Latest findings on polyphenols and antioxidants in Rooibos

Stellenbosch, 28 August 2012

In a three-year study supported by the SA Rooibos Council (2009 – 2011) Prof Lizette Joubert and her team at the Nietvoorbij Research Institute of Agricultural Research Council looked at the variation in phenolic content and antioxidant activity of fermented Rooibos tea, and how this is affected by different production seasons and quality grades.

The aim of this study was to generate representative content values for the principal monomeric phenolic compounds present in a ‘cup-of-tea’ rooibos infusion as normally consumed (regular, fermented Rooibos tea).

Samples were obtained from different geographical areas, and different producers, to capture as much potential variation in the phenolic composition and antioxidant activity as possible to create a representative data set suitable for inclusion in food composition databases. A total of 114 Rooibos samples were analysed over three productions seasons (2009, 2010 and 2011) and quality grades (A, B, C and D).

Their research article based on the outcomes of this study has been peer-reviewed and accepted for publication in the Journal of Agricultural Food Chemistry (published online on 24 Aug 2012 – see http://pubs.acs.org/doi/pdf/10.1021/jf302583r).

Key findings from the study

1. The major phenolic constituents in fermented rooibos are isoorientin and orientin (> 10 mg/L), with quercetin-3-robinobioside, phenylpyruvic acid glucoside and aspalathin present at > 5 mg/L. Isovitexin, vitexin and hyperoside were present at < 3 mg/L, rutin, ferulic acid and isoquercitrin at < 2 mg/L and nothofagin at < 1 mg/L. Only traces of luteolin-7-O27 glucoside and the aglycones quercetin, luteolin, and chrysoeriol were present. (See Table 5 in research article.)
2. Substantial variation was observed in the individual content values of the phenolic compounds and total antioxidant capacity within production seasons and quality grades.
3. Production season had no significant effect on the total polyphenol content.
4. The higher quality grade samples tend to be associated with higher levels of the phenolic compounds (Table 6). Grade A samples had the highest mean values for most phenolic compounds and also contained significantly higher levels of aspalathin, isoquercitrin, rutin, hyperoside and quercetin-3-O-robinobioside than the other grades.