Targeting the tongue's sweet spots

Scientists move close to finding the ideal sweetener


Steven Munger knows all about your sweet tooth.

As a neurobiologist at the University of Maryland School of Medicine, he explores how sweeteners tickle your tongue in his search for treatments for obesity and eating disorders.

One elusive goal - and a potentially lucrative one for many food scientists - is an artificial, low-calorie sweetener that really tastes like sugar.

"It's the sort of thing that would be worth millions," said Manfred Kroger, a retired Penn State food science professor and flavor expert.

This month, Munger published findings that shed new light on how two receptors on the tongue react with the confections we eat to send signals to the brain that produce the sensation of sweetness.

"This is the very first paper where someone has measured the binding of the sweet compound to the receptor itself," said Sue Kinnamon, a neurobiologist who also researches taste receptors at Colorado State University.

Munger's research was funded by the National Institutes of Health, but experts in the highly competitive artificial sweetener business are poring over his findings. "It's easily a billion-dollar industry, just in the U.S.," said Nicholas Fereday, a senior economist who tracks the food industry for LMC International, a marketing research company.

A variety of sweeteners compete for sales in our diet-crazed society. Aspartame, marketed as Nutrasweet and Equal, remains the top-selling artificial sweetener in the U.S., with about half the market, Fereday said.

Other top sellers include saccharin (commonly sold as Sweet'N Low), sucralose (Splenda), and acesulfame potassium (acesulfame K), which is often combined with other low-calorie sweeteners as an additive to foods and beverages.

Cyclamates, sold in Europe, have been banned in the U.S. since 1970 because of fears that the compounds might cause cancer.

But the ideal sugar substitute has remained elusive. Some sweeteners leave an aftertaste, while others have a tinge of bitterness. Most lack sugar's bulk, making them problematic for baking. Experts say there's still much to learn about food chemistry and how our taste buds work.

"These receptors and how they interact with all these molecules in all these foods is extremely complex," Kroger said.

Researchers at Senomyx, a California-based firm that develops flavors for the food industry, found last year that cyclamate bonded with one sweet taste receptor, known as T1R3, while aspartame bonded with the other one, called T1R2.

What Munger found was this: Sucralose and sugar bonded with both receptors, and sucralose bonded much more tightly to both of them - accounting for its sweeter taste. Sucralose bonded 300 times more effectively to T1R2 and up to five times more effectively to T1R3, Munger said.

For his research, Munger created an extract from proteins that activate rat and human sweet taste receptors. He then ran the extract through a spectrometer to measure how tightly the proteins bonded with natural sugar and sucralose. A sweetener's effectiveness is determined by how it bonds to a receptor.

Munger chose to analyze sucralose because it appeals to more than just humans. Rats, which are often used in sweetener studies, can't taste the sweetness of cyclamate and aspartame. But they can sense the sweetness in sucralose.

"We wanted to look at a sweetener that's the most broadly preferred," he said.

The results are being published this month in the journal Current Biology.

Five basic tastes

Although his research may lead to contracts from food companies, Munger says he does it for its scientific value. "We'd really like to understand how an animal makes sense of all these stimuli - how this sensory system works in its entirety," he said.

The system consists of bumps on the tongue, called papillae, that contain taste buds with dozens of taste cells. When the cells encounter molecules that give off flavors, they trigger a range of nerve signals to the brain.

A food's taste is determined by the sensations created by the taste cells and is enhanced by chemicals that create odors detected by olfactory neurons in the nose.

Scientists say humans can detect five basic tastes: sweet, sour, bitter, salty and umami, a recently discovered taste created by salty, acidic compounds such as the flavor enhancer MSG.

It's a myth that different regions of the tongue trigger bitter and sweet tastes, Munger said. There are a few more bitter taste receptors on the back of the tongue than elsewhere, but different types of receptors are scattered everywhere, he said. The myth is believed to have started with a mistranslation of a German report on receptors that made its way into U.S. textbooks.

"It was one of those things that once it appeared, it just kept re-appearing," Munger said.

'Learning new things'

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