Phenylethanoid glycosides as major antioxidants in Lippia multiflora herbal infusion and their stability during steam pasteurisation of plant material

Lippia multiflora, a perennial, aromatic shrub commonly known as bush tea has recently been identified as an African plant with high commercial potential due to its medicinal properties. The plant material was subjected to steam pasteurisation to improve its microbial quality. The major compounds of L. multiflora herbal infusion, i.e. the phenylethanoid glycosides (PhGs), verbascoside, isoverbascoside, nuomioside A and isonuomioside A, and the flavone, luteolin-7-O-glucuronide were quantified by HPLC. Verbascoside was the most abundant PhG. The PhGs are of interest due to their pharmacological properties. Liquid chromatography tandem–mass spectrometry (LC–MS2) was used to tentatively identify the compounds. The on-line DPPH (2,2′-diphenyl-1-picrylhydrazyl radical) scavenging assay (reaction time = 0.45 s) applied to the infusion in “quantitative” mode, showed the relative order of activity: isonuomioside A > isoverbascoside > verbascoside > nuomioside A. In the microplate assay (reaction time = 2 h), isoverbascoside and verbascoside had similar activity. Both compounds were less active in the latter assay than the well-known flavan-3-ol antioxidant, (−)-epigallocatechin gallate, but more active than caffeic acid and an ester, rosmarinic acid. Steam pasteurisation of L. multiflora leaves at maximum exposure (150 s at ca 99 °C) for improved microbial quality did not decrease the soluble solids content, phenolic content and antioxidant activities of the infusion compared to the untreated control (p < 0.05). The PhG content of the water soluble solids was as high as 15%, underscoring the potential of L. multiflora extract as functional ingredient.