The influence of fixation temperature on Longjing tea taste profile and the underlying non-volatile metabolites changes unraveled by combined analyses of metabolomics and E-tongue

Fixation is a critical step in green tea quality formation. In this study, an integrated analysis of sensory evaluation, electronic tongue, quantitative analysis of major chemical constituents and metabolomics analysis was performed to investigate the effect of fixation temperature (205 °C, 215 °C, 225 °C) on the taste profile of Longjing green tea and the underlying non-volatile metabolites changes. A total of 17 major chemical constituents and 100 non-volatile metabolites were investigated. 31 key differential metabolites were screened. With the increase of fixation temperature, the content of cis-catechins (epicatechin) and dimeric catechins (theaflavin, theaflavin-3′-gallate, theaflavin-3,3′-gallate) were significantly decreased, which may reduce the bitterness. The astringency of tea infusions varied depending mainly on the content changes in flavonol glycosedes, along with trans-catechins (gallocatechin, catechin gallate, gallocatechin gallate). Besides, the contents of amino acids, phenolic acids and carbohydrates were also varied. These substances were altered possibly due to variations in epimerization, oxidative polymerization, and thermal degradation. Longjing tea treated with an appropriate fixation temperature of about 215 °C rendered optimal taste quality. This study could advance the understanding of the fixation regulation of green tea taste and its underlying mechanism.