# 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
Trimethylpyrazine was converted by swapping out the methyl group in position 2 to 2-ethyl-3,5-dimethylpyrazine, which has a 4500-fold lower OT than trimethylpyrazine. The pyrazine molecule's ethyl groups in positions 3 or 5 were less successful in lowering the OT than those in position 2. The OT rose when a propyl and a butyl group took the place of an ethyl group in position 2 of 2-ethyl-3,5-dimethylpyrazine. When the propyl group was switched out for an isopropyl group in 3,5-dimethyl-2-propylrazine, the OT rose even higher. Their pea-like odour and extremely low OT, which supported earlier findings, showed that 3-alkyl-2-methoxypyrazines belong to a unique class of odourants. The pleasant odour was unaffected even though the OT rose when the 3-isopropyl-2-methoxypyrazine methoxy substituent was replaced by methyl and an ethyl group. Inferring that either their OT is substantially lower than those of the majority of alkylpyrazines or their quantities in foods are significantly higher, only a limited subset of alkylpyrazines have been evaluated by AEDA as potent odourants. For instance, although their OT values are rather high, alkylpyrazine trimethylpyrazine, tetramethylpyrazine and 3-ethyl-2,5-dimethylpyrazine have been found in cocoa mass and are generated in quantities that make up for their mild odour. Additionally, these findings were then used to specify the geometry of a hypothetical receptor. By superimposing the reduced structures of pyrazines with a low OT, the geometrical shape of a hypothetical receptor was obtained (Chem-Xforce-field minimization). By superimposing pyrazines with a high OT, they discovered sterically prohibited regions in the model [28]. By using QSAR analysis, [PERSON] et al. [69] confirmed that there are favourable steric locations 2 and 3 and that the inclusion of bulky groups in these regions will intensify the odour (lower the OT). The bulkiness, on the other hand, is unfavourable at positions 5 and 6 for the low OT. Additionally, a low Log P value of the substituent in position 2 was favourable for a low OT. The decreasing order of the OT for the dialkylpyrazines, as noted by [PERSON] and [PERSON] [66], is as follows: 2,5-, 2,6- and 2,3-positions. The order of the decreasing OT is inverted for methylpyrazines that contain a polar group, such as an ethoxy-, methoxy-, methylthio- or acetyl: 2,3-position, 2,6-position or 2,5-position. Additionally, it seemed that the reduction in the lipophilicity of the pyrazines caused the OT to drop. Out of all those findings, we can make a summary of the structure features of pyrazine that enhance great odour intensity:  
1. Positions 3 and 5 were not acceptable for an ethyl group; only position 2 was. In addition, a propyl, butyl, pentyl, isobutyl or hexyl group in position 2 was too bulky, and the OT was high in these compounds compared with 2,3-dimethylpyrazine, which was not substituted by them. However, positions 2 and 3 are more suitable for bulky groups than positions 5 and 6 regarding the OT value;
2. Replacing the H atom in position 2 with a methyl group reduced the OT, and the lengthening of the side chain to ethylpyrazine decreased the OT even further;
3. For compounds with a low OT, the only group allowed in position 5 is the methyl group;
4. For a low OT, position 6 should be unsubstituted;
5. For the dialkylpyrazines, the substitution position affects the OT by lowering the OT at 2,5-dialkyl-pyrazine, then 2,6-dialkyl-pyrazine and then 2,3-dialkyl-pyrazine. 6. The incorporation of a highly polar group among those presented in cocoa alkylpyrazines (such as methoxy) tends to lower OT values. As an example, 3-alkyl-2-methoxypyrazines belong to a unique class of odourants with very low OT values.