Flavor Enhancement

Flavor Enhancement
February 1996 -- Cover Story

By: Scott Hegenbart
Editor

    Flavor enhancers seem to be surrounded by controversy. If consumer advocates aren't debating health issues related to them, food scientists and other researchers are considering questions about the very nature of these ingredients. Do they potentiate existing flavor, or contribute one of their own? Do they actually affect aroma? Do they really do anything at all? No one seems to be able to agree on anything.

Basic taste, not so basic

  Part of the reason our understanding of flavor enhancers is so confused is that knowledge about the mechanism of taste itself is always changing. The most commonly believed mechanism for taste perception is that taste consists of four main sensations: sweet, salty, bitter and sour. Thousands of receptor cells sensitive to these taste sensations are grouped together in taste buds located mainly on the surface of the tongue. The various chemical compounds in food trigger these specialized nerve cells and form a basic taste sensation.

  Generating an overall flavor experience requires more than basic taste. Aromas released in the mouth stimulate olfactory receptors and transmit odors to the brain. Finally, chemical feeling factors such as astringency and heat are registered on the lips and in the mouth. The total flavor experience, therefore, is thought to be a combination of taste, aroma and chemical feeling factors.

  Although deeply rooted in and strongly supported by empirical evidence, this mechanism technically is still mostly theory. In recent years, however, supportive research has been published. One example showed that cations, especially sodium cations, produce the physiological response recognized as saltiness. It is believed that the cations travel through special channels in the outer membrane of taste cells and enter receptor cells. This changes the voltage across the receptor cell membrane, electrically exciting the cell and causing the release of neurotransmitters that stimulate nerve cells to signal saltiness to the brain. In addition, anions have been found to contribute to the sweet response. In sodium saccharin, for instance, the sweetness is attributed to the saccharin ion.

  But this sort of research has raised as many questions as it has answered. For example, it has been found that certain other cations -- namely those of potassium and ammonium -- are perceived not just as salty, but bitter as well. When comparing salt with the same cation, bitterness seems to increase as the molecular weight of the anion -- the negatively charged ion -- increases.

  Also confusing the issue is that dilute concentrations of sodium chloride are perceived as sweet. One theory holds that this effect is caused by the water molecules organized around the sodium ion. This produces the same structure as that which produces a sweetness response in sweeteners such as sugar.

  Animal studies further demonstrate that a receptor may respond to chemicals that represent more than one taste quality. It has been demonstrated in mammals that fibers that respond the best to salt often show a secondary response to sucrose. Some salt sensors may exhibit a secondary response to acid, but not at all to sucrose.

  Promising though the research may be, the only conclusion that can be drawn from these advances is that the entire mechanism of taste is much more complex than usually assumed.

The quest for definition

  The fact that the understanding of flavor perception is in a state of flux makes it all the more challenging to discuss flavor enhancers. It doesn't help that these ingredients historically have had a murky definition.

  " 'Flavor enhancer' has been a term that's been a catch-all, and a lot of ingredients are dumped into that category," says Steve Box, marketing manager -- flavor ingredients, Cultor Food Science, New York. "I don't think there is a good understanding of what they are."

  Still, a better understanding of where and how flavor enhancement occurs would certainly help guide product designers to use such ingredients more effectively. To this end, many researchers have been seeking to unlock the secret of flavor enhancement for years.

  "It's certainly not an easy issue to address," says George Stagnitti, manager food applications, Cultor Food Science, Groton, CT. "Looking at the ingredients and the mechanism of their function involves tremendous crossover from the various scientific disciplines."

  Indeed, scientists ranging from chemists, biologists and biophysicists to food scientists, sensory specialists and psychologists have tried explaining what flavor enhancers do.

  These efforts have yielded many diverse theories and a great deal of data, but few answers.

  The first of these theories is the amplification theory. The idea behind this theory is that an enhancing ingredient either physically holds a flavor substance to a taste receptor to allow a stronger electrical signal to be sent, or somehow opens a greater number of channels to taste receptors for the same effect.

  One proponent of this theory is Tilak Nagodawithana, director of R&D, Red Star Bioproducts, Milwaukee. His theory for the mechanism behind flavor amplification is that flavor enhancer molecules help anchor the flavor of a food to the receptors on the taste bud which is interpreted by the brain as a more intense flavor.

  "The theory is that the enhancer attaches to the flavor particle and helps retain it on the surface for a longer period of time to give more stimulation," says Nagodawithana. "Some of our proprietary research has pointed in this direction."

