## 5 Conclusions  
By analyzing the quality properties, microbiota composition and their functions in NXDQ with S and N grades, we determined that their characteristics are different. The N-NXDQ had higher acidity, esterification power, a predominant bacterial and fungi composition that is similar to that in high-temperature _daqu_, and a higher abundance of higher alcohols, aldehydes and ketones, while S-NXDQ had higher liquefaction power, saccharification power and ferment power, and had a typical bacterial and fungal composition of NXDQ. In 36 differential volatile compounds, the alcohols, aldehydes and ketones accounted for approximately 50% of the total, and had higher abundances in N-NXDQ. Furthermore, most of them were positively correlated with the bacterial biomarkers. The metabolic pathways of bacterial community uncovered that S-NXDQ had the higher expression abundance of key enzymes involved in lactic acid and ethanol fermentation, while N-NXDQ had the higher expression abundance of key enzymes involved in amino acid synthesis and long-chain fatty acid and lipid metabolism. Based on the above results, we suggest that the evaluation standard of NXDQ quality should not only focus on sensory indices and biochemical properties, but microbial community position and microbial biomarkers might be a more important contribution for quality of _daqu_. In addition, the rational mixture of S and N grades of _daqu_ is also a way to stabilize or improve the quality of fresh _Baijiu_. Meanwhile, our findings also provide effective information for the quality improvement of NXDQ.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/foods13060914/s1](https://www.mdpi.com/article/10.3390/foods13060914/s1)]([https://www.mdpi.com/article/10.3390/foods13060914/s1](https://www.mdpi.com/article/10.3390/foods13060914/s1)). Table S1: Identification and relative abundance of volatile compounds in the S-NXDQ and N-NXDQ samples.  
Conceptualization, X.H., M.H. and R.L.; methodology, X.H. and M.H.; writing--original draft preparation, X.H. and X.S.; data curation and validation, X.G., X.S. and C.F.; software, X.G., C.F. and J.W.; investigation and resources, X.L.; writing--review and editing, X.H. and R.L.; supervision, R.L.; funding acquisition, C.P. and R.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Key Project of Scientific and Technological Research in Henan Province (222102110242), R&D and Demonstration Application of Key Technologies for Ecological Brewing of Henan baijiu (231111112000), and the Innovative Funds Plan of Henan University of Technology (2020 ZKCJ23).  
Not applicable.  
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.  
The authors thank Henan Songhe Distillery Co., Ltd., (Luyi, China) and Henan Yupo Distillery Co., Ltd., (Xiping, China) for their materials support. Sequencing was performed by Majorbio Co., Ltd. (Shanghai, China).  
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.