# Effect of Fermentation Humidity on Quality of Congou Black Tea
## 3 Results and Discussion
### Effects of Fermentation Humidity on Congou Black Tea Volatile Compound Composition  
In this study, ninety-eight volatile compounds were identified from the five treatments using HS-SPME-GC-MS, including twenty-seven alcohols, twenty-six esters, twenty-five aldehydes and ketones, seven alkylenes, five acids, four phenols and four other compounds (Table S2). As shown in Table 2, alcohols were the main aroma substances, accounting for about 59% of the total, followed by aldehydes and ketones at 21%, and the content of phenols was relatively low at only about 1%. The tea aroma varied significantly with different humidity treatments. With the increase in fermentation humidity, the alcohol, alkene hydrocarbon, aldehyde plus ketone and acid contents all gradually increased, in contrast to a content decrease in phenolic substances. The proportion of each substance is shown in Figure 5A, and their proportion was relatively stable. As the fermentation humidity increased, the samples also showed a significant uptrend in the total amount of volatile compounds, with an increase of 6.4% in RH95 relative to RH55. There was a significant difference between low fermentation humidity groups (RH55, RH65) and high fermentation humidity groups (RH85, RH95), with 75% fermentation humidity as the demarcation point. Based on the aforementioned results, all five treatments could be divided into a high and a low humidity group: group 1 (low humidity) (RH55, RH65) and group 2 (high humidity) (RH75, RH85, RH95). These two groups were investigated by OPLS-DA and the results are shown in Figure 5B. Both R\({}^{2}\) and Q\({}^{2}\) values exceeded 0.5, denoting an acceptable fit of the model. After 200 times of permutation tests, the intersection of the Q\({}^{2}\) regression line with the vertical axis was below zero (Figure 5C), denoting that the model was not over-fitted and validated. The above results showed a good separation effect for the samples fermented under high and low humidity conditions, indicating significant aroma composition variations in congou black tea fermented at different humidity levels. Sixteen differential aroma components under different humidity treatments of congou black tea were screened based on the criteria of VIP >1 and \(p\) < 0.05 (Figure 5D). As the fermentation humidity increased, a downtrend was observed in the contents of eugenol, 2,4-di-t-butylphenol, furnace, trans-2-hexenyl hexanoate and nerolidol, while an uptrend was found in the contents of nonoic acid, hexanoic acid, linalool, trans-pyranoid linalool oxide, \(\beta\)-cyclocitral, nerol, \(\alpha\)-farnesene and (E,E)-2,4-nonadienal. Meanwhile, the contents of limonene, (E)-2-octanal and jasmone exhibited a trend of increasing first and then decreasing. Many studies have shown the formation of key aroma substances contributing to black tea odor during the fermentation process [(39; 40)]. In this process, the aroma precursors, including fatty acids (such as linolenic and linoleic), amino acids (such as phenylalanine and methionine), geranyl/farnesyl pyrophosphate and carotenoids (such as \(\beta\)-caroteneand lutein), are correspondingly converted into fatty acid-derived volatiles (FADV), amino acid-derived volatiles (AADVs), volatile terpen (VTs) and carotenoid-derived volatiles (CDVs). These volatile compounds make an important contribu