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

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.

Feature Reduction for the Classification of Bruise Damage to Apple Fruit Using a Contactless FT-NIR Spectroscopy with Machine Learning

“…There is some possibility that higher performance may be obtained using deep learning, which has been applied successfully in other agro-product classification scenarios [36]. However, effective training of a deep learning classifier typically requires a much larger dataset than was available for this study [32,37]. Furthermore, many studies that apply machine learning (including deep learning) do not include error bars in their performance evaluations, so there is some question as to whether reported improvements in classification are statistically significant [38].…”

“…The precision of the different classifiers were evaluated using the testing set. In order to obtain error bars, jackknife with leave-out-one was implemented [32,33]. For the training set error, the left-out instances were 1/3 of the training set, while for the testing set all instances were left out one-by-one to obtain the jacknife estimate of the standard deviation.…”

The effects of a green Rooibos (Aspalathus linearis) extract on metabolic parameters and adipose tissue biology in rats fed different obesogenic diets

“…Other green Rooibos extracts with different relative proportions of constituents may therefore be more efficacious at reversing DIO and DIMD than the GRT used in the present study. The composition of Rooibos extracts is influenced by many factors, including oxidation status of the plant material, the solvent used (methanol vs. water) 55 and the temperature of the water used to prepare aqueous extracts, 56 and it is therefore essential to use extracts with standardised preparation methods and known composition in studies of this kind. It is also important to consider that GRT was administered against the background of ongoing obesogenic feeding, and it may be valuable to understand whether green Rooibos extracts could contribute to enhanced recovery from obesity or reversal of “obesogenic memory” in adipose tissue 57 upon cessation of obesogenic feeding.…”

Phenolic Profiling with Antioxidant Activity of Thai Traditional Medicines: Phikud Bencha Thien, Kot and Keson Remedies

“…Several studies have shown that phenolic constituents derived from plants are potential antioxidant agents. 14 Among extraction solvents, polar solvents such as water, methanol, and ethanol are more effective in extracting phenolic compounds than non-polar solvents 15 , implying that the different solvent selected for an extraction can affect the phenolic constituents in an extract and its antioxidant properties. Therefore, the phenolic acid and flavonoid phytochemical components and the antioxidant activity were determined for BT, BK, and BKS extracts prepared by using different solvents in this study.…”