## 3 Results and Discussion
### Phenolic Content Estimation Assays (TPC, TTC, TCT)
Some previous studies have documented the biotransformation process of conjugated hydroxycinnamic acids via the fermentation of lactic acid bacteria or yeast decarboxylases/reductases in various plant sources [22; 24; 25]. According to [PERSON] et al. [26], following fermentation, the decrease in phenolic acid concentration could be attributed to an increase in volatile phenol levels. This is the result of phenolic acid catabolism, in which the breakdown of l-phenylalanine via the Ehrlich pathway is mediated via yeast, contributing to a 1.9-fold increase in 2-phenylethylenol levels. Simultaneously, the reductions in the levels of alkanes, acids, and aldehydes can be attributed to the unique hydrophobic substrate metabolizing pathways. Since all the sample beans used in this study were commercially roasted coffee beans, it is important to note that during the roasting process of coffee beans, the Maillard reaction leads to the accumulation of melanoidins. These melanoidins can react with the Folin-Ciocalteu reagent used in total phenolic content (TPC) analysis, thereby potentially increasing the measured results. This could lead to confounding effects in the context of fermentation [27]. In the results of total flavonoid content (TFC), the fermented coffee beans showed slightly higher levels of both free and bound phenolic compounds compared to the unfermented coffee samples. Additionally, in the results for total condensed tannin (TCT), the fermented coffee samples exhibited lower TCT levels compared to the unfermented coffee beans. However, it should be noted that both TFC and TCT assays were unable to accurately determine the bound phenolic content, as the levels of flavonoids and condensed tannins were too low to produce a discernible effect on the absorbance of the reagent used in the assays. These two results align with the findings of a study conducted by [PERSON], [PERSON], and [PERSON] [21], where all groups of fermented coffee beans exhibited higher TFC values compared to the unfermented control group. The increase in flavonoid content can be attributed to the accumulation of acidic compounds during the fermentation process. This accumulation may liberate bound flavonoids from the food matrix, leading to the transformation of flavonoids into their aglycone forms, a process that occurs during coffee fermentation of lactic acid bacteria (LAB) and yeast [28]. Also, the fermentation effect on increasing flavonoid content in coffee beans has correlated to the TFC results from several previous studies on different plant sources [29]. For TCT, condensed tannins from the condensation of flavonoids could be metabolized by several species of LAB, for example, _Latchobacillus plantarum_ and _Lactobacillus pentosus_, fungi and yeasts that possess tannic acid hydrolase into the free gallic and ellagic acid monomers [30; 31]. Therefore, the fermented coffee beans exhibit lower results in TCT compared to unfermented coffee beans due to the microbiologic metabolism. But as intensive commercial roasting could also break the condensed tannin structure into the flavonoids in a lower molecular mass, like anthocyanin, this breakdown could potentially increase the free TFC values for both fermented and unfermented coffee beans [32; 33].