Fermentation-associated compositional changes in hot-brewed Chenopodium formosanum leaf infusion under defined microbial conditions.

UNLABELLED: Chenopodium formosanum leaves are an underutilized agricultural by-product rich in phenolic-related constituents. This study evaluated fermentation dynamics and compositional changes of brewed C. formosanum leaf infusion (CFLI) during defined kombucha-type fermentation. An infusion prepared at 90 °C for 20 min was selected based on phenolic-related parameters and antioxidant activity. Fermentation with yeasts, acetic acid bacteria, and lactic acid bacteria showed stage-dependent microbial changes, characterized by early proliferation, an active middle transition, and later population decline. These changes were accompanied by acidification, sucrose hydrolysis, organic acid accumulation, and increases in total phenolic content, total flavonoid content, rutin, and DPPH radical-scavenging activity. In an exploratory Drosophila melanogaster assay, CFLI and day 6 fermented samples showed changes in climbing performance and antioxidant enzyme activities; however, these responses cannot be attributed specifically to fermentation-derived effects because no sucrose-supplemented, matrix-matched unfermented control was included. These findings support CFLI as a substrate for defined fermented beverage systems. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10068-026-02221-w.