Effect of Pressing Process on Metabolomics Profiling and Sensory Properties: Comparative Study of Fu Brick Tea Versus Fu Loose Tea from Identical Raw Dark Tea

Fu tea is a unique microbially fermented tea from China, and has two types. The primary distinction between these two types—Fu brick tea (FBT) and Fu loose tea (FLT)—is that FBT is compressed into bricks, whereas FLT is maintained as loose leaves. To investigate the differences in the chemical composition and sensory characteristics between the two types of Fu tea, this study utilized samples produced from the same batch of raw dark tea material to ensure comparability. Multiple analytical approaches were applied, including main active component analysis, sensory flavor evaluation, metabolomics, and differential characteristic component analysis. These methods were employed to comprehensively compare and characterize the two tea types. The results showed that compared to FBT, FLT exhibited a longer-lasting fungal flower aroma and a mellower taste. Furthermore, the quantity of Eurotium cristatum in FLT was 56.7% higher than that in FBT, indicating a significant difference. Untargeted ultrahigh-performance liquid chromatography–mass spectrometry was used to screen metabolites in Fu tea samples, and after multivariate statistical analysis, 12 differential metabolites were finally identified, including phenolic acids and their derivatives, coumarins and their derivatives, phenylpropanoids, and flavonoids and their glycosides. Subsequently, a targeted UHPLC-QQQ-MS/MS-based method was established and validated for the main differential metabolites, phenolic acids and catechins. The results indicated that gallic acid, catechin, and epicatechin can serve as characteristic markers for distinguishing between FBT and FLT. Notably, the content of gallic acid in FLT was 168.6% higher than that in FBT. These findings elucidate the impact of the pressing process on Fu tea, provide guidance for discriminating between FBT and FLT, and are significant for quality control in the industrial production of Fu tea.