Bitter receptors for stevia sweeteners discovered

Why stevia tastes not only sweet but also bitter

Stevia is considered a healthy alternative to sugar. The stevia products recently approved as sweeteners by the European Union also have disadvantages, for example a long-lasting, bitter aftertaste. The responsible taste receptors on the human tongue have now been identified by scientists from the Technical University of Munich and the German Institute for Nutrition Research Potsdam-Rehbrücke. With cell culture experiments and sensory tests, the scientists were also able to show that steviosides, which contain many glucose components, taste particularly sweet. The researchers report on their results in the Journal of Agricultural and Food Chemistry.

There is only one type of receptor on the human tongue that is responsible for the perception of sweet tastes, whereas there are around 25 different receptors for bitter aromas. Scientists from the Technical University of Munich (TUM) and the German Institute for Nutritional Research Potsdam Rehbrücke (DIfE) have now identified the two taste receptors hTAS2R4 and hTAS2R14, which are responsible for the bitter aftertaste of stevia.

The extracts of the subtropical plant are up to 300 times sweeter than conventional sugar. They hardly contain any calories and are easy on the teeth. Nevertheless, the “honey herb” has an aftertaste: In high concentration it provides licorice-like, bitter notes.

The scientists examined nine so-called steviol glycosides, which provide the intense taste of extracts from the stevia plant. The researchers first tested how sweet or bitter the different glycoside variants appear in the test tube: Specially grown cells take on the function of taste receptor cells and react to the glycoside molecules like an artificial tongue. This enabled the scientists to identify those types of receptors that are activated by stevia.

In addition, sensory tests were carried out in which specially trained test persons assess the taste intensity of the stevia components depending on their concentration. The result of the combined taste tests: The structure of the glycoside molecules is a decisive factor for the degree of sweetness or bitterness of stevia. "The more glucose that is bound to the molecule, the sweeter and less bitter," explains Prof. Thomas Hofmann, who holds the TUM Chair for Food Chemistry and Molecular Sensor Technology. The stevia component rebaudioside D, for example, contains five glucose components and is about five times sweeter and two thirds less bitter than dulcoside A with only two glucose components.

"The bitter aftertaste of the steviol glycosides arises when the glycosides activate the two types of bitter taste receptors on the human tongue," explains Anne Brockhoff from the German Institute for Nutritional Research. These new findings could help minimize the bitter taste of stevia products at an early stage. "For example, breeding measures or purification in the production of stevia products can be focused on the best sweetener candidates," says TUM scientist Thomas Hofmann with certainty.

Publication:

Caroline Hellfritsch, Anne Brockhoff, Frauke Stähler, Wolfgang Meyerhof, Thomas Hofmann: Human Psychometric and Taste Receptor Responses to Steviol Glycosides, Journal of Agricultural and Food Chemistry, May 2012 http://pubs.acs.org/doi/abs/10.1021/jf301297n

Source: Potsdam-Rehbrücke [DIfE]