Changes Of Microbial Flora And Main Components Of Pericarp During Liquid Peeling Of Pepper By Aeration

Pepper is known as "king of spices".White pepper is the main product in China that enters the domestic and foreign markets.At present,the most widely used is the traditional retting process,but the white pepper produced by the method has strong odor and low quality.Although the sales volume is high,the price is low,and it is difficult to reflect the economic value of white pepper.In this paper a new method is proposed to improve significantly the odor of white pepper by aeration to the traditional retting system.However,the process is still not perfect,and it lacks theoretical support from all aspects.In this study,high-throughput sequencing technology based on the next generation sequencing platform was applied to analyze the complex microbiota structure and its changes in the aerated fermentation process(0-7d).And the change of main chemical components in the pericarp was also studied in order to provide a theoretical basis for subsequent process improvement.The main results are as follows.(1)The process conditions of aerated fermentation were explored.The optimized conditions were found to be at 30?,pH=3.2 adjusted using HCl,pepper:water(kg/kg)=1:5,and the ventilation being 2.5L/min.(2)PCoA analysis based on weighted UniFrac distance showed that the bacterial flora changed drastically during pepper peeling.At the genus level,Azospirillum,Pseudomonas and Acinetobacter played a key role in the whole process.We screened the core OTUs in each time point sample and integrated them to study the changes in the genus of the entire process through changes in core OTU at each time point.A total of 17 core OTUs were integrated during the aerated fermentation process,mainly belonging to Azospirillum.Acinetobacter,Niveispirillum,Arcobacter,and Pseudomonas.These genus were all aerobic bacteria or facultative aerobic bacteria.In the non-aerated fermentation,19 core OTUs were integrated,mainly belonging to Lactococcus,Clostridium,Pectobacterium,Lactobacillus,Klebsiella,Anaerosinus.These genus are basically anaerobic or facultative anaerobic bacteria.(3)PCoA analysis based on weighted UniFrac distance showed that the fungal flora structure changed during the pepper peeling.At the genus level,Fusarium and Wickerhamomyces played a key role in the whole process.We selected the core OTU in each sample at each time point and integrated it.A total of 15 core OTUs were identified during aerated fermentation,mainly belonging to Fusarium,Candida.A total of 28 core OTUs were identified in the non-aerated fermentation,mainly belonging to Wickerhamomyces,Candida,Fusarium,Trichosporon.(4)Based on functional gene analysis,it was found that the functional genes related to pepper peeling were xylanase-related gene COG3693 and pectin esterase-related gene COG4677.The expression of COG3693 increased sharply in the middle and late stages of peeling,and the expression level of COG4677 also decreased after reaching the peak in the peeling process,and these two enzymes were also proved to be closely related to pepper peeling in the previous studies by our team.(5)There was a big difference in the pH value of the environmental water in the process of peeling.In the experimental process,the pH value of the water body was adjusted to 3.2 for peeling.In the initial stage of fermentation,the pH of the water body increased significantly,but the pH value of the aerated environment stabilized quickly at around 8,and the water body was weakly alkaline.The water environment of the control group was slightly lower than 6 when the pH was stable,and it was weakly acidic.The dissolved oxygen content in the aerated process was 4-5 mg/L,while the non-aerated control group was less than 0.5 mg/L.The highest content of monosaccharide in pepper peel is aminogalactose,and the monosaccharide components mainly composed of pectin in the peel are galacturonic acid,arabinose,galactose and rhamnose.In the process of aerated process,the degradation rate of pectin backbone was the highest at 4-6d fermentation.The change of branched galactose was the most obvious at 0-2d.The degradation rate of branched arabinose was faster at 2-4d fermentation.In the non-aerated control group.the degradation rate of pectin backbone was the highest when at 2-4d,and the change of branched galactose was 2-4d,and the degradation rate of abranched arabinose was 4-6d.