Anaerobic Fermentation of Specialty Coffee Through Microbial Metabolism, Chemical Transformations, and Aroma Precursor Formation

Anaerobic fermentation has emerged as an increasingly important post-harvest processing strategy in specialty coffee production, offering opportunities to modify the biochemical composition of coffee cherries and influence the sensory characteristics of the resulting beverage. Unlike conventional fermentation approaches, oxygen-restricted processing can create distinct microbial and physicochemical environments that affect substrate utilization, metabolite production, and the development of aroma-related compounds. This review examines the biochemical and chemical mechanisms underlying anaerobic fermentation in specialty coffee, with particular emphasis on microbial ecology, carbohydrate metabolism, organic acid production, amino acid transformations, and the formation of volatile and non-volatile metabolites. The metabolic contributions of yeasts and bacteria are considered in relation to changes in the chemical composition of coffee during fermentation. Particular attention is given to esters, higher alcohols, aldehydes, ketones, and other volatile compounds associated with aroma development, as well as the potential role of fermentation-derived metabolites as precursors of roasting-related flavor compounds. The influence of fermentation parameters, including temperature, duration, pH, oxygen availability, and microbial composition, is also critically discussed. Furthermore, analytical approaches such as gas chromatography–mass spectrometry, liquid chromatography, and metabolomics are examined for their contributions to chemical characterization and process evaluation. Finally, current knowledge gaps concerning microbial interactions, metabolic pathways, process reproducibility, and sensory outcomes are identified. A more integrated understanding of microbial metabolism and chemical transformations is essential for developing controlled, reproducible, and scientifically optimized anaerobic fermentation strategies for specialty coffee.