Scientists unlock 6th basic flavour—the signature taste of salt licorice candy!

US scientists found evidence of a the fifth basic taste — umami — in addition to sweet, sour, salty and bitter, eight decades after Japanese scientist Kikunae Ikeda first proposed it in the early 1900s.

Now, almost half a century later, researchers at the University of Southern California's Dornsife College of Letters, Arts and Sciences have found that the tongue responds to ammonium chloride through the same protein receptor that signals sour taste—which might mean we can detect six, rather than just five basic tastes!

And it is certainly somewhat of an acquired taste, as anyone not a lover of Scandinavian black licorice candy, aka salmiakki or salt licorice—often found in the shape of little black fish—can attest.

Because sal ammoniac, a rather pungent substance found in encrustations near volcanic fumaroles, in smoky coal seams, and in guano deposits, is rather distinctive.

Emily Liman, professor of biological sciences, speculates that the ability to taste what we now call ammonium chloride might have evolved to help organisms avoid eating harmful biological substances that have high concentrations of ammonium.

"Ammonium is found in waste products — think of fertiliser — and is somewhat toxic, so it makes sense we evolved taste mechanisms to detect it,” she said, in a paper published in the journal Nature.

It is, however, used in small doses in food too: it can be used to support bread yeast in baking, and adds crispness even to commercially prepared fried items like jalebi and samosa.

In some northern European countries, salt licorice has been a popular candy at least since the early 20th century. The treat counts among its ingredients salmiak salt, or ammonium chloride.

Scientists have for decades recognised that the tongue responds strongly to ammonium chloride. However, despite extensive research, the specific tongue receptors that react to it remained elusive.

In recent years, scientists have uncovered the protein responsible for detecting sour taste. That protein, called OTOP1, sits within cell membranes and forms a channel for hydrogen ions moving into the cell.

Hydrogen ions are the key component of acids, and as foodies everywhere know, the tongue senses acid as sour. That's why lemonade (rich in citric and ascorbic acids), vinegar (acetic acid) and other acidic foods impart a zing of tartness when they hit the tongue. 

The researchers introduced the OTOP1 gene into lab-grown human cells so the cells produce the OTOP1 receptor protein. They then exposed the 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," Liman said. "It activates as well or better than acids." But while the same receptor is involved, it is not the same as tasting something 'sour'.

Ammonium chloride gives off small amounts of ammonia, which moves inside the cell and raises the pH, making it more alkaline, which means fewer hydrogen ions.

The research team then turned to a technique that measures electrical conductivity, simulating how nerves conduct a signal. 

Using taste bud cells from normal mice and from mice the lab previously genetically engineered to not produce OTOP1, they measured how well the taste cells generated electrical responses called action potentials when ammonium chloride is introduced.

Taste bud cells from wild-type mice showed a sharp increase in action potentials after ammonium chloride was added, while taste bud cells from the mice lacking the OTOP1 gene failed to respond to the salt. This confirmed the hypothesis that OTOP1 receptors respond to the salt, generating an electrical signal in taste bud cells.

Then the team went one step further and examined how mice react when given a choice of either plain water or water laced with ammonium chloride to drink. (For these experiments, they disabled the bitter taste-detecting cells that also contribute to the taste of ammonium chloride.) 

Mice with a functional OTOP1 receptor protein found the taste of ammonium chloride unappealing and did not drink the solution, while mice lacking the OTOP1 receptor protein did not mind the alkaline salt, even at very high concentrations.

With inputs from IANS