In addition to sour, sweet, bitter, salty and umami, there may be a sixth basic taste!

Produced by: Science Popularization China

Author: Denovo Team

Producer: China Science Expo

For a long time, it has been generally accepted in the scientific community that humans can perceive five basic tastes: sour, sweet, bitter, salty and umami.

Are there any other flavors?

We often use "sour, sweet, bitter, and spicy" to describe the different feelings of life, including the taste of "spicy". However, this is actually a misunderstanding. In fact, spiciness is a painful feeling, which is caused by chemical substances stimulating cells and causing a stimulating feeling in the brain. It is not the taste felt by taste buds. So the real five basic tastes are still "sour, sweet, bitter, salty, and fresh".

But recent research suggests that the taste of ammonium chloride (NH4Cl) could potentially be considered the sixth basic taste that humans can sense.

How do humans perceive taste?

Taste is not a single, isolated sensation, but a complex experience resulting from the interaction of multiple chemical stimuli. The five basic tastes (sour, sweet, bitter, salty, and umami) each have their own unique perceptual mechanisms and historical background.

For example, umami was first proposed in 1908, but it was not generally recognized by the scientific community until nearly a century later in 2002. It was originally discovered by Japanese scientist Kikunae Ikeda as the fifth taste, the taste of glutamate. Since adding glutamate to food or soup can significantly increase the umami taste, glutamate was later made into a commercial product, monosodium glutamate, called Ajinomoto in Japan and MSG in China.

Regarding the distribution of taste receptors on the tongue, it is generally accepted that different taste receptors are distributed in different parts of the tongue, such as the sweet receptor is at the tip of the tongue, the salty and sour receptors are at the sides of the tongue, and the bitter receptor is at the root of the tongue. This distribution model is usually called a "taste map".

However, recent studies have shown that this traditional "taste map" theory may not be accurate. When the "taste map" was drawn, umami was not yet widely accepted, so it was not included in the discussion.

Traditionally, it is believed that different taste receptors are located in different parts of the tongue.

(Source: drawn by the author)

Modern research shows that the taste receptors (taste buds) on the tongue are actually able to sense a variety of tastes, rather than just one specific taste. Each taste bud contains cells that can detect different tastes, so different areas of the tongue can sense sour, sweet, bitter, salty and umami tastes.

A true taste map: different areas of the tongue can sense sour, sweet, bitter, salty and umami

(Image source: drawn by the author)

What are the mechanisms of different tastes?

At present, all receptors for the five basic tastes have been scientifically identified.

For example, sourness is perceived through hydrogen ion channels on the surface of taste buds (acids dissociate to form H+ ions); saltiness is perceived through sodium ion channels (table salt NaCl dissociates to form Na+ ions); and sweetness, bitterness and umami are perceived through a special membrane protein - G protein-coupled receptors.

What’s even more interesting is that these basic tastes also have their own quantitative units.

For example, sourness is defined as "1 unit of acidity" based on the acidity of citric acid; saltiness is defined as "1 unit of saltiness" based on the saltiness of sodium chloride (table salt); sweetness is defined as "1 unit of sweetness" based on the sweetness of sucrose; and bitterness is defined as the bitterness of a drug called quinine (for basic taste, the threshold values ​​of each typical representative are generally considered to be: sucrose is 3 g/L, citric acid is 0.2 g/L, quinine is about 16 mg/kg, and sodium chloride is 2 g/L).

Umami is special. Although MSG (monosodium glutamate) is generally considered to have a very strong umami taste, there is currently no official definition of "umami". It is worth mentioning that in addition to glutamate, guanylic acid and inosinic acid can also produce umami, and when these substances are mixed with glutamate in a certain proportion, their umami taste will even be enhanced.

A possible sixth basic taste for humans: ammonium chloride

In addition to the above five basic tastes, according to the latest research results, the taste caused by ammonium chloride may become the sixth basic taste of human beings. So, why can ammonium chloride be crowned as such?

Ammonium chloride is not an unfamiliar compound. It is widely used in agriculture as a fertilizer and plays a variety of roles in medicine. It is also used as a food additive and can be found in some European desserts, such as salty liquorice.

When our tongue comes into contact with ammonium chloride, it produces a unique taste experience, like a complex of bitter, salty and slightly sour taste. Due to its strong taste irritation, ammonium chloride has been used as a reference in taste research for decades. However, the precise mechanism of how human taste buds sense ammonium chloride has been unclear.

Recently, a joint research team from the University of Southern California and the University of Colorado made a breakthrough and published it in the journal Nature Communications. They found that a protein called otopetrin1 (OTOP1) located in taste cells can be activated by ammonium chloride. OTOP1 protein is an ion channel that is known to play a role in sour taste perception.

Taste buds precisely sense ammonium chloride

(Image source: Reference 7)

By exposing cultured human cells to ammonium chloride and acidic substances, the researchers observed that the OTOP1 protein had similar responses to both. This result provided us with an important clue: the transmission of ammonium chloride taste may be mediated by the OTOP1 protein.

It is worth mentioning that in animal model experiments, mice lacking the OTOP1 gene had little response to food containing ammonium chloride, while mice with this gene avoided such food.

Regarding how ammonium chloride activates OTOP1 protein, the researchers proposed a feasible biochemical mechanism: when cells take up ammonium chloride, ammonium ions (NH4+) can be converted into ammonia (NH3). NH3 enters the cell and is then converted into NH4+, causing alkalinity in the cell. The extracellular H+ concentration is greater than the intracellular H+ concentration. The H+ concentration difference causes the OTOP1 ion channel to open, thereby allowing the taste of ammonium chloride to be perceived, similar to the perception of sour taste.

Possible mechanism of ammonium chloride activation of OTOP1

(Image source: Reference 7)

This study revealed that the taste of ammonium chloride is perceived through the ion channel mediated by the OTOP1 protein. This is a new discovery because this taste can stimulate the taste perception system of human taste bud cells, rather than just causing stimulation in the brain like spicy taste, which means that we may need to list the taste of ammonium chloride as the sixth basic taste of humans.

Conclusion

Taste is one of the important ways for humans to understand the world and interact with the environment. From "sour, sweet, bitter, salty" to today's "sour, sweet, bitter, salty and fresh", people's understanding of taste is constantly deepening. If more research in the future can confirm that ammonium chloride does have the potential to become a basic taste, then how should this new "sixth taste" be named? We can only wait and see.

All of this may not only change our understanding of food, medicine and even agriculture, but will also bring new perspectives and challenges to taste science research.

References:

1.Ikeda K. New Seasonings. Chemical Senses. 2002, 27 (9): 847-849.

2.Huang AL, et al. The cells and logic for mammalian sour taste detection. Nature. 2006, 442. 934-938.

3.Bachmanov AA, et al. Taste receptor genes. Annu Rev Nutr. 2007, 27: 389-414.

4.Wong GT, et al. Transduction of bitter and sweet taste by gustducin. Nature. 1996, 381, 796-800.

5.Inoue M., et al. Soa genotype selectively affects mouse gustatory neural responses to sucrose octaacetate. Physiol. Genomics. 2001, 5, 181-186.

6. Zhao Xinhuai. Food Chemistry: Chemical Industry Press, 2006.

7.Liang et al. The proton channel OTOP1 is a sensor for the taste of ammonium chloride. Nature Communications. 2023, 14, 6194.

8. Teng et al. Cellular and neural responses to sour stimuli require the proton channel Otop1. Current Biology. 2019, 29, 3647-3656.