In Search of the Sixth Taste

Salty, sour, sweet, bitter, and umami (pleasant) – this is the current official list of the five basic tastes. For most ordinary civilized people, this spectrum of gourmet experiences is entirely satisfactory. If they were to crave another taste, it would perhaps only be the flavor of victory over the criminal Putin regime. However, no one will award a Nobel Prize for a burnt Kremlin, so scientists worldwide are racking their brains to discover yet another, hitherto unknown – the sixth taste.

Photo: The current official list of the five tastes, Source: Freepik.com

Recently, a group of scientists from the University of Southern California (Dornsife) claimed that Ammonium Chloride, commonly known as sal ammoniac, might unexpectedly be the sixth taste. In a study led by neurobiologist Emily Liman, published in the journal Nature Communications, it is stated that the human tongue reacts to ammonium chloride and does so through a specific receptor, which is also responsible for signaling the sour taste.

Phew, Spit it Out!

Of course, this news does not mean that one should rush to swallow ammonium chloride to check if it really has a distinct taste – remember, this substance is extremely toxic. The fact that the human tongue strongly reacts to ammonium chloride has been known to scientists for several decades, but they struggled to understand how this happens. Now they have figured it out.

The key is that certain receptors on the tongue have a protein called OTR1. It forms a channel for hydrogen ions to enter cells. Hydrogen ions are a crucial component of acids (such as citric or acetic acid). It was also known that ammonium chloride affects the concentration of hydrogen ions in the cell. So, researchers conducted an experiment to see if it could interact directly with OTR1. In the laboratory, they created human cells with this protein and treated them first with acid and then with ammonium chloride. Voila – both substances activated the receptor equally effectively.

Salty Licorice

Photo: Salty licorice, Source: Pixabay.com

Dr. Emily Liman suggests that the tongue developed a specific receptor to signal ammonium chloride not without reason but as a result of evolution – to make the brain react with strong aversion and discourage humans from swallowing toxic food. However, the taste of ammonium chloride is well known to Scandinavians. Salty licorice is a popular confectionery item there. Adding a certain dose of ammonium chloride makes these sweets, derived from the root of the sweet wood, unique in taste – bitter-salty-sour.

To recognize ammonium chloride as the sixth taste, many different studies, checks, and heated debates at scientific conferences will be needed. And there is no promise that this will be officially acknowledged, as practice shows: not every newly discovered taste gets official recognition. It may take many years for this to happen. The official fifth taste – umami – is a vivid example.

Meaty Deliciousness

Anyone who has ever eaten fried meat has noticed that it is not salty, sour, sweet, or bitter – but tasty. This taste signaling a high protein content in food is precisely called "umami." In the early 19th century, Japanese chemist Ikeda Kikunae noticed that when he added kombu (an edible seaweed) to vegan dishes, they became similar to meat. Later, the scientist found that the specific taste of kombu (Kikunae called it "umami," meaning "pleasant taste") is due to the content of glutamic acid. In 1908, he synthesized sodium glutamate from kombu and patented it as a new seasoning. Today, it is present in almost every fast-food joint, making generally inedible food so delicious that people want to eat it again and again.

But not only adding sodium glutamate creates umami. Espresso, soy sauce, fried meat, walnuts, many types of cheeses, and more – have it in their taste ensemble. Umami itself does not possess any divine qualities, but it enhances other flavors. A very pleasant aftertaste that is difficult to describe in words and mild salivation – these are the main features of umami.

For a long time, there were debates about whether sodium glutamate (as well as several other compounds, such as guanosine monophosphate or inosine monophosphate) is the fifth taste in addition to the four recognized by humanity since ancient times. In 1985, a special conference on umami was even held in Hawaii, where it was decided that sodium glutamate and nucleotides constitute its official taste. However, only in the early 90s did umami become internationally recognized as the fifth basic taste, for which humans have separate glutamate receptors. 

