# Antimicrobial and Odour Qualities of Alkylpyrazines Occurring in Chocolate and Cocoa Products
## 4 The Quantification of the Impact of Alkylpyrazine Compounds on the Aroma of Cocoa and Chocolate
### Structure-Odour Relationship in Alkylpyrazines  
Multiple studies of the structure-activity connection of the odour properties of pyrazine derivative products point to the possibility that differences in odour activities depend on the chemical structure of such molecules, specifically on the kind of substituent groups and the placement of the substituents tied to the pyrazine ring. The structural correlations [PERSON] et al. [12] observed were as follows: the incorporation of a highly polar group among those presented in cocoa alkylpyrazines (such as methoxy) tends to lower OT values; the incorporation of a bulky alkyl side chain tends to decrease the OT; the introduction of the strong electron inductive group (-OR) as an osmophoric group has a massive impact on both the relative OT and olfactory perception. Additionally, pyrazine and 2-methylpyrazine had a higher OT than other more substituted pyrazines in their study (OT methylpyrazine = 60,000 ug/L > OT ethylpyrazine = 6000 ug/L > OTisobutylpyrazine = 400 \(\upmu\)g/L > OT 2-methoxy-3-methylpyrazine = 4 \(\upmu\)g/L > OT 2-methoxy-3-isobutylpyrazine = 0.002 \(\upmu\)g/L). [PERSON] et al. [70], using comparative molecular field analysis and 3D modelling of pyrazine structures with the green odour, also discovered that adding a substructure or a negatively charged group, such as a functional group that contains an oxygen atom, is preferred to boost the odour intensity. This was also proved by [PERSON] et al. 's QSAR analysis [67], which evaluated the significance of electronic characteristics based on the influence of the polar substituents in the pyrazine rings, which can have a major impact on the odour intensity. The more odour-active pyrazine compounds have a greater lone-pair orbital energy, according to correlation studies between olfactory activities and orbital energies. These findings imply that polar substituents at position 3 in the pyrazine ring may have a slight electrical impact that helps to produce odour activity. However, it is logical to assume that steric factors are the primary causes of pyrazine's odour activity and that the lesser electronic effects are overshadowed by the dominating steric contribution. [PERSON] et al. [65] examined the OT in the water of a number of methoxypyrazines connected to 2-methoxy-3-isobutylpyrazine. The OT rises when methyl groups are added to the ring, indicating a significant drop in odour strength, while the aroma typically stays that of green peppers. When compared with 2-methoxy-3-isobutylpyrazine, the OT is raised by 105 times when the methoxy group is removed. It is also clear that the OT of 2-methoxy-3-alkylpyrazines is significantly lowered or that the odour intensity is increased as the chain length rises. As the side chain's carbon atom count decreases, the rise becomes steadier. Regarding the methoxypyrazines, the OT gradually diminishes or the smell intensity increases as the side chain length increases. However, unlike the methoxypyrazines, the OT was lowered rather than raised by the inclusion of additional groups to the pyrazine ring. However, methoxypyrazines typically have thresholds that are substantially lower than those of alkylpyrazines, suggesting that the methoxy group has a crucial role in relation to the OT. By using gas chromatography-olfactometry, [PERSON] et al. [28] measured the OT of 80 alkylpyrazines and developed some hypotheses regarding the relationship between odour and structure. Amongst mono-, di-, tri- and tetramethylpyrazine, trimethylpyrazine had the lowest OT. In the case of comparing 2,6-dimethyl- and trimethylpyrazine, the OT was reduced by a factor of roughly 34 when a third methyl group took the place of a hydrogen atom in 2,6-dimethylpyrazine. When this substitution took place in position 3, comparing 2,5-dimethylpyrazine and trimethylpyrazine, there was almost the same decrease, whereas in position 5, comparing 2,3-dimethylpyrazine and trimethylpyrazine, there was an 18-fold reduction in the OT. In every instance, the aroma transitioned from nutty to roasted.