Antioxidant activities of aqueous extract from Stevia rebaudiana stem waste to inhibit fish oil oxidation and identification of its phenolic compounds.

We investigated the potential for exploiting Stevia rebaudiana stem (SRS) waste as a source of edible plant-based antioxidants finding for the first time that the hot water extract of SRS had significantly higher antioxidant activity against fish oil oxidation than that of the leaf, despite SRS extract having lower total phenolic content, DPPH radical scavenging activity and ORAC values. To locate the major antioxidant ingredients, SRS extract was fractionated using liquid chromatography. Five phenolic compounds (primary antioxidant components in activity-containing fractions) were identified by NMR and HR-ESI-MS: vanillic acid 4-O-β-d-glucopyranoside (1), protocatechuic acid (2), caffeic acid (3), chlorogenic acid (4) and cryptochlorogenic acid (5). Further analysis showed that, among compounds 2-5, protocatechuic acid had the highest capacity to inhibit peroxides formation, but exhibited the lowest antioxidant activities in DPPH and ORAC assays. These results indicate that SRS waste can be used as strong natural antioxidant materials in the food industry.