  He warns, however, that this idea still is being researched, "so we must be careful about applying this model."

  "Back in the 1950s when they started trying to describe and quantify the effects of MSG, they thought that it somehow turned up the volume on the taste of foods," says Michael O'Mahony, professor of food science and technology, department of food science and technology, University of California, Davis. "No one has done experiments that can prove it."

  Certainly experimentation has suggested that something is happening when a flavor-enhancing ingredient is added, but the outcome tends to vary a great deal and the results conflict. Even how the data is looked at can make a dramatic difference in the conclusion. For example, many experiments to demonstrate flavor enhancer functionality are performed using human subjects for sensory testing. A common experimental procedure is magnitude estimation. Here, a test component is added at different levels to a solution of a "pure" taste, and the test subject ranks intensity.

  One such test performed in the late 1970s (Yamaguchi, S., and Kimizuka, A., "Psychometric Studies on the Taste of Monosodium Glutamate," Glutamic Acid. Advances in Biochemistry and Physiology, L.J. Filer, et al, Eds., Raven Press, New York, 1979) showed that MSG did not significantly increase the intensity of the four basic tastes.

  When looking at general trends in the same data, however, Joseph Maga of Colorado State University, Fort Collins, discovered that while no basic tastes increased in intensity, all but salt decreased slightly with increasing MSG concentrations. (Maga., J., "Flavor Potentiators," Critical Reviews in Food Science and Nutrition, Volume 18, CRC Press, 1983.) This led Maga to propose that potentiation was actually the apparent result of certain flavors being suppressed by the enhancer.

Questions in testing

  Doubts have been raised as to whether sensory testing -- particularly that using untrained panelists -- can even demonstrate flavor enhancement. Harry Lawless, Ph.D., a professor at Cornell University, Ithaca, NY, attributes some of the confusing results of sensory tests to "halo effects." These occur when test subjects have no available descriptor that adequately describes a sensation. They then attribute positive aspects to other qualities they can describe. The opposite also holds true for negative perceptions in a "horns effect."

  To demonstrate, Lawless added a small amount of vanilla extract to lowfat milk near the perceptibility threshold. He then collected ratings on sweetness, thickness, creaminess and overall liking for both the test sample and a control.

  "In spite of the lack of a relationship between vanilla aroma and sweet taste and between vanilla and texture," says Lawless, "the introduction of this one positive aspect was sufficient to cause apparent enhancement in these other areas." In other words, if subjects lack the verbiage to describe a sensation or if the descriptors available are limited, they may "dump" the perceived changes in some other -- possibly inappropriate -- category.

  "The big problem is how you get sensible numbers," says O'Mahony. "How do you know that people are giving you responses about their sensations and not their imagination? Imagination plays a very strong part unless you can design experiments to factor it out."

  Proper screening and training of panelists, avoiding presenting too many samples in one sitting, and using ranked comparisons instead of verbal descriptors are some of the controls that can be used to bring order and balance to human sensory testing. Such controls were used in a test performed by Sarah Kemp, Ph.D., of the Givaudan-Roure Corp., Clifton, NJ, and Gary Beauchamp. Ph.D., director of the Monell Chemical Senses Center, Philadelphia, with the results published in 1994.

  In the study, five concentrations of salt (NaCI) or MSG were added to pure taste or flavor substances. Subjects were given only five samples at a time a of either MSG or salt.

  The results of this test found no true potentiation. Salt and MSG concentrations higher than their taste threshold, however, generally appeared to suppress pure tastes and flavors. Kemp and Beauchamp went on to suggest that this suppression may cause the apparent potentiation seen in more complex food systems. This seems to mirror Maga's interpretation of Yamaguchi's 1979 data.

  To eliminate this human factor completely, other researchers turn to animals as subjects. A recent example of such work confirmed the lack of MSG effect on sweetness -- either potentiation or suppression. Nirupa Chaudhari. Ph.D., associate professor of physiology and biophysics. and Stephen Roper, Ph.D., professor of physiology and biophysics at the University of Miami Medical School, published an abstract in Chemical Senses toward the end of 1995 which describes their investigation into whether glutamate enhances other primary flavors. To do this testing, they observed rat behavior to see whether MSG could enhance the sweetness of sucrose.

  "We were inspired by a lot of the early literature in the 1940s and 1950s stating that MSG was an enhancer for primary taste," says Roper. "Our preliminary results did not show any synergistic or antagonistic interactions between sucrose and MSG. They are independent taste qualities."

