The challenges of tricking the taste buds

The sweetness receptor is made of two proteins in what is believed to be a structure like a Venus’ flytrap

Written by New York Times | Published: June 17, 2012 3:06 am

The basic idea of what would make a good sugar substitute is simple. On the surface of the tongue,certain proteins act as detectors for specific tastes. The sweetness receptor is made of two proteins in what is believed to be a structure like a Venus’ flytrap. When a sugar molecule attaches to the receptor,the receptor jogs neurons that send a signal to the brain that says something sweet has just been tasted. An artificial sweetener is simply a calorie-free substance that excites the same sweetness receptors.

Even so,most people can instantly tell an impostor. One reason is that the sweetener molecules also sideswipe receptors for bitterness,leaving an aftertaste. Different people also have slightly different bitterness receptors,and some react more strongly than others to the artificial sweeteners. Terry Acree,a professor of food science at Cornell University,said he cannot stand diet soda made with aspartame,because “I have a bitter receptor that is highly active.”

“People don’t like it,” said Paul Breslin,a taste researcher at Rutgers and the Monell Center,which focuses on taste and smell,in Philadelphia. “If you give them something that’s sweet but different,they innately know they’re different.”

In regular sugar,the sweet taste hits quickly and dissipates quickly. To more closely mimic that taste sensation,food companies have combined various artificial sweeteners,using one to mask the shortfalls of another. Even though artificial sweeteners are estimated to be a $1.5-billion-a-year business,not many companies are searching for new ones. The path to the supermarket can be long,winding and littered with regulatory and commercial obstacles.

The scientist and engineer Gilbert V. Levin,who developed the experiment that reported life on Mars,also discovered a substitute that tastes like sugar,is almost as sweet and is mostly devoid of calories. The sweetener,tagatose,is in fact a sugar that occurs naturally,in minute quantities,in milk and beets. Clinical studies indicate it even works as a drug to treat adult diabetes. But Levin was never able to get it manufactured in quantity at a viable cost,and efforts to have it approved as a diabetes drug have also foundered.

Levin has spent much of his life trying to convince someone to manufacture and sell tagatose,which was briefly in Diet Pepsi Slurpees,but is not used in any foods or drinks. Another approach is not to replace sugar but to look for other molecules that magnify its sweetness. It takes a lot of sugar to activate a sweetness receptor,and Senomyx,a company in San Diego,believes it can find compounds that would reduce the amount needed. PepsiCo is one of the companies Senomyx is working with.

Ordinary table sugar is sucrose,which consists of two smaller sugars,fructose and glucose. (High-fructose corn syrup is also a combination of fructose and glucose,in almost the same proportions.) Sweetness enhancers could prove important if it turns out that the fructose portion of sugar is the core cause of diabetes,heart disease and cancer. Because fructose has to be broken down in the liver,the surge of sugar may be overworking the liver. With soda,“you’re just pouring fructose into your liver,” Breslin said. Glucose is broken down by many cells in the body and by itself would not put as much strain on the liver. But glucose alone is not nearly as sweet.

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