Reduction of bitterness and astringency in summer-autumn Longjing tea using a novel sieve mesh cover fixation device

The fixation process critically determines green tea quality, yet the mechanisms through which it alleviates bitterness and astringency remain unclear. This study developed a novel sieve mesh cover (SMC) fixation device and explored its impact on improving the quality of Longjing tea in summer-autumn by optimizing the operating parameters (mesh size: 20–80 mesh; pan temperature: 220℃; covering time: 1–3 min). Compared with conventional fixation treated samples, SMC treatments showed an increase in amino acid content by up to 7.1 %, while the catechin content and TP/AAs ratio decreased by up to 15.6 % and 6.8 %, respectively. Sensory evaluation showed that SMC treatments significantly reduced bitterness and astringency in tea samples. Among the SMC treatments, T1, T4 and T5 significantly improved the appearance, aroma and taste of the tea, and the sensory evaluation scores were significantly increased. Metabolomic analysis revealed that down-regulated non-volatile metabolites outnumbered up-regulated ones in treatments, with terpenoids, flavonoids and flavonoid glycosides being the most notably decreased. These findings offer theoretical and technical support for precision quality improvement and advance the mechanistic understanding of how fixation regulates bitterness and astringency taste of green tea. • A new sieve mesh cover (SMC) fixation process was designed for flat green tea. • The SMC fixation reduced the bitterness of summer-autumn Longjing tea. • The SMC fixation enhanced the aroma quality of summer-autumn Longjing tea. • The SMC fixation method raised amino acid levels in summer-autumn Longjing tea. • The SMC fixation decreased catechin content in summer-autumn Longjing tea.