Japanese scientist Kikunae Ikeda introduced the concept of umami as a fundamental taste in the 1900s. He newly introduced the umami or spicy taste alongside sweet, sour, salty, and bitter. It took almost eight decades for the scientific community to acknowledge this additional taste officially.
In a new study, researchers found there is a sixth basic taste sensation. This research was conducted by a team led by USC Dornsife neuroscientist Emily Liman.
Ammonium Chloride’s Link to Sour Taste
Liman and her team made an intriguing discovery about ammonium chloride, a compound well-known for its unique taste. In Northern European countries like Scandinavia, ammonium chloride is found in salt licorice, a popular treat for over a century.
Scientists have recognized that the tongue responds significantly to ammonium chloride for years. However, the exact tongue receptors responsible for this response remained elusive.
The Role of the OTOP1 Protein
Liman’s team identified the OTOP1 protein as the key player in detecting sour taste. This protein forms a channel for hydrogen ions, the essential components of acids, to enter taste receptor cells.
The researchers discovered that ammonium chloride effectively activates the OTOP1 channel more than acids. When ammonium chloride introduced, it releases small amounts of ammonia inside the cell, altering its pH and driving a proton influx through the OTOP1 channel.
OTOP1 is not just a sour receptor, it also mediates ammonium chloride taste. Never tasted NH4Cl? It’s in salty licorice – beloved in Scandinavian countries. Our paper in @NatureComms goes from channel biophysics to behavior. https://t.co/BiuDJZmzqM. pic.twitter.com/byZe2rj8sU— Emily Liman (@emily_liman) October 5, 2023
Confirming the Findings
The team conducted experiments with lab-grown human cells producing the OTOP1 receptor protein to validate their results. They found that ammonium chloride activated the OTOP1 channel. And it generates an electrical signal in taste bud cells.
Mice with a functional OTOP1 protein found ammonium chloride unappealing and refused to consume it when given a choice. On the other hand, mice lacking the OTOP1 protein showed no aversion to the alkaline salt, even at high concentrations.
Why Do We Detect Ammonium Chloride?
Liman speculates that our ability to taste ammonium chloride may have evolved to help us avoid ingesting harmful substances rich in ammonium. This compound commonly found in waste products. It has some toxic properties. Sensitivity to ammonium chloride appears to vary among species.
The researchers aim to explore whether other members of the OTOP proton family share this sensitivity.