## 3 Results and Discussion
### Classification of Wines by Geographical Origin and Vintage Based on Phenolic Compounds
Ningxia wines were mostly characterized by 12 phenolic compounds (quercetin, gallocatedchin, caffeic acid, catechin, epicatechin, procyanidin oligomers, dimer1, dimer2, dimer3, dimer4, dimer6, trimer1, and trimer2. Bordeaux wines were mostly characterized by protocatechuic acid and dimer5. California wines were mostly characterized by epicatechin gallate and syringic acid. Previous studies have shown that sufficient light and colder weather contribute to the accumulation of some phenols [13]. The high content of most monomeric phenols in Ningxia wine may also be caused by sufficient light and a large temperature difference between day and night in Ningxia. In addition, Yinchuan and Yongning had very similar phenolic profiles and formed a subset with Helan, which had a slightly different profile. The Ningxia sub-regions are adjacent, with the same longitude and in the order Shizuishan, Helan, Yinchuan, Yongning, Qingtongxia, and Hongsipu, from north to south, over a distance of \(\sim\)300 km (Figure 1). The phenolic profiles of these five sub-regions had generally similar phenolic profiles and were grouped relatively closely. The exception was Hongsipu, which had a relatively low phenolic content, compared with the other sub-regions. Most of the phenolic compounds in the Hongsipu wines were significantly higher than Bordeaux and California wines, but the phenolic profile was most similar to that of Bordeaux. The differences in Hongsipu wines may be related to it being the southernmost region and having a warmer, dryer climate than the other regions (Figure 2). The differences between the Ningxia, California, and Bordeaux regions were very clear; their phenolic profiles were markedly different. The phenolic compounds from the six consecutive Ningxia vintages (2013-2018) were analyzed using hierarchical cluster analysis (HCA) to compare differences between them (Figure 6b). The 19 phenolic compounds were clustered into 3 groups in the vertical clustering tree, and the wines from the 6 Ningxia vintages formed 3 groups in the longitudinal clustering tree. The phenolic profiles of the 2014 and 2015 vintages, and the 2016 and 2017 vintages were very similar, forming two groups, which were closely related to each  
Figure 6: HCA (hierarchical cluster analysis) of wines based on phenolic c ompounds from the six sub-regions of Ningxia, Bordeaux, and California (**a**) wines from the six vintages (2013–2018) from the Ningxia region of China (**b**). other and the 2013 vintage. The phenolic profile of the 2018 vintage was prominent and formed a separate group. The 2018 vintage had the highest overall phenolic content, with 13 identified phenolic compounds (caffeic acid, catechin, epicatechin, gallic acid, protocatechic acid, dimer1, dimer2, dimer3, dimer4, dimer5, dimer6, trimer1, and trimer2). This may be due to the relatively new vintage in 2018, sufficiently accumulated temperature, and abundant precipitation, compared with other years. The content of most phenolic compounds in the most aged vintages of 2014 and 2015 was significantly lower than the others, especially catechin, epicatechin, quercetin, dimer1, dimer2, dimer3, dimer4, dimer5, dimer6, trimer1, and trimer2. The phenolic content of the 2016 and 2017 vintages was lower than the 2018 vintage and phenolic content followed a downward trend from 2018 to 2013. The content of gallic acid, isorhamentin, epicatechin gallate, and myricetin in 2013 wine was the lowest of the vintages, followed by 2014 and 2015. The content of most individual phenolic compounds decreased markedly with increasing wine age, as for the anthocyanin content (Table 2).