LONDON - Scientists have found that the tongue responds to ammonium chloride as a sixth basic taste, in addition to detecting sweet, sour, salty, bitter, and umami flavours.

Research published on Thursday in the journal Nature Communications suggests that protein receptors on the tongue that help detect the sour taste also respond to ammonium chloride – a popular ingredient in some Scandinavian candies.

“If you live in a Scandinavian country, you will be familiar with and may like this taste,” neuroscientist and study co-author Emily Liman from the University of Southern California said.

Salt licorice has been a popular candy in some northern European countries at least since the early 20th century and its ingredients consist of salmiak salt, or ammonium chloride.

While scientists have known that the tongue responds in some ways to ammonium chloride, the specific protein receptors on the tongue that react to it have remained elusive despite decades of extensive research.

The process became more clear once recent research uncovered the protein responsible for detecting sour taste via a protein receptor in the tongue called OTOP1.

This protein is present within the membranes of cells in the tongue and forms a channel for hydrogen ions – a key component of acidic sour food – moving into the cell.

OTOP1 plays a key role behind lemonade – rich in citric and ascorbic acids – and other acidic foods like vinegar imparting a zing of tartness when they hit the tongue.

Since ammonium chloride also affects the concentration of hydrogen ions within a cell, researchers wondered if it could also trigger OTOP1.

To study this, scientists introduced the gene behind the Otop1 receptor into lab-grown human cells so the cells produce the OTOP1 receptor.

Researchers then exposed these cells to acid or to ammonium chloride and measured the responses.

“We saw that ammonium chloride is a really strong activator of the OTOP1 channel. It activates as well or better than acids,” Dr Liman said.

Small amounts of ammonia from ammonium chloride was found to move inside the cell.

Since ammonia is alkaline, it raises the pH leading to fewer hydrogen ions.

This pH difference, scientists say, drives an influx of hydrogen ions through OTOP1, which could be detected by measuring changes in electrical conductivity across the channel.

To measure this, scientists used taste bud cells from normal mice and from genetically engineered mice that do not produce OTOP1.

They measured how well the taste cells generated electrical responses when ammonium chloride was introduced.

While taste bud cells from the normal mice showed a sharp increase in action potentials after ammonium chloride was added, those from mice lacking OTOP1 failed to respond to the salt.

This confirmed that OTOP1 responds to the ammonium chloride.

Scientists also found that mice with a functional OTOP1 protein found the taste of ammonium chloride unappealing and did not drink water laced with the salt, while those lacking the protein did not mind the solution, even at very high concentrations.

“This was really the clincher. It shows that the OTOP1 channel is essential for the behavioral response to ammonium,” Dr Liman said.

Researchers also found that the OTOP1 channel appeared to be more sensitive to ammonium chloride in some species than in others.

They suspect the ability to taste ammonium chloride may have been an adaptation that helped organisms avoid eating harmful biological substances with high concentrations of ammonium.

“Ammonium is found in waste products – think of fertilizer – and is somewhat toxic, so it makes sense we evolved taste mechanisms to detect it,” Dr Liman explained, however, adding that further study is needed to understand species differences.