Microbiological, physicochemical and sensory studies of coffee beans fermentation conducted in a yeast bioreactor model

ABSTRACT Coffee growers use a traditional method of fermentation to remove the cherry pulp surrounding the beans. The aim of this study was to evaluate the microbiological, physicochemical, and sensory aspects of coffee beans fermentation conducted in a controlled yeast bioreactor model (New Brunswick™ BioFlo®). Fermentations were conducted with or without the addition of a selected yeast starter culture (viz., Pichia fermentans YC5.2) and different parameters, including microbial growth, bacterial diversity, inoculum persistence, sugar consumption, and metabolic compounds formation (organic acids, ethanol, and ethyl acetate), were investigated. The chemical composition of resulting fermented coffee beans was assessed by high‐performance liquid chromatography and gas chromatography–mass spectrometry, and sensorial analysis of coffee beverage was performed using the cupping test (SCA – specialty coffee association). The yeast bioreactor model enabled efficient yeast starter culture growth and ethanol (0.136 g/L.h) and ethyl acetate (1.039 mg/L.h) formation. The bacterial population was mainly represented by Pediococcus sp. and Leuconostocaceae family, as revealed by Illumina high-throughput 16S rRNA gene sequencing. The fermentation system also enabled the production of coffee beans with rich aroma composition (including D-Limonene, phenyl-acetaldehyde, and phenylethyl alcohol) and beverages with a remarkable increase in quality compared to the conventional process. With further refinements, the stirred-tank bioreactor (STR) model may be useful in designing novel bioreactors for the optimization of coffee fermentation with starter cultures.