Stevia Taste Science

Summary

• Tastants are individual stimuli that elicit one of five basic tastes: sweet, umami, bitter, salty and sour
• Binary taste interactions can change taste perception when multiple taste stimuli are present
• Taste perception can vary greatly among individuals due to genetic variation
• Taste is mediated by taste receptor cells, specialized cells located on the taste buds
• Research on taste receptor cells can be leveraged to optimize the perception of steviol glycosides

Introduction

Extracts from stevia, steviol glycosides, can be up to 350 times sweeter than conventional sugar while containing no calories. Some steviol glycosides may elicit a bitter taste that some people find unpleasant but this is overcome by using combinations that are designed for specific matrices and optimal great taste. An understanding of the science of taste helps identify ways to enhance the sweetness that stevia naturally brings.

Sweet and Bitter Tastes

Individual compounds that elicit basic tastes are referred to as tastants. In addition to sugars, sweet tastants include a variety of chemical compounds such as sugar alcohols, glycosides (including steviol glycosides), amino acids and proteins (Bachmanov, 2014). For bitter taste, more than 550 chemically diverse compounds have been identified (Wiener, 2011). Some commonly identifiable bitter tastants (and where they are found) include quinine (tonic water), caffeine (coffee), epicatechin (tea), tetralone (hops) and naringin (grapefruit) (Reed, 2010).

Taste perception can change when multiple taste stimuli are presented together in a food or beverage rather than when presented alone. This is referred to as a binary taste interaction (Keast, 2003) (figure 1). On a practical level, binary taste interactions are important in the development and modification of foods and beverages made with steviol glycosides. Sweet and bitter tastes found in steviol glycosides interact such that the presence of one suppresses the other (Hellfritsch, 2012).

Taste Perception

The tongue contains thousands of taste buds which are small structures that mostly reside on visible raised bumps (papillae) on the upper surface of the tongue and on the palate. Within each taste bud is a set of 50 to 100 specialized cells known as taste receptor cells. Within a taste bud, some taste receptor cells are specific to sweet, bitter, sour, salty or umami.

Sweet tastes are detected by a category of taste receptor cells called T1R and bitter tastes are detected by a family of taste receptors called T2R. For sweetness perception, the taste receptor cell T1R is comprised of two dimerized taste receptor proteins, T1R2 and T1R3 (Renwick, 2017). Bitter perception is much more complex: there are at least 28 bitter taste receptors within the T2R family (Bachmanov, 2014).

Genes encoding T2R bitter taste receptors begin with the nomenclature “TAS2R”. Human bitter taste receptor genes are named TAS2R1 to TAS2R64, with many gaps in the naming sequence due to the emerging nature of genetic science. The number of tastants perceived as bitter is much larger than the number of human TAS2R genes, implying that individual bitter taste receptors can respond to more than one bitter tastant (Behrens, 2006). Likewise, many bitter tastants can be discerned by more than one type of bitter taste receptor as is the case for caffeine which is recognized by receptors TAS2R7, TAS2R10, TAS2R14, TAS2R43 and TAS2R46 (Wiener, 2011).

Genetic Variability and Bitter Taste Receptors

Human taste perception, especially for bitter tastes, can vary greatly among individuals due to genetic variation (Bachmanov, 2014). This genetic variation can affect food perception, choice, and consumption and thus can influence nutrition and predisposition to certain diseases. Most genes that encode for taste perception are considered polymorphic, meaning that multiple forms of the gene can exist.

There are over two dozen genes that code for the ability to taste many other bitter substances. A bitter database at the Hebrew University of Jerusalem currently indexes over 680 bitter compounds and their associated 25 human bitter taste receptors genes (Wiener, 2011). For instance, polymorphisms of the genes TAS2R43 and TAS2R44 are associated with the perception of bitter taste in several artificial sweeteners including saccharin and acesulfame K.

Bitter Taste Receptors for Steviol Glycosides

A study published in the Journal of Agricultural and Food Chemistry characterized the sensory properties of steviol glycosides by combining sensory taste panels with cell-based receptor assays to assess how compounds are sensed by the tongue (Hellfritsch, 2012). Results indicate that two receptors, TAS2R4 and TAS2R14 mediate the bitter taste in steviol glycosides. The three main structural features that modulate the sweet and bitter taste in steviol glycosides were glycone chain length, pyranose substitution and the C16 double bond. For glycone chain length, steviol glycosides that had more glucose moieties attached to them were sweeter and less bitter. For example, the steviol glycoside rebaudioside D comprises five glucose molecules and is around five times sweeter and two-thirds less bitter than dulcoside A, which has just two glucose molecules. Rebaudioside A which has four linked glucose molecules is also sweeter and less bitter than dulcoside A. These findings are being utilized to optimize agronomy and processing to produce stevia extracts high in molecules like rebaudioside M which have six glucose moieties and a resulting clean, sweet taste.

Sweet Taste Receptors and Steviol Glycosides

Research on sweet taste receptor cells might also be leveraged to optimize the taste of steviol glycosides. The area of a taste receptor cell that tastants bind to is referred to as a docking site. The multiple docking sites present on taste receptor cells can create diversity in how they function (Masuda, 2012). Along with multiple docking sites, functional diversity stems from the ability of each heterodimeric sweet taste receptor subunit to induce independent taste signaling. In a 2015 paper published in the journal Phytochemistry, docking studies were performed on eight steviol glycosides by constructing models of the T1R2 and T1R3 subunits of human sweet taste receptors (Mayank, 2015).

Findings showed significant variation in the docking positions of all the steviol glycosides evaluated. Docking scores are able to predict the exact sweetness strength of various steviol glucosides, from most to least sweet: rebaudioside A, rebaudioside E, rebaudioside D, rebaudioside B, stevioside, steviolbioside and dulcoside. The interaction of the carbon molecules at positions C-13 and C-19 of the steviol backbone in a particular steviol glycoside with a specific set of active docking sites was responsible for producing its characteristic taste. Results suggest that modifying structures and thus enabling their binding towards a specific point in the sweet taste receptor cells may be useful to enhance the taste quality and sweetness index of steviol glycosides.

Conclusion

The benefits of stevia are numerous: 350 times sweeter than sugar, natural origin, sustainability, zero calories, zero glycemic load, heat stable and tooth friendly to name a few. Advances in agronomy and continued understanding of the taste differences and interactions of the steviol glycosides will result in even greater advances in stevia’s contribution to food science and taste science. Taste receptor science is an exciting new area that continues to provide new direction for optimizing the sweet potential of stevia.