Liberica coffee (Coffea liberica) represents only about ۲% of global coffee production and is often undervalued due to its inferior sensory quality compared to Arabica and lower durability compared to Robusta. Nevertheless, Liberica coffee possesses a unique flavor profile that can be enhanced through biotechnological post-harvest processing. Microbial fermentation offers a promising approach to improve both sensory attributes and stability of this underutilized coffee variety. In this study, Liberica coffee beans were fermented using
Streptococcus thermophilus (ATCC ۱۹۲۵۸) in de Man, Rogosa, and Sharpe (MRS) broth for ۲۴ h, followed by roasting and grinding. The effects of fermentation were evaluated through proximate analysis, sensory cupping tests, and thermal–kinetic characterization. Advanced analytical techniques, including thermogravimetric analysis (TGA), derivative thermogravimetry (DTG), differential scanning calorimetry (DSC), gas chromatography–mass spectrometry (GC-MS), and liquid chromatography–mass spectrometry (LC-MS), were employed to investigate changes in physicochemical properties, volatile compound profiles, and thermal behavior. The results showed that fermentation decreased carbohydrate, protein, and moisture contents, while increasing fat and ash levels. Sensory evaluation revealed an improvement in cupping score from “good” (<۸.۰) to “excellent” (≥۸.۰). Thermal analysis indicated enhanced stability in fermented samples, as evidenced by higher melting (Tm) and crystallization (Tc) temperatures. These improvements in flavor and durability are attributed to modifications in proximate composition, the formation of diverse volatile compounds, and increased resistance to oxidative and degradative processes. Overall, fermentation using Streptococcus sp. significantly improved the quality and stability of Liberica coffee, highlighting its potential as a competitive and sustainable coffee product.