Scientists have discovered a sixth flavor: what it tastes like
A new study shows that the tongue perceives ammonium chloride as a primary flavor.Researchers have uncovered the mechanism by which our taste buds detect ammonium chloride, an example being the distinct taste of salted licorice, popular in Scandinavia and the Netherlands. The discovery proves the existence of a sixth basic flavor, scientists are confident.
After Kikunae Ikeda defined umami in 1908, this taste was finally recognized as a separate taste in 1990. So it joined the canonical four tastes: sweet, sour, salty and bitter. Now researchers from USC Dornsife's College of Literature, Arts and Sciences have found evidence of flavor number six's primary flavor, ammonium chloride.
"If you live in a Scandinavian country, you are familiar with this flavor and may like it," says Emily Leeman, co-author of the study, in a press release from the university. She's referring to salted licorice, a confection laced with ammonium chloride, which gives it a distinctive flavor: bitter, salty and slightly sour.
Taste occurs when ingested chemicals interact with specialized taste receptor cells (TRCs) on the tongue and palate. Different TRCs respond to each of the five basic tastes and release neurotransmitters into the nerves. These send signals to the brain that allow the nervous system to determine whether what is eaten is perceived as bitter, sweet, umami, sour, salty, or a mixture of all five.
Sour-flavored foods are high in acids, low in pH and high in hydrogen ions. When sour TRCs are exposed to acids, they produce an electrical signal due to the movement of hydrogen ions across the cell membrane. Previously, the researchers found that acidic TRCs express the otopterin1 (Otop1) gene, which encodes the OTOP1 protein, to form a proton channel. It gives the cells the ability to detect low pH and sour taste.
In the current study, the scientists decided to test the contribution of acidic TRC and OTOP1 to the tongue's ability to sense ammonium chloride. They inserted the Otop1 gene into lab-grown human cells and exposed some of them to acid or ammonium chloride. It turned out that ammonium chloride activated the OTOP1 receptor as effectively as acid. Tests on mice confirmed that people with the Otop1 gene avoided ammonium chloride, while people with a "knockout" gene did not.
Ammonium and its gas, ammonia, the breakdown products of amino acids, are generally toxic to humans and other animals, many of which have the ability to detect and respond to ammonia/ammonia from the environment. The researchers hypothesized that the ability to taste ammonium chloride may have evolved to help organisms avoid harmful substances.
The researchers observed differences between species. Human and mouse OTOP1 channels were strongly activated by ammonium chloride, chicken OTOP1 channels were more sensitive, and danio fish were less sensitive to ammonium chloride. The researchers say these species differences reflect the ecological niches of each organism. For example, birds are known to be less sensitive to sour tastes, but they need to avoid ingesting ammonium chloride present in their droppings.
The researchers plan to further study the OTOP1 receptor's response to ammonium chloride in hopes of learning more about its evolutionary significance.
The study was published in the journal Nature Communications.