In the realm of human gastronomy, the term “basic taste” has long been associated with a quintet: sweet, sour, salty, bitter, and umami, the latter being a late addition recognized in the early 1900s thanks to the work of Japanese scientist Kikunae Ikeda. However, fresh revelations from the USC Dornsife College of Letters, Arts and Sciences suggest that our tongues might just be more sophisticated than we imagined.
A research team led by USC Dornsife neuroscientist, Emily Liman, unveiled evidence of a potential sixth basic taste. The culprit? Ammonium chloride, a compound well-acquainted with those in Scandinavian countries where it’s an essential ingredient in salt licorice, a candy that dates back to the early 20th century.
For many years, scientists have acknowledged the tongue’s pronounced response to ammonium chloride, yet the specific receptors enabling this remained a mystery. Liman’s team believed they held the key. In previous investigations, they had identified OTOP1, a protein responsible for detecting sour tastes.
“If you live in a Scandinavian country, you will be familiar with and may like this taste,” says Liman, professor of biological sciences. 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, or ammonium chloride.
This protein, which is located within cell membranes, facilitates the movement of hydrogen ions (the primary component of acids) into taste receptor cells. This movement is what gives acidic foods, like lemonade and vinegar, their characteristic tartness.
With the knowledge of how the OTOP1 protein functions, the team hypothesized a connection between this protein and the sensing of ammonium chloride, especially considering the latter’s influence on the acid concentration within a cell.
By introducing the Otop1 gene into lab-grown human cells, thus producing the OTOP1 receptor protein, and subsequently exposing these cells to ammonium chloride or acid, the team discerned that ammonium chloride was a potent activator of the OTOP1 channel — comparably strong, if not stronger, than traditional acids.
The experiments didn’t end there. The team furthered their quest by simulating the way nerves transmit signals using a technique that measures electrical conductivity. When subjected to ammonium chloride, taste bud cells from typical mice showed a noticeable uptick in electrical activity (action potentials). Conversely, those from genetically-engineered mice lacking OTOP1 showed no response. Similar results were observed when signals from taste-innervating nerves were examined.
Perhaps one of the most enlightening segments of their research was when they observed mice’s behavioral responses to ammonium chloride. Mice possessing the OTOP1 protein avoided drinking water mixed with ammonium chloride. Mice without the protein seemed unperturbed, even at high concentrations. This observation sealed the connection between OTOP1 and the sensing of ammonium.
Extending their investigation, the team questioned whether this taste sensitivity was universal across various species. They discovered that while the sensitivity of the OTOP1 channel to ammonium chloride varied among species, human OTOP1 channels, interestingly, were also sensitive.
Given the universality of this taste detection, one might ponder its evolutionary advantage. Liman postulates that this taste sensitivity evolved to deter the consumption of toxic biological substances rich in ammonium, such as certain waste products. This evolutionary rationale could be underscored by the varying sensitivities of different animals to ammonium based on their natural habitats.
The story of OTOP1’s interaction with ammonium doesn’t end here. The team was able to pinpoint a specific amino acid within the OTOP1 channel essential for its ammonium response. The conservation of this amino acid across species indicates that it’s crucial for survival. This finding hints at the broader significance of the OTOP1 channel’s ability to detect ammonium.
Liman’s team plans to expand their investigations to determine whether this ammonium sensitivity persists in other members of the OTOP proton family. As their research progresses, the world will keenly watch to see if the roster of basic tastes, currently standing at five, will soon welcome a new member: ammonium chloride.
Our tongues are intricate detectors, working tirelessly to discern the flavors of our meals. At the heart of this culinary exploration are the taste receptors, specialized cells that respond to specific flavors.
These receptors help us navigate the broad spectrum of tastes present in our foods, from the sweet bliss of a ripe strawberry to the piquant zing of a chili pepper.
Setting aside ammonium chloride for the moment, the human tongue recognizes five basic tastes, each attributed to its own set of receptors.
When you indulge in a dessert, it’s the sweet receptors that spring into action. These receptors respond to sugars and certain protein-based sweeteners, allowing us to appreciate the sugary delights of candies, fruits, and baked goods.
Biting into a pretzel? It’s the salty receptors that take charge. They react primarily to sodium ions, a primary component of table salt. This sensitivity to salt has evolutionary roots, as sodium is vital for many bodily functions.
The tang of a fresh lemon activates our sour receptors. These receptors detect acidity, specifically hydrogen ions in food. Foods rich in citric, malic, or other acids will trigger these taste buds, helping us identify potentially spoiled or fermented foods.
Many people shy away from overly bitter foods, and there’s a reason for it. Bitter receptors alert us to potential toxins, as many poisonous compounds have a bitter profile. Found in foods like dark chocolate, kale, and certain coffees, the bitter taste is complex and can vary widely.
The newest member of the taste family, umami, represents the savory or meaty taste. Discovered by Japanese scientist Kikunae Ikeda in the early 1900s, umami receptors detect amino acids like glutamate. Umami tastes are often found in broths, meats, and fermented products.
These receptors work in tandem with our sense of smell to create a comprehensive flavor profile for every bite we take. Upon activation, taste receptors send signals to the brain. The signals then translates these messages into the flavors we recognize and adore (or occasionally despise).
In recent years, with the advancement of molecular biology and genetics, researchers are gaining deeper insights into the world of taste receptors. Some scientists are even investigating the possibility of additional tastes, such as the aforementioned response to ammonium chloride.
For now, the next time you savor a meal, remember the hardworking taste receptors, the unsung heroes that let you journey through the world of flavors with every bite.
The full research paper was published in the journal Nature Communications.
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