Effect of Extraction Time and Additional Heating on the Antioxidant Activity of Rooibos Tea (<i>Aspalathus</i><i>linearis</i>) Extracts

Abstract

The antioxidant activity of aqueous extracts of rooibos tea (Aspalathus linearis) as affected by extraction time and additional exposure to heat was determined using the Rancimat and β-carotene bleaching methods. Two fractions of the aqueous extract, consisting of phenolic compounds soluble in ethyl acetate and, after removal of these substances, the remaining polymeric fraction only soluble in water, were analyzed for antioxidant activity. The effect of concentration of the soluble solids of rooibos tea on the oxidation of lard and linoleic acid were assessed by the Rancimat and β-carotene bleaching methods, respectively. Increasing extraction time resulted in increasing induction time for a fixed ratio of soluble solids to lard. Additional heat treatment of tea extracts also increased antioxidant activity according to the Rancimat method, without exhibiting a prooxidative effect at high concentrations. Antioxidant activity of rooibos was demonstrated for both the ethyl acetate soluble phenolic compounds and the highly polymerized polyphenols only soluble in water. Keywords: Antioxidant; rooibos; extraction time; heat treatment; Rancimat; β-carotene bleaching