Metabolomic Profiling Reveals the Impact of Yellowing Duration on the Flavor of Niangniang Tea

Yellowing is a critical post-fermentation step in the processing of Chinese yellow tea that strongly influences flavour quality. However, the underlying chemical changes in Niangniang tea (NNT) during yellowing remain unclear. In this study, a widely targeted metabolomics approach combined with sensory evaluation was employed to characterize flavour formation in NNT across five yellowing durations (0–32 h). A total of 111 volatile metabolites and 65 differential non-volatile metabolites were identified across all samples. Amino acids accumulated mainly in the early stage, whereas soluble sugars and several floral- and sweet aroma-related compounds, including neral, ionone and α-ionone, were enriched during the intermediate stage. In contrast, most metabolites declined in the late stage, resulting in a weaker flavour profile. Collectively, the intermediate yellowing stage (16–24 h) represented the most balanced phase for flavour formation, where taste and aroma were optimally coordinated. Shorter yellowing times (<16 h) retained grassy and bitter attributes due to incomplete precursor transformation, whereas extended yellowing (32 h) led to sensory deterioration accompanied by the accumulation of undesirable flavour notes. These findings provide new insights into the temporal regulation of flavour-active compounds during yellowing and offer a theoretical basis for optimising NNT processing.