Why does eating something sweet and then something sour produce an even more sour experience?

One of the most interesting phenomena you may have observed when eating a sweet dessert then taking a sip of wine, especially a dry wine with a high alcohol content, is that the wine tastes even drier and more sour and tannic than when sipped by itself. Why is this the case?

To understand what is happening we need to go back to a variant of the diagram in my book, "Three Basic Teas & How to Enjoy Them," and add an interesting property of "sweet" that you may have experienced with coffee and some teas.

First the diagram, of the taste cells in a taste bud:

As you can see, there are five different cells in this diagram. The first one, one the left is a Type I cell, and is sensitive to salt, especially to salt at low concentrations. The next three cells are Type II cells. Each Type II cell is sensitive to one taste modality, so in this diagram we have one cell each for bitter, sweet, and umami. Finally, on the right you find a Type III cell, which, as shown here, is sensitive to sour.

In addition to being sensitive to sour, Type III cells are also sensitive to heat, salt, and the burn of alcohol, because they have TRPV1 receptors on their surface.

With respect to salt and bitter (I haven't included an arrow for this interaction, for simplicity's sake): while we do not know exactly how Type I cells communicate their messages, their activation  turns off the Type II cells responsible for bitter sensations. That's why a tiny bit of salt will make tea, wine, and tequila less bitter.

Next to the connections shown in the diagram:

As indicated by the lines, Type II cells can communicate directly to the nervous system and brain. However the primary route of communication is through activation of Type III cells. Bitter, sweet, and umami messages can go through to the brain by either route so long as Type III cells are not activated by something sour or by something that activates TRPV1, such as high salt or alcohol.

However, if you eat something sour or something that activates TRPV1, Type III cells will inhibit messages coming through from the Type II cells, so perception of bitter, sweet, and umami is diminished.

Now to the interesting properties of "sweet:"

You may have had the experience of swallowing a mouthful of tea with lemon and then getting a lingering sweet aftertaste. When a sour compound is washed out and Type III cells are no longer inhibiting the Type II sweet cells, the brain interprets this condition as "sweet." The same thing can happen with coffee and artichokes: they contain compounds that inhibit Type II sweet cells, so that when washed out, you get a sweet aftertaste.

The opposite appears to happen when you wash a sweet compound away with something sour. When sweet goes away, something that is sour will seem more sour because nowType III cells are inhibiting Type II cells. Any sweet message is diminished and the brain interprets this condition as "very sour." This effect can be amplified by alcohol, especially by wine with its low pH and high titratable acid, because TRPV1 on Type III cells will be activated as well. 

In other words, sweet and sour exist in a balance, where the more sour something is the less sweet it will seem, and vice versa.

As a side note, I happen to be homozygous for a mutation in TRPV1 that increases this receptor's activation potential.* As a result, for me any wine with an alcohol by volume (ABV) greater than about 12% burns like crazy. Yet I can tolerate Harvey's Bristol Cream Sherry at 17.6% ABV. Why? Because that sherry is very sweet. The sweetness message is so great that the burn message can't get through. 

Apparently I am not the only person who has this experience: Tim Hanni MW and I have data that shows that people who avoid "big reds," dry wines with a high alcohol content such as California Cabernet Sauvignons, are happy with sweet cocktails that often have an even higher alcohol content than Harvey's Bristol Cream.**

Tim summarizes this pairing experience in the following diagram:

You can find this diagram and a whole lot more in his book "Why You Like the Wines You Like: Changing the Way the World Thinks about Wine." Go to http://www.timhanni.com/publications/.

Oh, and about teas for dessert: if you eat a sweet dessert, black teas will seem more astringent, because highly complexed polyphenols and a host of other compounds in the tea also activate TRPV1—activation of TRPV1 is required for astringency.***

Here's a very good summary reference for the way taste bud cells work: 
Bernd Nilius and Giovanni Appendino. Spices: The Savory and Beneficial Science of Pungency. Rev Physiol Biochem Pharmacol, doi: 10.1007/112_2013_11, Springer International Publishing, Switzerland, 2013. If you can't get the full reference on line, you can get relevant parts of it through googlebooks.

* Alissa L. Allen, John E. McGeary, and John E. Hayes. Polymorphisms in TRPV1 and TAS2Rs associate with sensations from sampled ethanol. Alcohol Clin Exp Res. 2014 October ; 38(10): 2550–2560. doi:10.1111/acer.12527.

** Tim and I have a summary of our report on the topic that you can find at https://www.jancisrobinson.com/files/pdfs/HanniUtermohlenSweetTolerant3.pdf. Thanks Jancis!

*** Nicole Schöbel et al. Astringency Is a Trigeminal Sensation That Involves the Activation of G Protein–Coupled Signaling by Phenolic Compounds. Chem. Senses 39: 471–487, 2014 doi:10.1093/chemse/bju014.