Here's What Makes Lemons Sour

How delicious is lemonade, with its contrasting flavors of tart and sweet. And what a perfect life lesson this simple beverage offers us: it's the sweet and the sour that make life balanced and interesting. Sugar of course, is what makes lemonade sweet, and the sour comes from the lemons themselves. But why are these bright, beautiful fruits sour and how do our brains even know this? 

First, an anatomy lesson. When scientists refer to "taste," they are talking about the five sensations that can be detected by the tongue: sweet, sour, salty, bitter, and umami, also known as savory (via Serious Eats). Remember those old "maps" of the human tongue you may have seen in grade school, showing us which parts of the tongue senses each of the tastes? Those were wrong. First, the bumps on your tongue aren't actually your taste buds at all. They are papillae, and contained on each of these are your taste buds — thousands of them. On each of these tiny buds are about 100 taste cells. Each taste cell detects just one of the five tastes. These become receptors, taking in information and passing it on to the brain, according to Wonderopolis

The bottom line? You taste sweet, salty, umami, bitter, and sour (as in the taste of lemons) with your whole tongue. But how?

Citric acid, pH, and the brain

According to Wonderopolis, lemons taste sour because they contain citric acid, a common organic acid found in many vegetables and fruits, including, of course, citrus fruits like lemons, limes, grapefruit, and oranges. The higher the concentration of citric acid, the more sour the particular fruit tastes. Lemons and limes, unsurprisingly, have more citric acid than an orange or tangerine.

What is an acid? According to the Australian Academy of Science, acidity measures the concentration of hydrogen ions. The pH scale (pH is short for potential of hydrogen ions) ranges from 1-14, with 14 being alkaline or "basic," and 1 being acid. The stronger and more highly concentrated the acid, the lower the number, and the more sour the substance will taste. For reference, water is neutral with a pH of 7. Lemons have a pH of 2, which is quite sour, just below stomach acid's pH of 1.

A series of signals In the brain tells us something is sour. According to Arizona State University, cranial nerves, via chemicals called neurotransmitters, send a message of taste from our tongues to the brainstem, and then into the thalamus, located near the center of the brain. The thalamus then sends the signal to the gustatory cortex. As the name indicates, this section of the cerebral cortex interprets taste. 

It's a lot to think about the next time you bite into a lemon square or enjoy some cold lemonade. Or, maybe just enjoy that wonderful tartness that lemons deliver.