The crucial role of yeasts in the wet fermentation of coffee beans and quality.

The objective of this study was to investigate the role of yeasts in the wet fermentation of coffee beans and their contribution to coffee quality using a novel approach. Natamycin (300 ppm) was added to the fermentation mass to suppress yeast growth and their metabolic activities, and the resultant microbial ecology, bean chemistry and sensory quality were analyzed and compared to non-treated spontaneous fermentation we reported previously. The yeast community was dominated by Hanseniaspora uvarum and Pichia kudriavzevii and grew to a maximum population of about 5.5 log CFU/g in the absence of Natamycin, while when Natamycin was added yeasts were suppressed. The major bacterial species in both the spontaneous and yeast-suppressed fermentations included the lactic acid bacteria Leuconostoc mesenteroides and Lactococcus lactis, the acetic acid bacteria Gluconobacter cerinus and Acetobacter persici and the Enterobacteriaceae Enterobacter, Citrobacter and Erwinia. For both fermentations, the mucilage layers were completely degraded by the end of the process and the absence of yeast activities had no significant impact on mucilage degradation. During fermentation, reducing sugars were consumed while lactic acid was accumulated inside the beans, and its concentration was significantly higher in the spontaneous fermentation (3 times) than that where yeasts were suppressed by Natamycin. Glycerol was detected with a concentration of 0.08% in the absence of Natamycin and was not identified when Natamycin was added. Green beans fermented with yeast growth contained a higher amount of isoamyl alcohol (21 times), ethanol (3.7 times), acetaldehyde (8 times), and ethyl acetate (25 times) compared to beans fermented in the absence of yeast activities, which remained higher in the former after roasting. Beans fermented without yeast activities had a mild fruity aroma, and lower sensory scores of fragrances (7.0), flavor (6.5), acidity (6.3), body (7.0) and overall score (6.5) compared to the former. These findings demonstrated the crucial roles of yeasts in wet fermentation of coffee beans and for producing high quality coffee.

Ecological diversity, evolution and metabolism of microbial communities in the wet fermentation of Australian coffee beans.

The microbial ecology in the fermentation of Australian coffee beans was investigated in this study. Pulped coffee beans were kept underwater for 36 h before air dried. Samples were collected periodically, and the microbial communities were analyzed by culture-dependent and independent methods. Changes in sugars, organic acids and microbial metabolites in the mucilage and endosperm of the coffee beans during fermentation were monitored by HPLC. Culture-dependent methods identified 6 yeast and 17 bacterial species, while the culture-independent methods, multiple-step total direct DNA extraction and high throughput sequencing, identified 212 fungal and 40 bacterial species. Most of the microbial species in the community have been reported for wet fermentation of coffee beans in other parts of the world, but the yeast Pichia kudriavzevii was isolated for the first time in wet coffee bean fermentation. The bacterial community was dominated by aerobic mesophilic bacteria (AMB) with Citrobacter being the predominant genus. Hanseniaspora uvarum and Pichia kudriavzevii were the predominant yeasts while Leuconostoc mesenteroides and Lactococcus lactis were the predominant LAB. The yeasts and bacteria grew significantly during fermentation, utilizing sugars in the mucilage and produced mannitol, glycerol, and lactic acid, leading to a significant decrease in pH. The results of this study provided a preliminary understanding of the microbial ecology of wet coffee fermentation under Australian conditions. Further studies are needed to explore the impact of microbial growth and metabolism on coffee quality, especially flavour.