Expanding the Palette: New Evidence Suggests a Sixth Fundamental Taste Linked to Ammonium Chloride

Scientists have found new indications supporting the existence of a sixth basic taste, associated with the sensing of ammonium chloride through the protein receptor OTOP1. This breakthrough has the potential to reshape our perception of taste and its evolutionary relevance.

In addition to the conventionally accepted tastes—sweet, salty, sour, bitter, and umami—new studies suggest that the human tongue may also be capable of detecting ammonium chloride as a fundamental taste.

Kikunae Ikeda, a Japanese researcher, was the first to introduce the notion of umami as a basic taste in the early 20th century. Despite his groundbreaking work, it wasn’t until almost eight decades later that the scientific community formally accepted umami as a basic taste.

Recently, a team of scientists, spearheaded by researchers at the USC Dornsife College of Letters, Arts, and Sciences, has found evidence pointing to a sixth basic taste. The research, led by USC Dornsife neuroscientist Emily Liman and published in the scholarly journal Nature Communications, revealed that the tongue reacts to ammonium chloride through the same protein receptor responsible for signaling the sour taste.

In northern European nations, particularly Scandinavian countries, ammonium chloride is a recognizable and sometimes favored taste, often found in the ingredient list of salt licorice candy, which has been popular since the early 1900s.

Comprehensive Study on Tongue Responsiveness

For many years, scientists have acknowledged the tongue’s strong reaction to ammonium chloride. However, the exact receptors on the tongue that respond to it have been hard to identify.

Liman and her team believed they might have discovered the answer. They had previously identified the protein responsible for the detection of sour tastes—OTOP1—which resides in cell membranes and creates a channel for hydrogen ions to enter the cell.

Hydrogen ions, the main components of acids, are what the tongue detects as sour. For example, substances like lemonade and vinegar, rich in citric and ascorbic acids respectively, are sensed as tart when they interact with the tongue’s taste receptors through the OTOP1 channel.

The team theorized that since ammonium chloride could alter the cell’s acid concentration, it might activate the OTOP1 receptor.

Animal Responses and OTOP1’s Role

To test this hypothesis, the researchers inserted the Otop1 gene into lab-grown human cells, causing them to produce the OTOP1 receptor. They then exposed these cells to either acid or ammonium chloride and recorded the responses. Liman observed that ammonium chloride was an exceptionally strong activator of the OTOP1 channel, even more so than acids.

Further experiments involved taste bud cells from genetically modified mice, both with and without the OTOP1 protein. The cells from mice with OTOP1 showed a marked increase in electrical signals, known as action potentials, when ammonium chloride was introduced, confirming the receptor’s role in detecting this compound.

Evolutionary Considerations

The detection of ammonium chloride may have evolved as a survival mechanism, allowing organisms to avoid consuming substances with high concentrations of ammonium, which is often toxic and found in waste products. Sensitivity to ammonium chloride varies between species, possibly reflecting their different ecological contexts.

The researchers intend to expand their studies to explore whether this sensitivity is maintained in other members of the OTOP proton family, which are found in different parts of the body, including the digestive system.

In summary, this groundbreaking study suggests that the official list of basic tastes may soon include a sixth entry, courtesy of ammonium chloride.

Reference: “The proton channel OTOP1 is a sensor for the taste of ammonium chloride” by Ziyu Liang, Courtney E. Wilson, Bochuan Teng, Sue C. Kinnamon, and Emily R. Liman, published on 5 October 2023 in Nature Communications.
DOI: 10.1038/s41467-023-41637-4

The research was financially supported by the National Institutes of Health.

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More about sixth basic taste

• Nature Communications Journal
• USC Dornsife College of Letters, Arts, and Sciences
• National Institutes of Health Funding
• Study on Basic Tastes
• Introduction to Taste Receptors and Sensation
• Emily Liman’s Research Profile