Impact of Cold versus Hot Brewing on the Phenolic Profile and Antioxidant Capacity of Rooibos (Aspalathus linearis) Herbal Tea.

Elisabetta Damiani,Biancamaria Senizza,Patricia Carloni,Elizabeth Joubert,Dalene De Beer,Luigi Lucini,Luca Tiano,Gabriele Rocchetti

doi:10.3390/antiox8100499

Abstract

Consumption of rooibos (Aspalathus linearis) as herbal tea is growing in popularity worldwide and its health-promoting attributes are mainly ascribed to its phenolic composition, which may be affected by the brewing conditions used. An aspect so far overlooked is the impact of cold brewing vs regular brewing and microwave boiling on the (poly) phenolic profile and in vitro antioxidant capacity of infusions prepared from red (‘fermented’, oxidized) and green (‘unfermented’, unoxidized) rooibos, the purpose of the present study. By using an untargeted metabolomics-based approach (UHPLC-QTOF mass spectrometry), 187 phenolic compounds were putatively annotated in both rooibos types, with flavonoids, tyrosols, and phenolic acids the most represented type of phenolic classes. Multivariate statistics (OPLS-DA) highlighted the phenolic classes most affected by the brewing conditions. Similar antioxidant capacities (ORAC and ABTS assays) were observed between cold- and regular-brewed green rooibos and boiled-brewed red rooibos. However, boiling green and red rooibos delivered infusions with the highest antioxidant capacities and total polyphenol content. The polyphenol content strongly correlated with the in vitro antioxidant capacities, especially for flavonoids and phenolic acids. These results contribute to a better understanding of the impact of the preparation method on the potential health benefits of rooibos tea.

Highlights

In recent years, the herbal tea prepared from stems and leaves of the endemic South African leguminous shrub—Aspalathus linearis, commonly known as rooibos or ‘Red Bush’ tea, has carved a niche for itself as a refreshing, caffeine-free, healthy drink, enjoyed by consumers of all ages from children to the elderly, and even by pregnant women and their new-born babies
Concerning inter-batch variations it can be noted that for the green rooibos samples, the brewing method plays a relevant role; the four long cut samples showed approximately half the levels of antioxidant capacity than the fine cut samples when prepared with the common, regular hot brewing method (R), but these inter-batch differences were less noticeable in the boiled brews (B) and were not observed at all in the cold brews (C)
The results reported here show that cold-brewed rooibos differs in antioxidant capacity and phenolic profile compared to the hot brews, and that the different brewing conditions have a different impact depending on the oxidation status of rooibos plant material considered

Summary

Introduction

The herbal tea prepared from stems and leaves of the endemic South African leguminous shrub—Aspalathus linearis, commonly known as rooibos or ‘Red Bush’ tea, has carved a niche for itself as a refreshing, caffeine-free, healthy drink, enjoyed by consumers of all ages from children to the elderly, and even by pregnant women and their new-born babies. Rooibos is characterized by the presence of aspalathin (a dihydrochalcone), which has been found to be a major compound contributing to the antidiabetic properties of this herbal tea [2,3]. This aspect makes rooibos extracts interesting as a condition-specific antioxidant, as such antioxidants that deliver more than just the ambiguous benefit of antioxidant activity, are in demand by the nutraceutical industry [4]. The fermented tea is characterized by a slightly sweet taste and flavor with honey, woody and herbal-floral notes [7], unlike the unfermented version which has a predominant grassy aroma and flavor

Objectives

Methods

Results

Discussion

Conclusion