Pyrazine odor threshold value

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 ... 

The odor threshold of 3,5-dimethyl-2-isobutylpyrazine and 3,6-dimethyl-2-isobutyl-pyrazine was published by Wagner, Czerny, Bielohradski and Grosh, to be >2000 ng/1 air (P), which indicates a relatively low cp value (77), of only ca. 82,000. 

Classes of chemical compounds having different function groups and odor descriptors, some of which are useful to the flavor or perfume industries were selected for this initial study. For example, alcohols, aldehydes, pyrazines and various benzenoid compounds which have been isolated in the volatiles of cooked meat as reviewed by Hornstein (22) were studied. For each class of chemical compounds literature threshold values obtained only from one laboratory were used in order to prevent errors associated with technique or methodology between laboratories that occur for threshold determinations as discussed by Guadagni al. ) and Powers and Ware (23). 

The other results in Table III are those of data sets not having noncolinear physical-chemical properties. Log P was highly correlated with these data sets as well. Ethylesters threshold data in air weis linearly related to log P (eq. 4) while 3-alkyl-2-methoxy pyrazines had threshold odor intensity which was parabolically related to log P (eq. 7). The pyrazine data indicates that 3-alkyl-2-methoxy pyrazines having a log P value of 2.43 would have the most intense odor of the series. 

The use of log P and HB parameters as a tool for predicting odor intensity seems promising. Although many excellent correlations were obtained as presented in Tables I-V further studies are needed to investigate several unresolved areas. The question on whether log P is linearly or parabolically related to odor intensity for a specific medium needs to be resolved. Six equations in Tables 1-V linearly related log P to odor intensity, while five parabolic relationships were observed which had an optimum hydrophobicity (log P) associated with maximum odor intensity. Log Po values observed were 3.17 and 2.90 for alcohols (threshold-aic). Alkanes had a log Po value of 5.33 (threshold-air). In aqueous media alcohols had a log Po value of 3.98 while 3-alkyl-2-methoxy pyrazines had a value of 2A3. The animal data indicates that rats had log Po values of 5.90 for acetates and 7.91 for alcohols. 

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