Scientists reveal the secrets of a sixth basic flavor

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Picture a table set with various dishes, each boasting one of the five fundamental tastes we’ve known: sweet, sour, salty, bitter, and umami.

Now, imagine adding a new dish to this culinary collection, offering a flavor quite unfamiliar yet fundamentally sensed by our taste buds.

This new contender in the realm of basic tastes comes from an unexpected source: ammonium chloride, a component that might be making a play to become recognized as the sixth basic taste, thanks to the meticulous research conducted by scientists at the USC Dornsife College of Letters, Arts and Sciences.

This intriguing revelation, published in Nature Communications, takes us on a flavorful journey from the theory to the lab and opens a new chapter in understanding our taste perceptions.

Emily Liman, a USC Dornsife neuroscientist, alongside her adept team, embarked on an exploration of how our tongue perceives the taste offered by ammonium chloride.

To some, especially those in Scandinavian countries, the taste might be vaguely familiar, reminiscent of a popular candy known as salt licorice, which contains salmiak salt or ammonium chloride.

However, despite being strongly sensed by the tongue, the precise receptors responding to ammonium chloride have remained hidden in scientific mystery until now.

With the knowledge that the tongue discerns acid, or sourness, through a protein called OTOP1 – a type of gatekeeper allowing hydrogen ions to enter taste receptor cells – Liman and her team wondered whether ammonium chloride could engage with OTOP1.

This curiosity stemmed from understanding that ammonium chloride can influence the concentration of hydrogen ions (acid) within a cell.

Through a series of experiments involving the Otop1 gene and lab-grown human cells, the researchers noticed a profound revelation: “ammonium chloride is a really strong activator of the OTOP1 channel,” according to Liman.

So potent, in fact, that it performed as effectively, if not more so, than acids.

The connection was made by observing that ammonium chloride releases small amounts of ammonia, which penetrates the cell, making it more alkaline, thus influencing the movement of hydrogen ions through the OTOP1 channel.

To ensure these findings weren’t merely coincidental, the team proceeded with additional experiments using taste bud cells from mice, revealing a sharp uptick in electrical activity (action potentials) when ammonium chloride was introduced to cells from typical mice.

In contrast, cells from mice genetically engineered to lack OTOP1 did not respond, solidifying the role of OTOP1 in sensing this unique taste.

Further confirmation came from behavioural experiments in which mice demonstrated a clear aversion to drinking water containing ammonium chloride, provided they possessed a functional OTOP1 protein. Those without it exhibited no such avoidance, indicating that the OTOP1 channel is crucial for perceiving and responding to ammonium.

As the team delved into the evolutionary aspects, considering why organisms might have developed a taste for ammonium chloride, Liman offered a speculation.

The capability to detect ammonium, commonly found in waste products and possessing a degree of toxicity, might be an evolutionary safeguard to avoid consumption of harmful substances.

Variations in sensitivity to ammonium among different species might be reflective of their respective ecological niches and the presence or absence of ammonium in their environments.

Liman notes a specific amino acid in the OTOP1 channel necessary for responding to ammonium and suggests that its conservation across species indicates an evolutionary pressure to retain it — an indication of its importance for survival.

The future of this research could explore whether sensitivity to ammonium extends to other members of the OTOP proton family found elsewhere in the body.

With such evocative findings, the prospect of introducing a sixth basic taste looms excitingly on the horizon, expanding our understanding of culinary sensations and possibly introducing a new flavor profile to our gastronomic adventures.

The science of taste thus takes a step forward, offering a novel perspective that challenges and enriches our sensory experiences.

And perhaps, in the near future, our dining tables will embrace a new, zestful addition to our flavorful feasts.

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