Sixth taste discovery? Scientists unlock ammonium chloride’s taste mechanics on the tongue

10 Oct 2023 --- A breakthrough in taste could be on the horizon as US-based researchers complete a study they hail that can reshape the way taste perception is understood. It suggests the tongue might detect ammonium chloride as a basic taste, which could lead to an expansion of current thinking that there are five tastes (sweet, sour, bitter, salty and umami or savory).

A team of scientists led by neuroscientist Emily Liman from the University of Southern California Dornsife, US, has uncovered that the tongue can sense ammonium chloride, a compound with a chemical tang, through the same protein receptor responsible for detecting sour tastes.

While ammonium chloride might not be a staple in every kitchen, it holds a particular place in making salted licorice candies in North Europe. Known as “saltlakris” or “salmiakk” in Norway, the candy provides a distinct flavor — a salty edge combined with a certain sharpness, creating a unique experience for the palate. 

Despite this widespread use of ammonium chloride in some regional sweets, the enigma of how the tongue perceives ammonium chloride remained largely unsolved until Liman’s team ventured into the exploration.

Cracking the code of ammonium chloride
In an interview with Food Ingredients First, Professor Liman details: “It has been known for nearly 100 years that ammonium strongly activates the taste system. What was not known was that it works, in part, through the same receptor-ion channel that detects sour/acids.”

Ammonium chloride is a common ingredient in making salted licorice candies in North Europe (Image credit: Licorice International).The human tongue, responsible for decoding a wide array of flavors, from sweet and salty to bitter and umami, relies on taste buds. 

As Liman’s research establishes, OTOP1, a protein receptor in the tongue, is sensitive to ammonium chloride, even more so than to acids, highlighting a discovery about our taste mechanics.

When ammonium chloride is encountered, it releases a small amount of ammonia within cells, subsequently altering their pH levels and initiating a proton influx through the OTOP1 channel, the study found. 

This eventually culminates in an electrical response, mirroring the mechanism of nerve signal conduction within the body.

Courtney Wilson, a postdoctoral research fellow from Liman’s research team, also tells us: “We’ll need more research into both how humans respond to ammonium chloride in foods and how the taste of ammonium chloride interacts with other taste stimuli to get the full picture of how ammonium chloride impacts flavor.”

The USC Dornsife scholars
This eventually culminates in an electrical response, mirroring the mechanism of nerve signal conduction within the body.

However, the application of this knowledge poses its own set of challenges. 

“Our study is in laboratory mice, who don’t tend to like the flavor of ammonium chloride. Humans are exposed to many more tastes and flavors throughout their lives and have the opportunity to develop a more complex set of taste and flavor preferences. The mice in our study tend to avoid ammonium chloride (unless they lack OTOP1 and Type II taste cells), but some humans enjoy ammonium chloride in salty licorice,” Wilson explains. 

A sixth taste? Further exploration needed
It might be tempting to christen this sensation elicited by ammonium chloride as a new, distinct taste modality — potentially a sixth taste. 

But Professor Liman is cautious: “We do not claim to have evidence that ammonium taste is a sixth taste.”

“We do not actually claim to have discovered a sixth taste. It’s been known for quite a while in the taste field that ammonium chloride generates a sizable taste nerve response in mice and other mammals and that mice avoid ammonium chloride solutions as the concentration increases in behavioral experiments.”

“We sought to identify the mechanism(s) behind this response and found that OTOP1 is sensitive to ammonium chloride in addition to sour/acidic stimuli.”

“As evidenced by our behavioral experiments, mice use OTOP1 as well as a population of Type II cells (we suspect a population of cells that respond to bitter stimuli) to detect and avoid ammonium chloride. So, in mice, the taste bud is required to detect and avoid ammonium chloride solutions. But whether the sensation elicited by ammonium chloride constitutes its own separate taste modality in mice or in any other animals remains to be seen,” she elaborates.

However, there could be developments if more research was carried out.The intricacies of the human tongue constantly challenge scientists to delve deeper into our taste perception.

Japanese scientist Kikunae Ikeda first proposed umami as a basic taste in the early 1900s. About eight decades later, the scientific community officially agreed this was the case. 

Unraveling the potential for new flavors and healthier foods
When considering whether ammonium chloride could be utilized to amplify saltiness, particularly in low-sodium or reduced-salt items, Wilson points out its distinctiveness from ordinary table salt (sodium chloride). 

“While the same cells that express OTOP1 have also been implicated in ‘high salt’ taste, the concentrations of sodium salt that activate these cells are generally aversive, rather than appetitive,” Wilson elucidates, signifying the unlikelihood of ammonium chloride serving as a palatable salt substitute. 

Exploring the potential of ammonium chloride to create new flavors or contribute to healthier food development unveils a complex tapestry of possibilities and limitations. 

As Wilson highlights, “Adding ammonium chloride to more foods could possibly enhance the flavor but could also do the opposite, depending on individual flavor preferences and the flavor context.” 

This entwines with Liman’s assertion, “Ammonium also activates other cell types, which we suspect are bitter sensing cells. To observe a complete loss of ammonium sensing mice, we needed to inactivate these other cells and eliminate the OTOP1 channel.”

By Sichong Wang