Effect of oxygen concentration on volatile compounds and their formation in Keemun black tea fermentation

Fermentation is a key process in the production of Keemun black tea (KBT), and oxygen, time, temperature, and humidity are key factors affecting black tea fermentation. However, the effect of oxygen concentration on the aroma quality of KBT remains unclear. To deeply investigate the effect of oxygen concentration (16 %, 21 %, 35 %, and 45 %) on the volatile compounds of KBT during fermentation, the aroma characteristics of tea fermented with different oxygen concentrations were assessed. A total of 80 volatile compounds were detected based on gas chromatography-mass spectrometry. Gas chromatography-olfactometry, modified detection frequency, odor activity value, and p-values demonstrated that ten aroma-active compounds, including phenylethyl alcohol, geranyl alcohol, and linalool, that were more significantly altered by oxygen were the key compounds contributing to the differences in the aroma of KBT, and their aroma contributions were confirmed by subsequent aroma addition experiments. Additionally, the role of oxygen in the formation of key compounds was further investigated. The results revealed that oxygen-enriched fermentation significantly increased the total concentration of volatile compounds and yielded stronger sweet and fruity aromas, whereas low-oxygen fermentation resulted in weak and single aromas. This study provides new insights into the effect of oxygen on the volatile compounds of KBT and provides theoretical support for the production of high-flavor KBT.