The Microbiota Profile And Functional Gene Analysis During Pepper (Piper Nigrum L.) Peeling

Pepper (piper nigrum L.), known as "the king of spice", is an important tropical spice plant. In our country, the main pepper product is white pepper. Although almost white pepper is made by traditional retting method, however it possesses low quality due to its strong odd-flavour and bring about disadvantage in the international market.Recent study reveals that biological peeling method shows great potential to deal with the problem of pepper peeling, however, it’s not mature enough and lacks more comprehensive theoretical foundation. In our research, we use high-throughout sequencing technology based on the second generation sequencing platform, to fully explore the complicated microbiota and its dynamic change, and the intricate enzyme system related to the peeling process, both in traditional retting process (0-6d) and solid state fermentation process(0-60h) developed in our lab. The results were as following.(1) For the traditional retting process, UniFrac distance-based principal coordinates analysis revealed significant changes in the pericarp microbiota structure during peeling, which were attributed to the increase in bacteria from the genera Selenomonas and Prevotella. We identified 28 core operational taxonomic units at each time point, mainly belonging to Selenomonas, Prevotella, Megasphaera,Anaerovibrio, and Clostridium genera. The results were confirmed by quantitative polymerase chain reaction. At the functional level, we observed significant increase in enzymes related to acetyl xylan esterase and pectinesterase for pericarp degradation during peeling.(2) For the solid state fermentation process, UniFrac distance-based principal coordinates analysis suggested that the inoculated Aspergillus niger was dominant throughout the fermentation stage, with the genus constituting more than 95% of the fungi present, and therefore the fungal community structure was relatively stable. The bacterial community structure fluctuated across different fermentation periods; among the bacteria present, Pseudomonas, Tatumella, Pantoea, Acinetobacter, Lactococcus,and Enterobacter accounted for more than 95% of all bacteria. Based on the correlations among the microbial community, we found that Pseudomonas and Acinetobacter were significantly positively related with Aspergillus niger, which showed strong synergy with them. In view of the microbial functional gene analysis,we found that these three bacteria and fungi were closely related to the production of pectinesterase (COG4677) and acetyl xylan esterase (COG3458), the key enzymes for pepper peeling.