Abstract
Post-harvest wet coffee processing is a commonly applied method to transform coffee cherries into green coffee beans through depulping or demucilaging, fermentation, washing, soaking, drying, and dehulling. Multiple processing parameters can be modified and thus influence the coffee quality (green coffee beans and cup quality). The present study aimed to explore the impacts of these parameters, including processing type (depulping or demucilaging), fermentation duration, and application of soaking, on the microbial community dynamics, metabolite compositions of processing waters (fermentation and soaking) and coffee beans, and resulting cup quality through a multiphasic approach. A large-scale wet coffee processing experiment was conducted with Coffea arabica var. Catimor in Yunnan (China) in duplicate. The fermentation steps presented a dynamic interaction between constant nutrient release (mainly from the cherry mucilage) into the surrounding water and active microbial activities led by lactic acid bacteria, especially Leuconostoc and Lactococcus. The microbial communities were affected by both the processing type and fermentation duration. At the same time, the endogenous coffee bean metabolism remained active at different stages along the processing, as could be seen through changes in the concentrations of carbohydrates, organic acids, and free amino acids. Among all the processing variants tested, the fermentation duration had the greatest impact on the green coffee bean compositions and the cup quality. A long fermentation duration resulted in a fruitier and more acidic cup. As an ecological alternative for the depulped processing, the demucilaged processing produced a beverage quality comparable to the depulped one. The application of soaking, however, tempered the positive fermentation effects and standardized the green coffee bean quality, regardless of the preceding processing practices applied. Lastly, the impact strength of each processing parameter would also depend on the coffee variety used and the local geographical conditions. All these findings provide a considerable margin of opportunities for future coffee research.
Highlights
It has only been 500 years since coffee acquired its worldwide popularity (Yilmaz et al, 2017)
Each step of this processing chain plays a significant role in the coffee quality, which can be evaluated by the quality of the green coffee beans as well as the sensory experience of the brewed coffees (Batista and Chalfoun, 2014)
The present study aimed to investigate the impacts of multiple processing parameters on the dynamics of the microbial communities, the metabolomic profiles of the coffee beans and processing waters, the metabolite compositions of the green coffee beans, and the sensory quality of the coffee brews upon post-harvest processing of C. arabica var
Summary
It has only been 500 years since coffee acquired its worldwide popularity (Yilmaz et al, 2017). To transform the coffee cherries into a cup of coffee, a complex chain of post-harvest events is needed, during which the cherries are first picked from the coffee trees, processed to green coffee beans, subsequently roasted, and brewed to generate a coffee beverage with pleasant aroma and taste (Batista and Chalfoun, 2014; Brando and Brando, 2014; Silva, 2014; Poisson et al, 2017; Waters et al, 2017). An extra soaking step, during which the washed beans are submerged in clean water, is applied to improve the visual appearance of the green coffee beans and to obtain a clean taste in the final cup (Murthy and Naidu, 2012). The added values of demucilaging and soaking are controversial, since their impacts on the green coffee beans and cup quality are still unclear, as they have never been studied in great detail
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