Threshold 3-alkyl-2-methoxy pyrazines

Alkyl methoxy pyrazines are an important class of aroma compounds exhibiting intense green bean/green pea aroma notes. Reineccius s group at the University of Minnesota show that mutant strains of Pseudomonas perolens produce the isopropyl isomer to a final level of 15 mg/L of culture. Since the threshold is 2xl0 ° ppm, the reported yield is substantial. 

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. 

More than 100 different pyrazines have been identified in various food products. The sensory properties of the pyrazines are quite diverse. The alkyl pyrazines (Figure 5.5a) generally possess roasted, nut-like notes while methoxypyrazines (Figure 5.5b) often possess earthy, vegetable notes [47]. The 2-isobutyl-3-methoxy pyrazine has a freshly cut green pepper flavor with a sensory threshold of 0.002 ppb in water. The acetyl pyrazines typically have a popcorn character, and 2-acetonyl pyrazine has a... 

As can be seen in Table 9.5, nutrient source had a strong influence on the production of the methoxy pyrazines by a selected mutant of Pseudomonas perolens. The end result is that an organism that initially produced a few ppb of the methoxy alkyl pyrazines was converted into an organism that produced ppm levels of the target compounds. While one may not think that ppm levels of an aroma compound is significant, these pyrazines have sensory thresholds in the pptr levels. 

There are many reports of Pseudomonas cultures producing musty, earthy, and potato-like odors ( l-6). The work of Morgan et al. (T) established 2-methoxy-3-isopropyl pyrazine to be partially responsible for these odors. Subsequently, 2-methyoxy-3-isopropyl pyrazine was found in bell peppers (8), a similar compound 2-methoxy-3-secbutyl pyrazine was identified in galbanum oil (9), and several 2-methoxy-3-alkyl pyrazines were identified in various raw botanicals (10). The odor threshold exhibited by 2-methoxy-3-isobutyl pyrazine (1 part in 10 indicates flavor significance for these compounds even at the exceptionally low concentrations in which they occur in foods and other natural products Producing these compounds from microbial fermentations could be an economical source of flavor for the food industry. 

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