## 3 Results and Discussion
### Correlation Analysis of Effects of Flavor Precursors and Volatile Components on Cupping Quality  
The quality of coffee cupping was closely related to the flavor precursor components and volatile components. The VIP was selected to identify the chemical composition and flavor precursor components that were significantly different between the different altitudes. The results of the VIP analysis are shown in the Supplementary Materials, and the correlation analysis with the significant components and cupping indicators are shown in Figure 5.  
Two of the seven trace elements showed significant differences at different altitudes, namely manganese and iron. The correlation analysis showed that the presence of manganese had a positive effect on coffee cupping, whereas the presence of iron had a negative effect. Although trace elements directly affected the coffee body and metal chelates were formed during the brewing process that affect coffee acidity [21; 43], the differences caused by altitude mainly had a very significant impact on the aroma and flavor indicators, and also had a significant impact on balance.  
Figure 5: Correlation analysis of effects of flavor precursors on cupping quality.  
Linoleic and palmitic acids are fatty acids that cause significant differences due to altitude. They had a positive impact on coffee cupping, mainly reflected in the aroma, flavor, and body, which also made the balance better. This result was also reflected in other research results [4]. This effect might stem from the natural aroma of these fatty acids and their ability to create bubbles and a creamy texture in the mouth, enhancing the aroma by capturing and holding volatile compounds [32]. Although a positive impact on body was noted, no clear trend emerged in cupping, potentially due to the high scores associated with the Pu'er coffee body itself.  
Alkaloids (caffeine and trigonelline) were significantly different due to altitude, while only 3-CQA among the six different chlorogenic acids was significantly different due to altitude. Their presence negatively affected the cupping testing, affecting the aroma, flavor, aftertaste, balance, and overall quality [25; 27]. Alkaloids and CGAs generally decreased with rising elevation, while their concentrations decreased with rising elevation. The coffee cupping score also supports this view.  
Monosaccharides and organic acids could directly influence acidity and sweetness during cupping. According to the VIP analysis, there were three kinds of monosaccharides significantly affected by altitude, namely D-(+)-talose, D-(+)-mannose, and DL-arabinose, while there were two kinds of organic acids, namely malic acid and quinic acid. The correlation analysis showed that monosaccharides had a positive effects on coffee cupping, and only D-(+)-talose had significant effects on aroma and balance. Organic acids had a negative effect on coffee cupping, especially malic acid, with a significant effect on aroma, flavor, and balance. While quinic acid had a major negative impact, it also had a positive effect in aftertaste. It might be that the contents of quinic acid and malic acid were reduced at high altitudes, reducing the bitterness of the coffee and improving the quality of the high-altitude coffee [29; 30].  
The relationship between aroma and increasing altitudes was inconsistent with the levels of volatile components. The aroma index encompasses not only the fragrance of the coffee powder (volatile components) but also the aroma released after brewing [19]. The components that significantly changed with altitude resulted in a decrease in nutty and roasted aromas while enhancing sweet and caramel notes. Although the concentration of the coffee aroma decreases, the overall flavor perception improves, leading to higher scores.