Human taste biology is a complex process, much of which scientists have only recently discovered, thanks in part to the Human Genome project. It is amazing to realize that formal publication of the discovery of genes that code for bitter taste receptors occurred a mere seven years ago. This research opened the door for additional discoveries and identification of other chemosensory receptors, such as those associated with sweet taste. Although the effect of chemicals found in food and food ingredients on taste receptors is not yet fully understood, biologists and food scientists continue to collaborate on ways in which food product development involving sweet taste can be optimized for consumer preference.
In the meantime, food scientists continue their search for the “Holy Grail” of ingredients that both function and taste exactly like sugar. Sugar is and has been the preferred choice of sweetener but often is replaced in low- or no-sugar added products targeted toward the consumer, who either faces or fears the perils of obesity and diabetes. Not only are nutritive sweeteners such as sugar or corn syrup often replaced fully or in part by non-nutritive sweeteners and/or polyols (sugar alcohols), but also by flavoring agents and/or natural compounds that impart sweetness.
Sensing Sweet TasteWhat is now known about taste perception is far different from that previously thought. As food scientists, many of us were shown a map of the tongue with certain regions delineated as locations specific to certain taste qualities (i.e., sweet, salty, bitter, sour or umami). Although there are certain areas of the tongue that are more sensitive to certain tastants, taste buds can detect all five tastes.
Back in March and April of 2000, two separate groups of researchers affiliated with the Howard Hughes Medical Institute (HHMI) published their findings of bitter taste receptor genes in the esteemed science journals Cell and Nature, respectively. An April 6, 2000 HHMI research news brief describes the taste receptor mechanism as follows: “Receptors are proteins that nestle in the cell’s surface and bind specific chemicals in much the same way that a key fits into a lock. When activated by a chemical, receptors trigger molecular signals within a cell that alter the cell’s metabolism. In the case of taste receptors, chemicals impinging on the taste buds trigger nerve impulses that travel to the brain, where taste information is processed.”
One of the research teams based its investigation of bitter taste receptor genes on the previous research of another HHMI investigator, Robert Margolskee, who had discovered evidence that the receptors for both bitter and sweet chemicals are coupled to a common G-protein signaling molecule. What is now known: “Sweetness detection is most like bitter perception at a molecular level, because bitter and sweet use essentially the same receptor and transduction system; the receptor types are similar, and the way the signals inside the cell are coded and conveyed to nerves is similar,” explains Danielle Reed, Ph.D., associate member, Monell Chemical Senses Center.
Gustducin is the name given to the G-protein signaling molecule. According to John W. Kimball, Ph.D., professor emeritus of immunology at Harvard University, the complex of G-proteins was named as such because of its similarity in function to that manifested by the G-protein transducin, which plays an essential role in rod vision. Each G-protein coupled receptor (GPCR) is composed of two sub-units designated as T1R2 and T1R3, which are coupled to the G-protein gustducin. Activation of gustducin by compounds that bind with these two sub-unit receptors results in the sensing of sweetness.
“Recent research has focused on understanding what happens inside the taste receptor cells at the molecular level, when chemicals are present,” says Reed. “For sweet, we have learned that the receptor is made from two proteins that intertwine and offer a complex external surface that can interact with a variety of sweeteners. Although the details are not completely understood, this discovery is consistent with a puzzling observation that both simple, small molecules such as glucose, as well as large, complex proteins such as monellin, can taste sweet.”
Tastes Like SugarSugar or sucrose, a disaccharide comprised of glucose and fructose, remains the gold standard when formulating a product’s sweetness profile. Product developers search for the blend of sweeteners that most closely matches sucrose’s temporal profile, or its onset, intensity and duration in a given food matrix. Other factors such as functionality, mouthfeel, ingredient interaction, processing and storage stability are naturally considered as well. Sugar and sugar alcohols have similar temporal profiles in that sweetness intensity increases with progressively higher concentrations of sweetener, notes Ronald C. Deis, Ph.D., vice president of technology for a well-known sweetener supplier. While high-intensity sweeteners follow a similar course at low-use concentrations, sweetness intensity eventually levels off at higher concentrations.
Food scientists are taking a closer look at a wide range of binary and ternary blends of sweeteners to assess how closely their temporal profiles mimic that of sucrose. Blending sweeteners reduces the probability of a phenomenon known as adaptation, where repeated exposure to a particular sweetener results in a loss of sensitivity, notes Deis. In addition, many combinations of sweeteners have demonstrated synergistic properties. For instance, the combination of sucralose and acesulfame-K gives a more sucrose-like profile than either ingredient used alone.
Researchers from Duke University recently collaborated with scientists from a high-profile sweetener supplier on a study of binary and ternary sweetener blends, in an attempt to discover the optimal profile that most closely matched that of sucrose. The study incorporated sucralose, neotame, mannitol, acesulfame-K, sorbitol, saccharin, fructose, sucrose, cyclamate (not approved in the U.S.), stevioside (a compound found in stevia with a slightly bitter component), aspartame, rebaudioside-A (a compound found in stevia that is more sucrose-like than stevioside), neohesperidin dihydrochalcone (NHDC), alitame and the protein thaumatin. Blends that contained thaumatin were associated with the latest time to maximum sweetness intensity, perhaps because of the size and complexity of the protein’s structure. Blends containing stevioside, neotame, NHDC, alitame or rebaudioside-A reportedly had later times to maximum sweetness than that exhibited by blends containing sugars or sugar alcohols.