Fatty to the Touch

In the past decade, the quest for the sixth taste has turned into a genuine scientific frenzy. For instance, in 2014, the scientific community began discussing the possibility of a fatty taste. On one hand, if our tongue has learned to identify fat through separate receptors, it is entirely understandable from an evolutionary perspective. Fat was scarce yet highly desirable for our ancestors on the African savannah due to its high energy content. Over the last decade, scientists have gathered numerous pieces of evidence indicating that humans can indeed perceive the taste of fat. However, these pieces of evidence suggest more that we do not sense the fatty taste in our mouths but rather register fat tactually. In other words, we do it not through taste receptors but through touch receptors.

This discussion revolves not around the molecules of fat themselves but around the so-called "triglycerides" (when three fatty acid molecules attach to a certain basic molecule). For example, triglycerides are present in cream butter or fresh olive oil. As for free fatty acids, humans perceive them not as something tactually pleasant for their tongue but as something repulsively bitter or sour. If you doubt this, try using rancid sunflower oil for frying – it will taste like pure fat.

Starch, My Love

Millions of people around the world are willing to endure obesity and diabetes just to avoid giving up baked goods. In 2016, scientists from Oregon State University pondered whether humans have specific receptors for the taste of complex carbohydrates. It was previously believed that, for example, we like bread because when it enters the mouth, it immediately starts to be processed by saliva, whose enzymes break down complex carbohydrates into simple ones (i.e., sugars that we perceive with sweet receptors).

To test whether the human mouth can distinguish complex carbohydrates, researchers injected volunteers with substances that block saliva secretion and then fed them foods with carbohydrates of various chain lengths. It turned out that the participants perceived short and long chains as fresh, while medium-length carbohydrate chains were perceived as something starchy, resembling either starch itself, flour, or rice. Unfortunately, despite a promising start, enthusiasts have not yet found the corresponding receptors to officially recognize "starchy" as the sixth taste.

Water with a Hint of Sourness

Once in Ukraine, attempts were made to launch a brand of mineral drinks called "Blue Water." However, people were not eager to buy such a beverage because its name sounded like the Ukrainian word "blyuvota" (meaning "vomit"). This case illustrates that whether something tastes good to us is not determined only by taste receptors on the tongue but also by the brain, eyes, nose, ears – words, sounds, aromas, colors, and more can be tasty. Nevertheless, for the official recognition of a certain taste as basic, there must be specific receptors for it in the mouth – without this, it won't happen.

In 2017, scientists from the University of California claimed that pure H2O has a slightly sour taste and can be considered basic. Researchers noticed that in mice, when they drank pure water, sour receptors on the tongue were activated. To confirm that mice indeed considered water sour, scientists replaced it in their drinkers with blue light (using a method called optogenetics that stimulates sour receptors). When the mice saw the blue light, they started "drinking" it because sour receptors signaled to them that there was water in the drinker. However, even if pure water is perceived as slightly sour, it is not a new taste but a well-known one.

What about Drosophila?

When scientists find themselves in a dead-end and don't know where to go next, they often start experimenting with fruit flies, Drosophila. Firstly, the life and rights of fruit flies are not as fiercely defended as those of mice, as animal rights activists argue. Secondly, fruit flies are convenient for experiments, and they have given scientists various interesting ideas many times. For instance, in 2018, scientists from the University of California in Santa Barbara found that fruit flies can detect the presence of calcium in food and refuse to consume it if there is too much.

The experiment looked like this: in one feeder, researchers placed pure fructose, and in another – fructose with added calcium chloride. Ordinary fruit flies, which had three specific taste receptors, immediately sensed something wrong in the second feeder and stopped eating from it. However, genetically modified fruit flies – lacking at least one of those three receptors – continued to gorge on glucose with calcium and died from its excess. Scientists suggest that a similar mechanism (possibly involving all three receptors simultaneously) to detect excessive calcium in food and save one's life could have evolved in humans as well.

However, verifying this is difficult because creating genetically modified humans is challenging, and killing them with calcium for the sake of scientific experiments is even more challenging. So, we can only hope that someday in the future, scientists will come up with a humane way to finally take and find this elusive sixth taste, dreamed of by generations of researchers.