Effect of Oxygen Concentration in Fermentation on Black Tea Quality and Optimization of Oxygen-enriched Fermentation Process

Fermentation is a critical process of black tea quality formation and oxygen is the key factor affecting the fermentation, so it is important to analyze the effect of oxygen in fermentation on the quality and metabolites of black tea. One bud and two leaves of 'Longjing 43' tea varieties were used as materials for low oxygen fermentation (5%) , natural fermentation (21%) and oxygen-enriched fermentation (36%) treatments, and the effects of oxygen concentration on sensory quality, non-volatile and volatile metabolites of black tea were analyzed by sensory evaluation combined with gas chromatography-mass spectrometry (GC-MS) and ultra-high performance liquid chromatography-mass spectrometry (UPLC-MS), and the parameters of oxygen-enriched fermentation of black tea were optimized by single factor combined with response surface analysis. Results showed that oxygen-enriched fermentation could significantly improve the taste and aroma quality of black tea compared with nature fermentation (P<0.05). Compared with natural fermentation, the content of key taste compounds catechins and gallic acid (GA) decreased significantly (P<0.05), total theaflavins (TFs) and its monomer increased significantly (P<0.05), and amino acids did not change significantly (P>0.05) in oxygen-enriched fermentation. A total of 25 volatile compounds differed significantly in three treatments, including 12 aldehydes, 2 ketones, 3 alcohols, 3 alkenes, and 5 esters, and the content of most differing compounds increased with increasing oxygen concentration. The optimized parameters of oxygen-enriched black tea were: Oxygen concentration of 40%, oxygenation time of 1.5 h, fermentation time of 4 h. And the contents of TFs, TF, TF3G, TF3'G and TFDG of black tea were 2.86%, 0.25%, 1.71%, 0.24% and 0.68%, respectively. The results of this study would provide an important basis for guiding black tea processing and quality control.