## 2 Results and Discussion
### Volatile Compounds
#### 2.8.1 Acetic Acid  
The compounds present in a mature coffee cherry are degraded during the fermentation process increasing the formic, lactic, glycolic, and acetic acid content [31]. This occurs because the microorganisms degrade the pulp and mucilage by petolytic enzyme action, increasing the content of the alcohol and acids, including acetic acid [32]. Acetic acid brings an acceptable flavor of fruit and wine and a fermented aroma [33]. [PERSON], in 2012, established that the increment in acetic acid indicates overfermentation, which causes bad coffee quality. Based on that information, Figure 4A shows that the anaerobic and aerobic dry fermentation processes, after 48 h of fermentation, obtained a high content of acetic acid, different to the fermentation in an atmosphere modified with CO\({}_{2}\), because this  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Compound** & **TR (min)** & **KI** & **KI\({}_{\mathbf{R}}\)** & **Aroma and Taste Description** \\ \hline Acetic acid & 3.11 \(\pm\) 0.04 & 602.90 & 605 & Acid [27], bitter [28], vinegar [6] \\
2-methylpyrazine & 9.55 \(\pm\) 0.1 & 878.18 & 876 & Nut [6,20,29], chocolate [28] \\
2-furancarboxaldehyde & 10.14 \(\pm\) 0.1 & 892.89 & 851 & Floral [30] \\
2-furanmethanol & 11.11 \(\pm\) 0.6 & 915.31 & 891 & Candy [20], burnt [28] \\
2,6-dimethylpyrazine & 14.20 \(\pm\) 0.1 & 972.52 & 930 & Cocoa [20], chocolate [29] \\
5-methylfurfural & 17.01 \(\pm\) 0.1 & 1028.15 & 982 & Spicy [20], candy [29] \\
2-methoxy-4-vinylphenol & 34.48 \(\pm\) 0.04 & 1390.26 & 1330 & Clove [20], spicy [30] \\ \hline \end{tabular}  
* RT: retention time, KI: Kovats index calculated for the TR-5 MS capillary column, KIs: Kovats index from the National Institute of Standard Technology (NIST) database: [[https://webbook.nist.gov/chemistry/name-ser/](https://webbook.nist.gov/chemistry/name-ser/)]([https://webbook.nist.gov/chemistry/name-ser/](https://webbook.nist.gov/chemistry/name-ser/)) (accessed on 20 December 2021).  
\end{table}
Table 1: Volatile compound identification.  
Figure 3: Correlation between fermentation time, pH, and acidity.  
gas inhibited acetic acid bacteria developing, as explained in Section 2.6. During aerobic dry fermentation, the acidity increased faster, so it permitted a better consumption of the mucilage substrate and a reduction in acetic acid bacteria due to the acidic conditions of the media [34], decreasing the acetic acid content of the coffee bean by limiting the development of the precursor microorganisms of acetic acid and the consumption of the substrate.  
The acetic acid content caused a high decrease in pH in aerobic and anaerobic dry fermentation because the microorganisms developed in optimal conditions [35]. In the dry anaerobic fermentation process, the predominant microorganisms present in the mucilage were lactic acid bacteria _Lactococcus_, _Leuconostoc_, and _Weissella_, which increased the content of lactic and acetic acid [36]. These bacteria were microaerophile aerobic, which means that they can develop in a low oxygen environment; but in the absence of oxygen, these microorganisms ferment sugars from the media [37], which explains the increase in acetic acid in the dry anaerobic fermentation. Otherwise, in the dry aerobic fermentation process, _Acetobacter_ and _Gluconobacter_ were predominant due to the aerobic conditions [38]. The metabolic processes of these bacteria, which are part of the acetic acid bacteria, increased the content of the acetic acid at a long fermentation time [39].