Identification and Evaluation of Colour Change in Rosemary and Biluochun Tea Infusions

The colour of tea beverages during processing and storage significantly influences their visual quality. However, natural pink tea products are rare. This study investigated the mechanism behind the pink colouration in the mixed infusion of Biluochun (a green tea) and rosemary.

Infusions of Biluochun (B), rosemary (R), and their mixture (BR), brewed with boiling water for 10 min, were analysed using liquid chromatography-mass spectrometry (LC-MS). Additionally, the pH value and tea pigment content were measured.

A total of 134 differential metabolites (DEMs) were detected. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that phenylalanine metabolism and tyrosine metabolism pathways were enriched with abundant DEMs. Some amino acids in BR showed degradation. The content of pelargonin, a compound in the anthocyanin biosynthetic pathway, was significantly elevated in BR compared to that in B and R. DEMs related to fatty acid metabolism were at low levels in BR. Other compounds, such as quercetin, caffeate, rosmarinic acid, and isoferulic acid, were also more abundant in BR. No significant differences in pH value and tea pigment content were found among the three infusions.

A model of pink colouration formation in BR was proposed based on the results of this study. Some substances in Biluochun and rosemary were released during the brewing process. Tyrosine was converted into p-coumaric acid, which further reacted to form pelargonin. Pelargonin, an orange-red (pH ≈ 5.0) anthocyanin, was the primary contributor to the pink colouration in BR. Additionally, p-coumaric acid formed co-pigments such as quercetin, caffeic acid, rosmarinic acid, and isovaleric acid. These co-pigments stabilised or enhanced the colour of pelargonin through co-pigmentation. The findings provide a theoretical basis for optimising tea processing techniques and improving quality control in beverage production.