Saturday, March 5, 2016

The SWEETNESS of ROOIBOS TEA

Have been reading Alexander McCall Smith's "The Woman Who Walked in Sunshine," his most recent No. 1 Ladies' Detective Agency novel. The heroine of the novel, Mma Ramotswe, keeps her spirits up by drinking red bush tea—rooibos.

Rooibos is inherently sweet, thanks to a dihydrochalcone unique to it called asplathin (2′,3,4,4′,6′-pentahydroxy-3′-C-β-d-glucopyranosylhydrochalcone). The chemical structure looks like this:


The dihydrochalcone part is the structure with the circle marked B.

Dihydrochalcones are sweet because they interact with the sweet receptor, as I show in this little animation:



Starting with pink sugar molecule: it comes in and binds in the bowl-like parts of the receptor molecules; this causes the receptor molecules to change shape, which in turn causes the tail ends of these molecules inside the cell to send off a signal (the yellow arrow) that something sweet has come along. But once the receptor molecules change shape, the sugar molecule pops away, the receptor molecules regain their original shape, and the taste cell doesn't experience "sweet" anymore.

The situation with the very very sweet dihydrochalcone glycoside molecules, like the ones in rooibos, is quite different. Here the sweet molecule works its way into the cell membrane to attach onto the receptor molecules where they cross the membrane. This process takes a little time, so it takes a beat or two before we taste the sweetness of these molecules. However, once attached, the dihydrochalcones take a long time to pop off, so the "sweet" message is sent repeatedly. That is why the sweetness of these compounds lingers. ENJOY! (and let me know what you think!)

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