Handing out salty liquorice – known as salmiak candy – at Halloween might be a trick or a treat, depending on your palate.
The Scandinavian sweet is certainly an acquired taste. Now scientists publishing in Nature Communications, present evidence the candy’s key ingredient – salmiak salt, or ammonium chloride (NH4Cl) – might one day join sweet, sour, salty, bitter and umami as a sixth basic taste.
A series of experiments by University of Southern California neuroscientists show the tongue responds to the taste of ammonium chloride through the same protein receptor it uses to detect sour.
Ammonium chloride has a uniquely strong acrid taste. Although high concentrations can be noxious to humans and other animals, people can develop a penchant for smaller amounts, such as in the form of salty liquorice.
“If you live in a Scandinavian country, you will be familiar with and may like this taste,” says paper author Emily Liman, a professor of biological sciences at the University of Southern California.
Taste buds constitute 50 to 100 receptor cells. Those called Type II detect bitter, sweet or umami. Type III taste receptor cells detect sour through the protein called OTOP1.
The paper’s authors previously demonstrated how OTOP1 detects sour taste via hydrogen ions, common to acidic foods like lemon juice or vinegar.
Their new study shows the same protein acts as a receptor for ammonium chloride.
Ammonium chloride releases small amounts of ammonia, which has the effect of increasing alkalinity and reducing hydrogen ions. This difference in pH levels also activates OTOP1, generating an electrical signal.
The scientists demonstrated the effect by measuring and comparing electrical conductivity in normal mice and mice genetically engineered to lack OTOP1, when given ammonium chloride. Those lacking OTOP1 failed to respond to the salt.
They also measured nerve signals related to taste, recording a response to ammonium chloride in the normal mice, but none in the mice without OTOP1.
In a further test, mice were offered water laced with ammonium chloride and plain water. Mice with the functional OTOP1 protein preferred to avoid the water with ammonium chloride, whereas those without the gene did not mind drinking the solution.
Another experiment tested the response to ammonium chloride in lab-grown human cells specifically designed to produce the OTOP1 protein.
“We saw that ammonium chloride is a really strong activator of the OTOP1 channel,” Liman said. “It activates as well or better than acids.”
The paper suggests the ability to taste ammonium chloride might have evolved in humans and other animals to help them avoid consuming substances with high, potentially toxic, concentrations of the salt.
Ammonium chloride is found in waste products, like fertilizer, Liman says. “So it makes sense we evolved taste mechanisms to detect it,” she says.
It took 8 decades for umami to be officially recognised as a basic taste. In the future ammonium chloride might be recognised as a sixth.
In the meantime, a gift of salty liquorice on 31 October might give those trick-or-treaters the chance to taste for themselves.