutions of the original extract. CHARM-values are directly proportional to odor units. 

On applying the concept of odor units with our current knowledge we can designate 15 to 20 compounds with high aroma values which only amount to 10 - 20 ppm. 

The intensity of aroma compounds found by AEDA and CHARM may be determined with further accuracy by subjecting them to odor unit (also called odor activity value) measurement.71,72 This is done by first measuring the odor threshold of a compound while the concentration of this compound in the specimen is determined using internal standards (for instance, isotopic samples in contemporary approach). Dividing the latter concentration with the odor threshold will give the odor unit value, naturally being higher when a compound better contributes to the total aroma. 

The odorous air sample is diluted automatically with neutral air by use of an olfactometer. This procedure leads to the definition of an odor concentration according to Eq. 7-9 that is applied for the determination of mixing ratios of known flow rates V (dynamic dilution method). The abbreviation ou stands for odor unit (Treitinger and Meyer-Pittroff 1997). The odor concentration is inversely proportional to the dilution. Actually, odor concentrations are dimensionless (see Eq. 7-9) and they simply reflect a dilution to threshold ratio. However, it is common to express odor concentration in odor units per cubic meter ([odor]/ou m ). 

Therefore, 1 ou mT is defined to be equivalent to the odor concentration at which 50% of the test people can recognize the difference between neutral air and the odorous air sample. If a waste gas was determined to have 90 ou this waste gas must be diluted 90 times with neutral air to reach the odor threshold. In other words, the larger the value of the odor unit per the more intensive is the odor. However, an increase of odor concentration from 100 ou m to 1000 ou m increases the sensory impression of the human nose only by a factor of about two, in accord with a logarithmic dependence. 

Tab. 8.6 Odor abatement (diminution of odor units, ou, c.f. Eq. 7-9) by photo-induced oxidation of waste air streams. For specification of the photoreactor, see Figure 8-21 (examples received from HDN-Technik GmbH, Rednitzhembach, Germany)...

Ferreira, V, Ardanuy, M., Lopez, R., and Cacho, J.F. (1998a). Relationship between flavor dilution values and odor unit values in hydroalcoholic solutions Role of volatihty and a practical rule for its estimation. J. Agric. Food Chem., 46, 4341-4346. 

Analysis of the vacuum volatile constituents of fresh tomatoes was carried out using capillary GLC-MS and packed column GLC separation with Infrared, NMR and CI-MS analysis. Evidence was obtained for the presence of the unusual components 3-damascenone, 1-nltro-2--phenylethane, 1-nltro-3-methylbutane, 3-cyclocltral and epoxy-3-1onone. A method for the quantitative analysis of the volatile aroma components In fresh tomato has been Improved and applied to fresh tomato samples. The quantitative data obtained have been combined with odor threshold data to calculate odor unit values (ratio of concentration / threshold) for 30 major tomato components. These calculations Indicate that the major contributors to fresh tomato aroma Include (Z)-3-hexenal, 3-lonone, hexanal, 3-damascenone, 1-penten-3-one, 3-methylbutanal, (E)-2-hexenal, 2-lso-butylthlazole, 1-nltrophenylethane and (E)-2-heptenal. 

Sensory Approach. The main sensory studies applied have been in the determination of odor thresholds of components and the calculation of odor unit values (Uo), the ratio of the concentration of the component in the food to its odor threshold in water. The results of these studies are summarized in Table I. It can be seen that (Z)-3-hexanal shows the most odor units... 

Concentrations of major volatile fresh tomato components using blending procedure, odor thresholds in water solution and Log Odor Units. Compounds listed in descending order of their Log odor units... 

The volatiles of fresh pineapple (Ananas comosus [L] Merr.) crown, pulp and intact fmit were studied by capillary gas chromatography and capillary gas chromatography-mass spectrometry. The fnjit was sampled using dynamic headspace sampling and vacuum steam distillation-extraction. Analyses showed that the crown contains Cg aldehydes and alcohols while the pulp and intact fruit are characterized by a diverse assortment of esters, h rocarbons, alcohols and carbonyl compounds. Odor unit values, calculated from odor threshold and concentration data, indicate that the following compounds are important contributors to fresh pineapple aroma 2,5-dimethyl-4-hydroxy-3(2H)-furanone, methyl 2-methybutanoate, ethyl 2-methylbutanoate, ethyl acetate, ethyl hexanoate, ethyl butanoate, ethyl 2-methylpropanoate, methyl hexanoate and methyl butanoate. 

Table III. Odor Thresholds and Odor Units for Selected...

Constituent Odor Units Odor Description (Fenaroi(5))... 

An odor unit is defined as the ratio of concentration to threshold. This unit quantifies the contribution of a specific component or a fraction to the total odor of a mixture however, it says nothing about the odor quality of the final mixture, and it does not imply anything about the relationship between the stimulus concentration and the intensity of sensation above the threshold (Teranishi et al., 1981). 

As gas chromatography evolved in sophistication, progress was made in the characterization of flavor compounds and in the elucidation of their chemical pathways. Initial researches focused on the identification of long list of flavor compounds, which has resulted in the identification of more than 7,000 aroma compounds in foods (S). Because, it was impossible to recreate food flavors from the laundry lists of chemicals, earlier attempts were made to determine character impact compounds. Except for few foods, the flavors of many foods are not determined by character impact compounds, rather results from a combination of several key flavor constituents. Today, there are several established methods (CHARM, AEDA, Odor Unit Value) used in the determination of key aroma constituents in foods. 

Currently, there is much interest in flavor research in determining what compounds are actually responsible for the characteristic flavor of a food. Researchers have utilized odor unit values (compound concentration/odor threshold) to determine the contribution of individual constituents to the overall flavor of a food. For example, citrus peel oil aroma quality has been characterized using logarithmic odor unit values... 

Tamura et al. (this volume) discuss the suitability of the detection threshold and the recognition threshold in determining the limited odor unit for characterizing citrus aroma quality. 

The "odor-unit" value, which was proposed by Guadagni et al. (S) and Rothe and Thomas (9), was introduced to evaluate the importance of individual aroma constituents in food volatiles. This value is defined in the following equation [1] ... 

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