06 October 2023 , 14:10
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For centuries, people all around the world were familiar with four different tastes identified by the tongue - sweet, sour, salty, and bitter.
This changed over a century ago when Japanese scientist Kikunae Ikeda became the first person to identify the source of umami, which was officially recognised as a basic taste about eight decades later. But now, scientists from the University of California say they have discovered a sixth basic sense our tongues can feel - ammonium chloride.
A study published in Nature Communications yesterday suggests that the tongue responds to ammonium chloride through the same protein receptor that signals a sour taste. For decades, scientists have recognised that the tongue responds strongly to ammonium chloride but the receptors reacting to it remained elusive despite extensive research.
Neuroscientist and biological sciences Prof. Emily Liman said: "If you live in a Scandinavian country, you will be familiar with and may like this taste. 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 (ammonium chloride)."
We feel a taste when ingested chemicals interact with specialised taste receptor cells (TRCs) on the tongue and palate. These then release neurotransmitters onto nerves that send signals to the brain, which in turns helps the nervous system to determine if we are eating something that is bitter, sweet, umami, sour, salty or a mix of all five.
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Salt licorice, with added ammonium chloride, gets a distinct flavour that is a mix of bitter, salty, and a little bit sour. When we taste something sour, it means what we are eating is high in acids - low in pH and high in hydrogen ions. Previous research found that sour TRCs express the otopterin1 (Otop1) gene that encodes a protein, OTOP1, to form a proton channel which gives cells the ability to detect low pH and sour taste.
In their recent study, scientists introduced the gene behind the Otop1 receptor into lab-grown human cells so that they can produce the OTOP1 receptor. They then exposed these cells to acid or to ammonium chloride and analysed the responses.
Dr Liman explained: "We saw that ammonium chloride is a really strong activator of the OTOP1 channel. It activates as well or better than acids." She said that small amounts of ammonia from ammonium chloride were found to move inside the cell and raise the pH, meaning there were fewer hydrogen ions.
The group of scientists then went on to examine how mice react when ammonium chloride was introduced to their drink. They discovered that normal mice showed a sharp increase in action potentials after ammonium chloride was added, while mice that were genetically engineered without producing OTOP1 failed to respond to the salt.
This experiment led scientists to confirm that OTOP1 responds to the ammonium chloride. They also found that mice with a functional OTOP1 protein avoided drinking the water laced with ammonium chloride, while those lacking the protein drank the water laced with it, even at high concentrations.
Dr Liman said: "This was really the clincher. It shows that the OTOP1 channel is essential for the behavioural response to ammonium." The expert explained that ammonium is found in waste products such as fertiliser, and is somewhat toxic, which would explain why "we evolved taste mechanisms to detect it". But despite their recent discovery, scientists now plan to study further in order to understand how different species react to ammonium chloride, as they believe the OTOP1 channel is more sensitive in some species than others.