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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.

HEARTY ROOIBOS NEWS

Results from a clinical trial showing that Rooibos significantly reduces the risk of heart disease were announced at a Rooibos Science Café at the MTN ScienCentre in Cape Town on 26 November 2008. About 150 members of the media, health sector and the Rooibos industry attended the event and were delighted about this new and conclusive evidence of the health promoting properties of Rooibos in humans.

Researchers traced the protective effect of Rooibos by looking at two important markers in the blood, as well as the oxidative status of the 40 adults who participated in the study. They found a significant decrease in conjugated dienes and malondialdehydes of 35% and 50% respectively – two blood markers that indicate oxidative damage – in the group that drank six cups of Rooibos per day for 6 weeks. “This means that Rooibos may help to slow down atherosclerosis, or the hardening of arteries,” explained Dr Jeanine Marnewick, who led the clinical trial at the Oxidative Stress Research Centre at the Cape Peninsula University of Technology. On top of this, Rooibos also increased the levels of the body’s own ‘super anti-oxidant’ called glutathione and helped to reduce the levels of “bad” LDL-cholesterol significantly.

“This is incredible news for Rooibos and the public,” said Mientjie Mouton, a director of the South African Rooibos Council. “We need scientific evidence to substantiate what we have always known – that Rooibos is good for you!”

Dr Marnewick also explained that they asked study participants for feedback on how they felt during the clinical trial. “Many of them reported feeling irritated during the washout period when they could not drink Rooibos, and much calmer once they were enjoying their six cups of Rooibos per day. That is why she will continue the clinical trial to look at the effect of Rooibos and stress.”

At the same science café Dr Carl Albrecht, head of Research at the Cancer Association of South Africa (CANSA) gave an overview of nearly a decade of research at South Africa’s Medical Research Council into the ability of Rooibos to prevent or slow down cancer. He also emphasised the importance of the ability of Rooibos to reduce oxidative stress in the body, as shown by the results of a study on rats, published in 2003. “I am elated that Dr Marnewick and her team were now able to prove that Rooibos also has this effect in the human body,” he added. Oxidative stress plays a role in the development of a whole range of diseases, including cancer, stroke, heart and liver disease, diabetes and Alzheimer’s. Another important milestone was the discovery, published in 2004, that Rooibos can prevent and slow down skin cancer in mice. “The next challenge is to prove that Rooibos can also prevent cancer in people, and I believe that there is a good chance that we’ll be able to prove this,” Dr Albrecht said.

This Rooibos Science Café was organised by the South African Rooibos Council who invests in Rooibos research, along with funding partners such as South Africa’s National Research Foundation as well as the Medical Research Council and CANSA.