Effect of Red Galangal–Citronella Grass Decoction Ratios on Phenolic Content, Flavonoid Content, and Antioxidant Activity

Whether the plants in polyherbal decoctions interact is rarely tested. Red galangal (Alpinia purpurata) and citronella grass (Cymbopogon nardus) were decocted at ratios of 100:0, 75:25, 50:50, 25:75, and 0:100 (w/w) and analysed for total phenolic content (TPC), total flavonoid content (TFC), and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical-scavenging activity; mixtures were compared with additive expectations. TPC decreased from 593.84 to 375.75 mg gallic acid equivalents/L as citronella grass increased, whereas TFC peaked at 25:75 (49.90 mg quercetin equivalents/L). The half-maximal inhibitory concentration (IC₅₀) ranged from 1106.64 µg/mL (red galangal) to 1682.97 µg/mL (citronella grass). Every mixture’s activity matched the additive prediction (observed/expected 0.96–1.05, p > 0.3), whereas TFC deviated strongly (0.74–1.41). Activity correlated with TPC (r = 0.97, p = 0.007) but not TFC. The ratio therefore adjusts antioxidant capacity proportionally, without synergy, and phenolics rather than flavonoids govern it.