Bitter taste impact and thermal conversion of a naringenin glycoside isolated from Cyclopia genistoides

A naringenin derivative, isolated from Cyclopia genistoides, a bitter tasting herbal tea, especially when in green (unoxidized) form, was identified as (2S)-5-[α-l-rhamnopyranosyl-(1→2)-β-d-glucopyranosyloxy]naringenin (1). The compound partially epimerizes to (2R)-5-[α-l-rhamnopyranosyl-(1→2)-β-d-glucopyranosyloxy]naringenin (2) when heated at different temperatures (80, 90, 100, 110, and 120 °C) for a prolonged period in a phosphate buffer at pH 5. The fractional conversion model predicted the decrease in the concentration of compound 1 the best. The activation energy of the conversion reaction was calculated as 99.16 kJ mol–1. Prolonged heating resulted not only in formation of compound 2 but eventually a decrease in its concentration and the formation of another conversion product, (E)-2′-[α-l-rhamnopyranosyl-(1→2)-β-d-glucopyranosyloxy]-4′,6′,4-trihydroxychalcone (3). In contrast, naringin, glycosylated at C-7, remained stable when heated under the same conditions (100 °C for 6 h at pH 5). The bitter intensity of compound 1 was substantially less than that of naringin, both tested at 0.04 mM, a concentration typical of compound 1 in an herbal tea infusion of green C. genistoides. This comparison indicates that the position of the sugar moiety plays an important role in determining both bitter intensity and heat stability of naringenin glycosides.