Science of Taste and Flavor Design

Sometimes, flavor technology starts with a physiological investigation of how the human body senses taste. Today, the science of taste has advanced far beyond mapping the areas of the tongue that sense sweet, salty and so on.

Cutting-edge taste technology

Investigations conducted by Senomyx, Inc., San Diego, CA, into a “lock and key” biological model of taste—where the tastant binds to the receptor and triggers the neurological response signaling a specific taste, or a “channel and pore” system where ions that create sour and salty move through pores to trigger the response—have netted the company partnerships with some of the industry’s leading players, including Nestlé, Coca-Cola, Campbell’s Soup, Ajinomoto, Cadbury Schweppes, Firmenich and Solae.

While the logistics of the company’s efforts may be mind-bending, the logic is downright intuitive. It all starts with the basic mediator of taste, the taste bud. Over the past 10 years, says Mark Zoller, Ph.D., chief scientific officer, Senomyx, the company has gained two key insights about this structure. “One is that a taste bud is composed of about 50 to 100 cells, and it appears that each taste has its own type of cell,” he explains. These cells intermingle within the taste buds, in contrast to the older “tongue map” model whereby we tasted sweetness exclusively at the front of the tongue, bitter at the back and so on. “You certainly can find that there’s prioritization of certain tastes in certain regions—for example, the tip of the tongue tends to be more sensitive to salty,” Zoller says. “But you can find salty-type protein receptors all over the tongue.”

The second new finding revealed there’s a cellular projection bearing characteristic proteins on the tip of each taste bud. “The proteins that we’re interested in are the taste receptors,” Zoller says, “and they fall into two different categories.” One physically binds the tastant, an example being the receptor that selects for sugar. In doing so, it “triggers the inside of the sweet cell to say, ‘I’ve been activated and now I release neurotransmitters that send signals to the brain that I’m tasting sweet,’” Zoller says. Some call this a lock-and-key system, with sugar the key and the receptor the lock, and it’s also the mechanism that describes our ability to taste bitter and umami compounds, as well.

“The other type of protein, rather than being a lock and key, is more like a channel, or pore,” Zoller continues. These channels permit the passage of the ions that generate salty and sour tastes. “So, if you think of something acidic, it would be a proton or something small like that going through this sour channel and triggering a sour cell to respond. And it’s the same with salt: Salt is sodium chloride, and the sodium ion goes through the ion channel into the salt cell, and that activates the signal that you’re tasting something salty.”

The artificial taste bud

What does all this mean for taste modulation? According to Zoller, it allows his team “to use those receptors to look for new things that are sweet or salty, or that block bitterness.” By applying their understanding of the biomechanics of taste to actual mechanical instrumentation, Senomyx has built an “artificial taste bud” that can suss out substances that trigger specific sensations. “So we take a sweet receptor, we put it in a specialized cellular system, and then we can use that cell system to screen all of our different samples to see if we can find something that’s sweet—something that triggers or enhances the sweet receptor,” Zoller says.

Because a cellular system can’t actually tell you when it tastes something sweet, Senomyx researchers have also developed a method for the artificial taste bud to signal what sensation it’s picked up. “In the case of the receptors for sweet, bitter and umami,” Zoller says, “if you trigger the receptor, it actually increases the intracellular concentration of calcium.” By placing these cells in a calcium-sensitive fluorescent dye, the company can tell when they’re binding a tastant by their degree of calcium-induced fluorescence. The ion channel mechanism works similarly. “You’re increasing the intracellular ion concentration, and there are dyes that also change their fluorescence when that happens, too.”