## 4 Conclusions  
This study investigated the evolution of sugars/acids ratio, free and bound volatile compounds and IBMP content near ripening time of four grape varieties (Cabernet Sauvignon, Cabernet Gernischet, Cabernet Franc and Merlot) in Ningxia Helan Mountain's east foothill. According to data of the sugars/acids ratio, Cabernet Sauvignon, Cabernet Gernischet and Cabernet Franc ripened at almost the same time, which was a week later than Merlot; Merlot more easily accumulated a number of sugars.  
For the free volatile compounds, the content of carbonyl compounds decreased continuously in the process of ripening. The contents of alcohols, esters and terpenes increased in the ripening stage and decreased in the stage of over-ripening. Other compounds fluctuated in the process. For the bound volatile compounds, carbonyl compounds and alcohols showed the highest proportion; the contents of terpenes and esters were significantly higher than those of free aroma compounds. Carbonyl compounds decreased in the process of ripening, while alcohols, esters and terpenes increased. Furthermore, esters and terpenes tended to decrease in the process of over-ripening. Other compounds fluctuated in the whole period. Through PCA analysis, it was found that bound volatile compounds were able to distinguish the different grape varieties studied in this paper.  
During both the ripening and over-ripening periods, the IBMP of whole berries showed a consistent downtrend in content. IBMP in Cabernet Gernischet and Cabernet Franc was always higher than in Cabernet Sauvignon and Merlot.  
To our knowledge, this is the first report to offer an insight on the evolution of the volatile compounds of these four grapes near harvest time in the Ningxia region. The present findings demonstrated that the best aroma maturity time of four studied grape varieties was later than the sugars/acids ratio in Ningxia region. Aroma maturity should be taken into account during harvest time determination. Modification of viticulture practices and wine style designs are encouraged. This may improve the aromatic quality of Ningxia wine by serving as a reference for determining the best harvest time and winemaking strategies.  
The following are available online at [[https://www.mdpi.com/article/10.3390/foods11182804/s1](https://www.mdpi.com/article/10.3390/foods11182804/s1)]([https://www.mdpi.com/article/10.3390/foods11182804/s1](https://www.mdpi.com/article/10.3390/foods11182804/s1)), Figure S1: The temperature (a) and rainfall (b) of the vineyard during the ripening process of the grape, Table S1: Standard curves and related parameters of IBMP in juice samples, Table S2: Qualification and quantification information on free volatile compounds in grape berries during ripening, Table S3: Qualification and quantification information on bound volatile compounds in grape berries during ripening.  
Conceptualization, Investigation, Writing--Original Draft, Formal analysis, X.G.; Methodology, Investigation, Writing--Original Draft, X.Z.; Methodology, Formal analysis, K.W., X.L. and R.L.; Conceptualization, Supervision, Funding acquisition, Writing--Review and Editing, W.M. All authors have read and agreed to the published version of the manuscript.  
Figure 4: Changes of IBMP during grape ripening. (The ordinate represents the content of IBMP, and the abscissa represents the sampling date explained above).  
**Funding:** Financial assistant is provided by National Natural Science Foundation of China, No. 32160577 and Ningxia Hui Autonomous Region Key Research and Development Program, Grant No. 2021 BEF02014.  
**Data Availability Statement:** Data are contained within the article or Supplementary Materials.  
**Conflicts of Interest:** The authors declare that they have no known competing financial interest or personal relationship that could have appeared to influence the work reported in this paper.