Oxidation of the Dihydrochalcone Aspalathin Leads to Dimerization

Abstract

Aspalathin and nothofagin are typical ingredients of unfermented rooibos (Krafczyk, N.; Glomb, M. A. J. Agric. Food Chem. 2008, 56, 3368). During oxidation these dihydrochalcones were degraded to higher molecular weight browning products under aerated and nonenzymatic conditions. In the early stages of browning reactions aspalathin formed two dimers. These two compounds were unequivocally established as atropisomers stemming from oxidative A to B ring coupling. Multilayer countercurrent chromatography (MLCCC) and preparative high-performance liquid chromatography (HPLC) were applied to obtain pure substances. The purity and identity of isolated dimers were confirmed by different NMR experiments, HPLC-DAD-MS, and HR-MS. In parallel to the formation of chromophores during the fermentation of black tea, the formation of aspalathin dimers implies an important mechanistic channel for the generation of color during the processing of rooibos.

Green Rooibos Extract from <i>Aspalathus linearis</i>, and its Component, Aspalathin, Suppress Elevation of Blood Glucose Levels in Mice and Inhibit α-amylase and α-glucosidase Activities <i>in vitro</i>

“…On the other hand, unfermented rooibos tea, known as "green rooibos tea", has been the focus of research because it contains higher levels of flavonoids and polyphenols than fermented rooibos tea, in which flavonoids and polyphenols are degraded and form dimers by oxidation during the fermentation process (Krafczyk et al, 2009, Heinrich et al, 2012. In particular, aspalathin (Asp), a major flavonoid in rooibos, is present at twice the level in unfermented compared to fermented tea (Bramati et al, 2002, Bramati et al, 2003.…”

Antioxidants of Rooibos Beverages

“…Following fractionation of an HPLC eluate of an ethyl acetate fraction of fermented rooibos and incubation of the fractions to measure color formation (absorbance at 450 nm based on maximum absorbance for fermented rooibos), aspalathin was identified as the most important substrate for the formation of colored products (Krafczyk et al, 2009a). Incubation of aspalathin under aerated and deaerated conditions confirmed that the browning reaction is oxygen-dependent.…”

“…Beelders et al (2012b) studied the composition of hotwater infusions of both fermented and green rooibos, identifying 28 compounds of which all, except for the C5′-hexosyl derivative of aspalathin, have been previously identified in fermented rooibos. The oxidation mechanism of aspalathin, as recently elucidated by Glomb and co-workers (Krafczyk and Glomb, 2008;Krafczyk et al, 2009a;Heinrich et al, 2012), will be discussed in the section Processing of Fermented Rooibos and Oxidative Conversion of Aspalathin, below. It is a novel compound and its structure suggests that it may be an oxidation product formed during fermentation.…”

“…Recent studies by Glomb and co-workers (Krafczyk and Glomb, 2008;Krafczyk et al, 2009a;Heinrich et al, 2012) provide further insight into the oxidation of aspalathin and its role in color formation. Reaction products from the oxidation of aspalathin are depicted in Figure 14.1.…”

Bräunungsmechanismen pflanzlicher Lebensmittel

“…Weder Eriodictyole, Isoorientin, Orientin, noch die Dimere sind jedoch gefärbte Substanzen. Sie stellen lediglich Intermediate innerhalb der Fermentationskaskade dar, und werden im weiteren Verlauf zu braunen Melaninen abgebaut .…”