BITTERNESS & HOW WE PERCEIVE IT in WINE & TEA (& COFFEE)

One of the outstanding characteristics of both tea and wine is their bitterness, yet the compounds that confer bitterness to each of these beverages are quite different chemically.  In tea, caffeine and the catechins are major bitter compounds (example below: (+)-catechin), 

while in wine it’s primarily alcohol (ethanol).

How can such different chemicals yield pretty much the same sensation that we call bitter? and why do people differ so much in their perception and appreciation of bitterness? The answers to these two questions lie in the genes for bitter receptors.  

We humans have some 43 different functional genes for bitterness receptors in a family of genes called TAS2R.  Each of these genes codes for a specific protein. Bitter-sensing taste bud cells use to these proteins to catch bitter molecules as they pass by.  Once caught, the cell sends a non-specific message — “bitter” — to the brain.

As you can quickly see, this number of different proteins (and therefore different receptors) is far lower than the number of different chemicals we call bitter. In other words there is no receptor that responds to just one bitter compound. For example,  the receptor protein produced by gene TAS2R38 responds to ethanol, but also to phenylthiocarbamide (PTC), the compound that you may (or may not) have tasted on a piece of paper when you were in biology class.

At the same time, ethanol binds not just to TAS2R38, but also to other bitterness proteins, for example the one from a gene called TAS2R13. A different TAS2R gene — TAS2R7 — produces a protein that responds to caffeine, and at least four different proteins from four different TAS2R genes respond to epicatchins. None of these epicatechin-responsive proteins appears to be the same as the protein that responds to either caffeine or alcohol!

I used the word functional above because we have at least six TAS2R genes that don’t produce a receptor protein. We know that these were once functional genes because of their DNA sequence—mutations that accumulated during evolution have rendered them non-functional. 

Evolution has also affected the functional TAS2R genes in humans—in fact TAS2R genes are among the most variable that we have! Above I mentioned PTC. This is a chemical that some people taste as extremely bitter, while others can’t taste it all. The reason for the difference lies in variations in the TAS2R38 gene: some people produce a protein from this gene that can bind successfully to PTC while others produce a protein that cannot.

You may have noticed that ethanol binds to TAS2R38…and yes, you are right in thinking that if you have a non-functional version of the gene ethanol will taste less bitter.  But that’s not the whole story: there is at least one other protein that responds to ethanol with a bitter signal, from gene TAS2R13. This gene also has variations. Importantly, variations in TAS2R13 determine the overall bitterness of ethanol, and modulate the overall intensity of the experience of ethanol—both burn and bitterness. 

From this ethanol example, you can quickly guess that people differ in their experience of tea bitterness as well. As noted above, TAS2R7 dictates the extent of our ability to sense the bitterness of caffeine. There is a variant of this gene that leads to loss of function, that is, a protein that does not sense caffeine. An initial single short steeping time extracts caffeine from tea without extracting significant amounts of catechins. People who have the non-functional version of this gene won’t taste bitterness in tea that is steeped for a short time, and won’t taste much bitterness in coffee either.

That isn’t the whole story with respect to coffee, however, because there are at least four other genes, TAS2R3, TAS2R4, and TAS2R5, and possibly more, that influence its bitterness. Each of these genes has variants that lead to function or non-function. Importantly, these variants are inherited together in what is called a haploblock. In other words, if you have the non-functional variant of one, you will almost always have the non-functional variant of the others.

In my research with Tim Hanni MW, we found that people who tolerate high alcohol “big red” wines drink their coffee black, while people who prefer lower alcohol white wines and sweet wines prefer tea and put a lot of sugar in their coffee if they drink it at all — activation of sweet taste bud cells inhibit bitter taste bud cells. 

Bottom line: the intensity of bitterness you experience in tea, wine, and coffee, and therefore the need for sweet or salt to modulate the bitterness, may be very different from the bitterness your friends experience. Makes sense?

PS: if you want the references to this article, please contact me—the list is very long so I haven’t included it here.

PPS: couldn't resist including this image, from: Noever, R., J. Cronise, and R. A. Relwani. 1995. Using spider-web patterns to determine toxicity. NASA Tech Briefs 19(4):82. Also: New Scientist magazine, 29 April 1995 and http://www.caffeineweb.com/?p=15.