Changes and metabolic mechanisms of organic acids in the fermentation of pu-erh tea

Organic acids are one of important flavor and bioactive components in tea beverages, which changed significantly during the microbial fermentation of pu-erh tea, but their metabolism is unclear. In this research, pu-erh tea was industrially fermented, and changes in 74 components including 26 organic acids were measured. After fermentation, the contents of 20 components decreased [Variable importance in projection (VIP) > 1.0; p < 0.05 and Fold-change (FC) FC < 0.67], Meanwhile, contents of 13 compounds increased (VIP >1, p < 0.05, and FC > 1.5). These changes lead to a reddish-brown color, mellow taste, and stale aroma of tea infusion. Tannases, laccases, and tyrosinase of Aspergillus, Pigmentiphaga, Agrobacterium, and Rasamsonia were assumed to catalyze the hydrolysis and oxidation of polyphenols, which resulted the decrease in gallate catechins and increase in gallic acid and theabrownins. The pathways of organic acid metabolism are composed of phenylalanine, tyrosine, and tryptophan biosynthesis (ko00400); citrate cycle (ko00020); and C5-branched dibasic acid metabolism (ko00660), which are catalyzed by 26 enzymes mainly from Pantoea, Staphylococcus, and Rasamsonia. Together, microbial functional genes associated with changes in characteristic components especially organic acids through the microbial fermentation of pu-erh tea were systematically revealed. These provide novel insights about the fermentation mechanism of ripen pu-erh tea.