# Impact of Cooking Processes on Volatile Flavor Compounds and Free Amino Acids in Fish Sauce
## 3 Results and Discussion
### Key Flavor Compounds Identified by OAV Analysis in the Fish Sauce  
To further evaluate the contribution of each flavor compound to the overall aroma profile of fish sauce, OAVs, which account for both concentration and odor threshold, were calculated. Ten volatile compounds were screened as key aroma-active compounds based on their high odor activity values (OAV \(\geq\) 1) (Table 1). Trimethylamine exhibited the highest average OAV, followed by decanal, methional, 2,3,5-methyl pyrazine, benzene acetaldehyde, 2-ethyl-5-methylpyrazine, nonanal, 2,6-diethylpyrazine, phenylethyl alcohol, and \(\gamma\)-nonalactone. The total OAVs of key odorants increased in all samples following heating. These findings differ from the total concentration of volatile substances, likely due to the high acid content in fish sauce, which significantly impacts the total volatile content. However, total OAVs are primarily driven by compounds with low odor threshold values. The observed changes in total OAVs align with a previous study, which reported increased total odor intensity scores, quantification, and OAV values in soy sauce after cooking [3].  
The enhancement of food flavor during heat processing mainly depends on the Maillard reaction, which is critical for improving food acceptability [30]. This reaction involves pyrazines and aldehydes as key flavor compounds [30]. During the Maillard reaction, \(\alpha\)-dicarbonyl compounds from sugars or \(\alpha,\beta\)-unsaturated aldehydes from lipids react with amino acids to form structure-related aldehydes and \(\alpha\)-amino ketones, which serve as precursors of pyrazines, an N-containing heterocyclic aromatic compound [31]. In boiled fish sauce, the concentration of methional increased with extended heating, while the levels of other aldehydes and pyrazines initially increased and then decreased. In stir-fried fish sauce, excluding nonanal and 2,6-diethylpyrazine, the concentrations of other aldehydes and pyrazines first increased, then decreased, and later increased again. The OAV values of seven key flavor aldehydes and pyrazines did not increase consistently with heating time and temperature. This may be because aldehydes are key intermediates in the Maillard reaction, which contribute to melanoid formation [32]. Pyrazine formation competes with melanoid formation [32]. In stir-fried fish sauce, these Maillard products are highly volatile and evaporate with water vapor [31]. Phenylethyl alcohol, with phenylacetaldehyde as its precursor, is also influenced by heating temperature during its formation and degradation [33]. During black tea drying, the concentration of phenylethyl alcohol is lower with sun drying (20-34 \({}^{\circ}\)C) than with hot-air drying (110-120 \({}^{\circ}\)C), but higher than with pan-fireddrying (180-250 \({}^{\circ}\)C) [34]. These results are consistent with our findings. Moderate heating increases the OAV of phenylethyl alcohol, while excessive heating causes its degradation. The precursor of \(\gamma\)-nonalactone is 4-hydroxy acids, and heat drives their conversion into \(\gamma\)-nonalactone [35]. During heat treatment, ATP-related compounds are converted into nucleotides, which are then thermally degraded to trimethylamine [36,37]. A previous study has shown that trimethylamine content increases with heat treatment [37]. Heat may also cause a decrease in trimethylamine levels [38]. In this study, an increase in the OAV value of trimethylamine was observed in B30, B60, and F10, while it decreased in other samples. This may be because the degradation rate of trimethylamine in B10 is higher than its synthesis rate, while the highly volatile trimethylamine in F30 and F60 is lost during evaporation.