Scientists discover how sour works

Posted by Chris Nova

Today, the day after Thanksgiving as you eat leftover cranberry sauce for breakfast, you might wonder why the sour taste is so strong.

There are five taste sensations, sweet, bitter, sour, salty, and umami. Sour is arguably the most powerful but is the least understood. The sensation of sour is conjured up by substances that are high in acid like lemons and pickles. The more acid in the substance, the more sour the taste it.
    
But the way that acids, and the protons they release, activate the taste mechanism has been beyond our understanding.
    
Fortunately Emily Liman, associate professor of neurobiology at USC College, and her team have discovered one of the ways that cells responsive to sour tastes detect protons.
    
The team of researchers was expecting to find sour protons attaching on the outside of the cell opening a pore in the membrane that allowed sodium to enter the cell, which producing an electric response. That electrical response would be transmitted to the brain
    
Well, the researchers instead found that the protons released by sour substances were not binding to the cell’s outer area, they were entering the cell.
    
Their work has revealed that it is the entry of the protons into the cell that is the cause of the electrical charge.
    
Liman’s research results were published in the Proceedings of the National Academy of Sciences journal. The paper was co-written by neuroscience Ph.D. student Rui B. Chang and research specialist Hang Waters, who is now at the National Institutes of Health.
    
“In order to understand how sour works, we need to understand how the sour-responsive cells detect the protons,” Liman said in a press release. “In the past, it’s been difficult to address this question because the taste buds on the tongue are heterogeneous. Among the 50 or so cells in each taste bud, there are cells responding to each of the five tastes. But if we want to know how sour works, we need to measure activity specifically in the sour sensitive taste cells and determine what is special about them that allows them to respond to protons.”
    
Liman and her research team created genetically modified mice to help them in their research; they marked their sours cells with a yellow florescent protein. They then recorded the electrical responses from just those cells to protons.
    
Liman speculated that the ability to sense protons with a method that does not rely on sodium entry has important implications for how different tastes interact.
    
“This mechanism is very appropriate for the taste system because we can eat something that has a lot of protons and not much sodium or other ions, and the taste system will still be able to detect sour,” she said. “It makes sense that nature would have built a taste cell like this, so as not to confuse salty with sour.”
    
In the future this research may be useful for chefs and the food industry.
    
“We’re at the early stages of identifying the molecules that contribute to sour taste,” Liman said. “Once we’ve understood the nature of the molecules that sense sour, we can start thinking about how they might be modified and how that might change the way things taste. We may also find that the number or function of these molecules changes during the course of development or during aging.”