Beyond SweetenersStriving for the ultimate flavor profile of sugar is not only accomplished by ingredients categorized as sweeteners, but by other ingredients as well. A host of products exist that either in or of themselves act as sweeteners or have a sweetness-enhancing effect on the food matrix within which they are used. The average consumer may consider them sweeteners, but in the U.S., some of these products fall under the “dietary supplement” category, while others are considered flavors or flavor enhancers.
Stevia, a glycoside formed by three molecules of glucose and one of steviol (made from the South American plant Stevia rebaudiana), is one such product. While considered a food additive in countries such as Japan, Brazil and China, it is labeled as a dietary supplement in the U.S. until additional information is gathered on its safety. Stevia, marketed as a more natural sweetening alternative, is approximately 300 times as sweet as sugar with a slower onset and a longer duration. It is also heat- and pH-stable, non-fermentable and does not invoke a glycemic response. The FDA could reportedly be petitioned to approve stevia as a food additive—not an unlikely scenario considering the fact that Coca-Cola Co. and its joint-venture partner, a well-known supplier of sweetener ingredients, have filed 24 patent applications for the ingredient in the U.S. There are several different variations of stevia products coming to market. Optimal varieties that have a clean sucrose-like taste without the presence of a bitter aftertaste are being produced through plant-breeding techniques.
Another product, neohesperidin dihydrochalcone (NHDC), is also not approved for use as a sweetener in the U.S., but is considered generally recognized as safe (GRAS) by the Flavor and Extract Manufacturers Association (FEMA) as a flavor modifier when used at low concentrations in foods and beverages. NHDC, a derivative of the bitter component of grapefruit rinds, has a slower onset, lingers longer and has a different overall flavor profile than sugar. However, NHDC reportedly has a strong synergistic effect when used with other sweeteners such as aspartame, saccharin, acesulfame-K, cyclamate and sugar alcohols.
A group of proteins that have been isolated from plants grown in tropical rainforests includes thaumatin, brazzein, monellin, curculin, mabinlin, miraculin and pentadin—each of which has sweet or taste-modifying properties. Although brazzein is the most heat-stable and pH-stable of the group, thaumatin is the only protein of those named that has GRAS status as a flavor modifier. Thaumatin is 10,000 times sweeter than sugar on a molar basis. According to Ravi Kant, miraculin acts as a taste-modifying protein in that it causes the sweet taste receptor to respond to acid, thereby causing the sour compound to taste sweet (Nutrition Journal, 2005, 4:5).
Ammoniated salts of glycyrrhizic acid are naturally occurring flavoring compounds characterized by their sweetness. Ammonium glycyrrhizinate (AG) is the fully ammoniated salt of glycyrrhizic acid. Both AG and monoammonium glycyrrhizinate (MAG), in and of themselves, contribute a sweetness that is 50 times that of sugar, but also possess a licorice aftertaste. Treatment with 5’-nucleotides represses this licorice flavor without suppressing the sweet character. One of the issues associated with AG and MAG is the slow onset of sweetness and the lingering sweet aftertaste. The addition of certain inorganic phosphates has reportedly resulted in an improvement in both of these temporal characteristics, such that sweetness onset is improved and the lingering aftertaste reduced.
Why Study Taste Perception?While enhanced consumer perception resulting in an increase in a product’s marketability certainly provides impetus for understanding human taste perception, other reasons exist as well. The reduced ability to taste—a condition known as hypogeusia—can indicate the presence of an adverse health condition. In addition, knowledge of taste perception may help food scientists develop products that have greater appeal to an aging population that theoretically suffers some loss of taste sensation.
“It is important to distinguish between the ability to detect sweetness and the degree to which people enjoy sweetness,” says Reed. “The inability to detect sweetness is extremely rare in humans and is not an important part of clinical medicine or health research. However, researchers do study the degree to which people ‘enjoy’ highly sweetened foods and drinks, and the results here indicate that children, especially during growth spurts, enjoy intensely sweet drinks more so than older people and children not actively growing. In adults, the preference for highly-sweetened foods and drinks is associated with both alcoholism and, perhaps, depression.”
There are many other ways in which further research might shed light on taste perception, such as ways in which medicines can be made more palatable by suppressing bitter compounds, or ways in which foods can be customized for specific populations based on known sweet taste preferences. Taste perception is not constant across gender, race and age populations, or even necessarily within common genetic origins. Reed and her colleagues address many of these issues in an article entitled “Diverse Tastes: Genetics of Sweet and Bitter Perception,” (Physiological Behavior, 2006, June 30; 88(3): 215-226). For instance, how might DNA variance in taste receptors affect sensitivity to taste perception? Another area of interest, as described by Reed, et al., is the effect of the concentration of the hormone leptin on human physiological and behavioral responses to sweets, since there is evidence that these responses are suppressed in studies conducted with rodents.
The world of genetics has opened up a vast array of possibilities for future research in the area of taste perception. For instance, one flavor ingredients company recently received a patent (U.S. Patent #7,223,551) for assays that detect the specific binding of compounds to the T1R3 taste receptor. This patent provides the company with a method for identifying compounds that “modulate or elicit” sweet taste. Collaborations between food scientists, biologists, psychologists and the like surely will produce some phenomenal results over the decades to come.