## 4 Conclusions  
In this paper, metagenomic sequencing and a HS-SPME method based on a GC-MS method were used to systematically study the changes of microbial flora and VOCs in paocai fermented with different vegetables in six rounds of fermentation, combined with a multivariate statistical method to explore the intrinsic associations of \"raw materials--microorganism--VOCs\". The results revealed that after multiple rounds of fermentation, a higher number of microbial species were present in the cabbage paocai than in the red radish paocai with a brine of the same age. At the species level, _S. paracollinoides_ and _F. siliginis_ were found to be the characteristic microbial species of the red radish paocai, as well as 13 species belonging to the Firmicutes phylum (namely, _L. parafarraginis_, _L. buchneri_, _L. zymae_, _L. namurensis_, _L. diolivorans_, _L. acidipiscis_, _L. casei_, _L. parabuchneri_, _L. nilgardii_, _L. farraginis_, _P. selangorensis_, _L. sunkii_ and _L. otakiensis_) and two species belonging to Proteobacteria (namely, _K. ascorbata_ and _P. carorovarum_) for the cabbage paocai. By constructing a PLS-DA model, it was found that there were 13 kinds of different VOCs in the paocai from two raw materials. In addition, the correlation between the microorganisms and  
Figure 5: (**A**) PLS-DA score scatter plot and (**B**) permutation tests of VOCs. (**C**) Heatmap of correlation between microbial species (relative abundance exceeding 5% and the nine biomarkers previously identified) and different volatile flavor substances in the two kinds of paocai. R indicates red radish paocai, B indicates cabbage paocai. * 0.01 < \(p\) \(\leq\) 0.05, ** 0.001 < \(p\) \(\leq\) 0.01, ** \(p\) \(\leq\) 0.001.  
VOCs in the cabbage paocai was stronger than that in the red radish paocai, and the most significant differences were found to be in 4-isopropylbenzyl alcohol, \(\alpha\)-cadinol, terpinolene and isobutyl phenylacetate. The metagenomic sequencing in this study allowed the detection, identification and screening of the microbial ecology as well as the characteristic microorganisms of two different fermentation systems of the vegetable paocais. Further, the study also effectively allowed the identification of the VOCs and the establishment of their relationship to the microorganisms, which provided insights into the microbial flavor profiles of the studied paocais.  
The following are available online at [[https://www.mdpi.com/article/10](https://www.mdpi.com/article/10)]([https://www.mdpi.com/article/10](https://www.mdpi.com/article/10)) 3390/foods11010062/s1, Table S1: Statistics profiles of the metagenome, Table S2: Content of volatile organic compounds in the two kinds of paocai.  
Conceptualization, L.J. and S.X.; methodology, L.J.; data curation, S.X.; resources, X.L.; writing--original draft preparation, L.J., S.X., G.S. and Z.Z.; visualization, X.H.; supervision, A.C.; project administration, A.C.; funding acquisition, A.C. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Research on Key Technologies of Food Processing of Characteristic Biological Resources in Arid Valley of Southwest China (No. 2017 YC0505106-3).  
Not applicable.  
The data presented in this study are openly available in NCBI database at accession PRJNA731323. deposited in [[https://www.ncbi.nlm.nih.gov/sra/PRJNA731323](https://www.ncbi.nlm.nih.gov/sra/PRJNA731323)]([https://www.ncbi.nlm.nih.gov/sra/PRJNA731323](https://www.ncbi.nlm.nih.gov/sra/PRJNA731323)) (accessed on 21 December 2020).  
We thank [PERSON] and Sichuan Liji Lebao Food Co., Ltd. for their technical support.  
The authors declare no competing financial interest.