Comparison of Phenolic Content and Antioxidant Activity for Fermented and Unfermented Rooibos Samples Extracted with Water and Methanol

Eslam A Hussein; Christopher Thron; Mehrdad Ghaziasgar; Mattia Vaccari; Jeanine L Marnewick; Ahmed A Hussein

doi:10.3390/plants11010016

Comparison of Phenolic Content and Antioxidant Activity for Fermented and Unfermented Rooibos Samples Extracted with Water and Methanol

Plants (Basel). 2021 Dec 22;11(1):16. doi: 10.3390/plants11010016.

Authors

Eslam A Hussein¹, Christopher Thron², Mehrdad Ghaziasgar¹, Mattia Vaccari³, Jeanine L Marnewick⁴, Ahmed A Hussein⁵

Affiliations

¹ Department of Computer Science, University of the Western Cape, Bellville, Cape Town 7535, South Africa.
² Department of Science and Mathematics, University-Central Texas, Killeen, TX 76549, USA.
³ Department of Physics and Astronomy, University of the Western Cape, Cape Town 7535, South Africa.
⁴ Applied Microbial and Health Biotechnology Institute, Cape Peninsula University of Technology, Symphony Road, Bellville, Cape Town 7535, South Africa.
⁵ Department of Chemistry, Cape Peninsula University of Technology, Symphony Road, Bellville, Cape Town 7535, South Africa.

Abstract

Rooibos is brewed from the medicinal plant Aspalathus linearis. It has a well-established wide spectrum of bio-activity properties, which in part may be attributed to the phenolic antioxidant power. The antioxidant capacity (AOC) of rooibos is related to its total phenolic content (TPC). The relation between TPC and AOC of randomly selected 51 fermented (FR) and 47 unfermented (UFR) rooibos samples was studied after extraction using water and methanol separately. The resulted extracts were assessed using two antioxidant assays, trolox equivalent antioxidant capacity (TEAC) and ferric reducing antioxidant power (FRAP). The results were analyzed using both simple statistical methods and machine learning. The analysis showed different trends of TPC and AOC correlations of FR and UFR samples, depending on the solvent used for extraction. The results of the water extracts showed similar TPC and higher AOC of FR than UFR samples, while the methanolic extracted samples showed higher TPC and AOC of UFR than FR. As a result, the methanolic extracts showed better agreement between TPC and AOC than water extracts. Possible explanations are given for these observed results. Although, the current literature demonstrates direct correlations of the TPC and AOC of rooibos water extracts. This study showed deviation and highlighted the importance of solvent selection and analysis methodology as an important factor in determining the TPC/AOC correlation and subsequently the expectation of the actual health benefits of rooibos herbal tea. In particular, unfermented and fermented samples can be accurately identified on the basis of a combination of assays (any two of TPC, FRAP and TEAC), especially if methanol is the solvent used. Machine learning analysis of assay data provides nearly identical results with classical statistical analytical methods. This is the first report on machine learning analysis and comparison of the TPC and AOC of rooibos herbal tea extracted with methanol and water, and highlights the importance of using methanol as a solvent to evaluate its AOC.

Keywords: Aspalathus linearis; antioxidant; baseline; classification; machine learning; methanol extract; phenolics; rooibos tea; small data; uncertainty quantification; water extract.