On Halloween, do you go straight for the chocolate? Or do Sour Patch Kids and other mouth-puckering treats call your name? Some folks find sour ever so sweet. And now a team of researchers has identified the gene that lets us taste those flavors.
In humans and other animals, taste occurs when ingested foods interact with specialized cells on the tongue, the palate and roof of the mouth, with different cells telling us whether we are tasting salty, sweet, sour, bitter or umami flavors. While researchers have known for a century that low pH (a high concentration of hydrogen ions) accounts for a sour taste in foods and beverages, no one has understood the precise mechanism that allows us to perceive the sour taste. (That taste can be key for wine fans, as it adds vibrance.)
A team led by Emily Liman, a professor of biological sciences at the University of Southern California, recently reported the how of sour taste perception in an issue of Current Biology.
From an evolutionary standpoint, the ability to detect too much acid helps deter us from eating unripe fruit or corrosive substances. And any time you get a good dose of a sour or acidic solution like vinegar, lemon juice, or even yogurt in your mouth, your salivary glands kick into high gear and flood the mouth with saliva in an effort to neutralize the acid and protect the enamel on your teeth.
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A year ago, Liman and her colleagues investigated and identified a list of 40 previously uncharacterized genes that could possibly encode a sour taste sensor. They eventually narrowed the list down to one gene, OTOP1, because it was the only gene which, when introduced into non-taste cells, could give those cells the ability to respond to acids.
Once Liman and her team had identified that gene, they needed to prove it was responsible for the ability to perceive the sour or acidic taste in the mouth. Graduate students Yu-Hsiang Tu and Bochuan Teng did that by demonstrating that mice genetically altered to have a deactivated OTOP1 gene did not respond to sour tastes.
Liman said the team’s results show that OTOP1 is a bonafide sour taste receptor. Their work is the first definitive evidence for a protein that is both necessary and sufficient for sour taste receptor cells to respond to acids and stimulate the nerves to enable sour taste perception.
The scientists also found, however, that mice with a nonfunctional OTOP1 gene could still produce a limited response to sour taste stimuli. Liman explained that while the sour taste receptors may not be functioning, “you also have a pain system that responds to low pH.” In other words, exposure to acids can be painful.
The researchers believe their work can assist food and flavor chemists in explaining why we like sour flavors, and in making food or medicines more palatable, making household products containing toxins less palatable, or guiding novel approaches to pest control.