Background: Zingiber zerumbet (L.) is a traditional Malaysian folk remedy that contains several interesting bioactive compounds of pharmaceutical quality.

Methods: Total flavonoids and total phenolics content from the leaf, stem, and rhizome of Z. zerumbet at 3 different growth stages (3, 6, and 9 months) were determined using spectrophotometric methods and individual flavonoid and phenolic compounds were identified using ultra-high performance liquid chromatography method. Chalcone Synthase (CHS) activity was measured using a CHS assay. Antioxidant activities were evaluated by ferric reducing antioxidant potential (FRAP) and 1,1-diphenyl-2-picrylhydrazyl (DPPH) assays. The antibacterial activity was determined against Gram-positive and Gram-negative bacteria using the disc diffusion method.

Results: Highest content of total flavonoid [29.7 mg quercetin equivalents (QE)/g dry material (DM)] and total phenolic (44.8 mg gallic acid equivalents (GAE)/g DM) were detected in the rhizome extracts of 9-month-old plants. As the plant matured from 3 to 9 months, the total flavonoid content (TFC) and total phenolic content (TPC) decreased in the leaf, but increased significantly in the rhizomes. Among the secondary metabolites identified, the most abundant, based on the concentrations, were as follows: flavonoids, catechin > quercetin > rutin > luteolin > myricetin > kaempferol; phenolic acids, gallic acid > ferulic acid > caffeic acid > cinnamic acid. Rhizome extracts from 9-month-old plants demonstrated the highest CHS activity (7.48 nkat/mg protein), followed by the 6-month-old rhizomes (5.79 nkat/mg protein) and 3-month-old leaf (4.76 nkat/mg protein). Nine-month-old rhizomes exhibited the highest DPPH activity (76.42 %), followed by the 6-month-old rhizomes (59.41 %) and 3-month-old leaves (57.82 %), with half maximal inhibitory concentration (IC50) of 55.8, 86.4, and 98.5 μg/mL, respectively, compared to that of α- tocopherol (84.19 %; 44.8 μg/mL) and butylated hydroxytoluene (BHT) (70.25 %; 58.6 μg/mL). The highest FRAP activity was observed in 9-month-old rhizomes, with IC50 of 62.4 μg/mL. Minimal Inhibitory Concentration (MIC) of Z. zerumbet extracts against Gram-positive and Gram-negative bacteria ranged from 30 to >100 µg/mL. Among the bacterial strains examined, Staphylococcus aureus was sensitive to the leaf extract of Z. zerumbet, with MIC of 30.0 μg/mL and other strains were sensitive to the rhizome extracts.

Conclusions: Three- and 9-month-old plants are recommended when harvesting the leaf and rhizome of Z. zerumbet, respectively, in order to obtain effective pharmaceutical quality of the desired compounds.