Optimising the polyphenolic content and antioxidant activity of green Rooibos (Aspalathus linearis) using beta-cyclodextrin assisted extraction

Abstract

Antioxidant activity associated with green rooibos infusions is attributed to the activity of polyphenols, particularly aspalathin and nothofagin. This study aimed to optimise β-cyclodextrin (β-CD)-assisted extraction of crude green rooibos (CGRE) via total polyphenolic content (TPC) and antioxidant activity assays. Response surface methodology (RSM) permitted optimisation of β-CD concentration (0–15 mM), temperature (40–90 ◦C) and time (15–60 min). Optimal extraction conditions were: 15 mM β-CD: 40 ◦C: 60 min with a desirability of 0.985 yielding TPC of 398.25 mg GAE·g −1 , metal chelation (MTC) of 93%, 2,20 -azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging of 1689.7 µmol TE·g −1 , ferric reducing antioxidant power (FRAP) of 2097.53 µmol AAE·g −1 and oxygen radical absorbance capacity (ORAC) of 11,162.82 TE·g −1 . Aspalathin, hyperoside and orientin were the major flavonoids, with quercetin, luteolin and chrysoeriol detected in trace quantities. Differences (p < 0.05) between aqueous and β-CD assisted CGRE was only observed for aspalathin reporting the highest content of 172.25 mg·g −1 of dry matter for extracts produced at optimal extraction conditions. Positive, strong correlations between TPC and antioxidant assays were observed and exhibited regression coefficient (R2 ) between 0.929–0.978 at p < 0.001. These results demonstrated the capacity of β-CD in increasing polyphenol content of green rooibos.