WHAT IS SOUR ALL ABOUT?

The following is an expansion of what I wrote on my Facebook page (Facebook.com/pairteas) on February 8, with additional new information, and some editing.

Have been interested in sourness, in part because I have an addiction for these:

Am fascinated by the succession of flavors that these sour-sweet gummies give — and as we shall see, the idea of a succession of flavors is critical to understanding what sour is all about. 

What is an acid?
Acid is a general term for compounds to which hydrogen ions can attach and detach. If the hydrogen ions are detached when the compound is in solution, the compound is called a strong acid—hydrochloric acid, for example. If only some of the hydrogen ions are detached, the compound is called a weak acid. Examples: acetic acid in vinegar, lactic acid in sour milk, and citric acid in lemon.

What are pH and titratable acid?
You may have heard the terms pH and titratable acid, particularly in connection with wines. 

• pH refers to the concentration of free unattached hydrogen ions in a solution —the lower the pH the higher the free hydrogen in concentration. 
• Titratable acid refers to the concentration of hydrogen ions that would be present in a solution if you were to separate them from the acidic molecule to which they are loosely bound. The process of measuring this concentration is called titration. Titratable acid concentration includes the concentration of both strong and weak acids in a solution. 

The tongue recognizes titratable acid as sour at fairly low concentrations. Surprisingly, you need much higher concentrations of a strong acid like hydrochloric acid, in which all the hydrogen ions are free (hence very low pH), to sense its sourness.

How do we sense hydrogen ions in the mouth?
We sense hydrogen ions through three different mechanisms. In the first two, the acid has to enter the cell with its hydrogen ion attached in order to elicit sourness. The third, a much less sensitive mechanism, senses free hydrogen ions.

The first mechanism involves cells in taste buds called Type III cells. Taste buds with these cells are abundant in the front and sides of the tongue, and are designed to sense sourness in foods and beverages as they come into the mouth.

The second mechanism involves a special receptor, TRPA1, on the nerve endings of the trigeminal nerve. This nerve has branches in taste buds, as well as throughout the mouth, nose, and throat. TRPA1 is a receptor for cold as well as for a number of different food compounds. Activation of this receptor causes us to feel pain as well as cold.

The third mechanism involves the hot receptor on trigeminal nerve endings, TRPV1. This receptor responds to free hydrogen ions on the outside of the nerve endings, giving a hot painful sensation. This receptor is abundant in the back of the mouth and throat.

How do the first two mechanisms, activated by weak acids, work?

To experience what the first two mechanisms do in response to a weak acid such as acetic acid, take a tiny spoonful of vinegar and dip the tip of your tongue into it. Don't put the vinegar into your mouth—you just want to get the sensation from your taste buds on the tip of your tongue. Your first sensation will be a sharp pain—that's activation of TRPA1—followed quickly by salivation and a sense of sourness, followed by...sweet (more about sweet in a second).

Vinegars at the supermarket. Image from Wikipedia

What is happening on your tongue?

First, TRPA1 on the trigeminal nerve ending cells take up the acetic acid with its hydrogen attached. In the cell the hydrogen ions separate from the acid, which drops the pH inside the cell, making the inside more acid, but in a controllable way. Then the nerve cell sends the pain message to the brain, and a message to salivate to the brainstem. Salivation dilutes the acetic acid so there is less acid for TRPA1 to sense, and the pain ceases.

=>> Note that it is important for the cell to control free hydrogen ions inside it because otherwise its metabolic processes would go awry. That's why the cell doesn't let free hydrogen ions in, just acids with their hydrogen ions attached—it can control exactly how many hydrogen ions are free inside it.

Acetic acid enters the Type III taste bud cell with its hydrogen ion attached as well. But here is a difference: the Type III cells respond to acid when the pH in the cell starts to go up again. In other words, it responds by sending the "sour" message when the acetic acid is diluted by saliva so that the amount of acid entering the cell drops! This is why the sense of sourness comes after the sense of pain when you dip the tip of your tongue into the vinegar.

While the message for sour is in force, Type III cells inhibit the taste bud cells for sweet, bitter and umami, but once the sour message ends, the other taste bud cells are released, and you get a rebound sense of sweetness. 

That's why I think sweet-sour foods are such a delight—you see-saw between the two tastes over time as you have the food in your mouth, and if TRPA1 is activated (for example with sauerkraut, which is acidic enough) you get a burst of coolness as well as slight pain, which dissipates only to return with the next bite.

What about the third response, with the hot receptor TRPV1?

If there are too many free hydrogen ions for your saliva to neutralize, then TRPV1 (hot) receptors in the back of the tongue and in the throat take over. These receptors sense free hydrogen ions directly. Furthermore, it is made more sensitive to these hydrogen ions by activation of TRPA1. TRPV1 sends its "burn" message, with the goal of making you stop drinking or eating, and making you want to grab some water to dilute the acid. 

What does this mean for tea?

When you add lemon to tea at low concentrations, citric acid doesn't activate the trigeminal nerve endings but it does enter the Type III cells in the taste buds. These cells send the "sour" message while turning off the "bitter" message. As you finish your sip, the tea may taste sweet. That's what you may need to decrease the bitterness of a bagged tea.
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If you put in too much lemon, you get a double whammy of painful sour, first from the cold receptor TRPA1 which responds to the citric acid, and then from TRPV1—lemon juice also has a lot of free hydrogen ions. If these hydrogen ions are not neutralized by saliva and tea, they hit the TRPV1 receptors in the back of the mouth and in the throat. The result is not pretty.
=>> In other words a little lemon is nice, but too much...Oh my!!!

And for those of you into wine...

I started this post by discussing titratable acid, one of the measures performed on wine for purposes of determining (among other things) the degree of fermentation. In my next post, I'll talk about how my friend Tim Hanni's system for pairing food and wine works by changing the balance among acid, salt, and ethanol. Stay tuned!

Friend of Pairteas and Master of Wine Tim Hanni