  In addition to showing that glutamate and sucrose did not interact, Chaudhari and Roper subjected glutamate and IMP to the same test in order to confirm the effectiveness of the methods they used. Glutamate and inosine 5'-monophosphate (IMP) have a well-documented synergy, which the method of these researchers confirmed.

  "We used the glutamate/IMP interaction as our bench control to show our method does pick up synergistic interactions when they occur," says Roper. "Had (synergy) occurred with glutamate and sucrose, we should have seen it."

From enhancer to taste

  Other researchers suggest that flavor enhancers don't actually modify flavor at all. More and more research indicates they may contribute a taste of their own. While this is a newer concept in the United States, the idea that flavor enhancers contribute a basic taste is more established in Japan (though the idea is not as widespread as has been reported.) Kikunae Ikeda, Ph.D., who first identified monosodium glutamate as the distinctive taste of sea tangle, originally named this taste, "umami," derived from a word meaning "savory" in Japanese.

  Many different types of studies have been performed to demonstrate the existence of umami. These include a variety of sensory approaches, as well as psychophysical tests in both humans and animals.

  Susan Schiffman, Ph.D., at Duke University, Durum, NC, is one of the many researchers who have been involved with such tests. In one test, subjects were asked to rate nucleotides, monosodium glutamate, glutamic acid, and a host of other substances on five scales: salty, sour, sweet, bitter, and amount left over. Umami substances tended to score very high in the "left over" category. When asked to describe the sensation of the umami substances, the subjects could not adequately verbalize the taste character, leading Schiffman to conclude that many tastes cannot be described completely in terms of four basic qualities.

  Schiffman later elaborated on these findings using "multidimensional scaling," a mathematical technique that can analyze the similarity among sensory stimuli without being dependent on verbal descriptors. Instead, multidimensional scaling arranges stimuli in a three-dimensional map on the basis of the similarity of their taste properties. Assuming only four basic tastes, these primary tastes would appear at the corners of a theoretical tetrahedron. Because other sensations would be combinations of these basic four, they should appear somewhere within the space of the tetrahedron. Multidimensional scaling experiments on flavor enhancers indicate that MSG, IMP and guanosine 5'-monophosphate (GMP) fall outside the region occupied by the four classic tastes, again indicating that their taste contribution is unique.

  To support the idea that umami substances have direct reception, neurophysical experiments have been performed on animals. In one such experiment, Schiffman sought to determine if a glutamate agonist could depress neural firing to glutamic acid in rats. Kainic acid is such a glutamate agonist and can bind to receptors with 50 times more potency than glutamic acid. By measuring neural response to glutamic acid both before and after treating the rat's tongue with Kainic acid, Schiffman found that the treatment did indeed depress neural firing, yet had no effect on other stimuli tested, such as salt or sucrose. This suggests that the tongue has receptor mechanisms specific to glutamic acid.

The direct connection

  So, if indications for a glutamate receptor exist, why not just check to see if there are receptors on the tongue and end the debate? A major problem with such research is the high level of technology required to perform it. The price tag often is richer than that of typical food research, so obtaining funding through food industry corporate grants is difficult. In addition, because issues of taste are not considered critical to human health, other grants and funding also are scarce. Recently, though, some progress on this situation has been made.

  In 1991, a group of researchers isolated and cloned receptors for glutamate synaptic transmission in the brain. (Glutamate is an important component of synaptic signal transmission.) While working on a project to support this research, University of Miami Medical School's Chaudhari and Roper began wondering if a similar receptor might be used to perceive dietary glutamate.

  "Our strategy was based on what was discovered in the brain," says Roper. "And we did find a brain-like receptor expressed in the taste buds. We don't know if it's alike, but it's very similar."

  With the receptor isolated, behavioral studies in rats were necessary to determine if this receptor did indeed function in taste.

  "Just because it is in the tissue, doesn't mean it's expressed in sublingual tissue and involved in taste," says Roper.

  Chaudhari and Roper designed a pharmacological ligand specific to the newly isolated receptor. Being designed in this way, the ligand will bind with the receptor in question and, if the receptor is involved in taste, generate an MSG-like taste response. The ligand was fed to rats, then their behavior was observed.

  "When you apply the drug to the rats, it tastes very similar to MSG," says Roper. "In fact, they react identically as if they received glutamate even though (the ligand) isn't glutamate; it is a pharmacological drug."

  Roper emphasizes that he and Chaudhari don't claim to have found the receptor nor to have the full explanation for glutamate taste. Currently, they are working to solidify their preliminary results with further tests funded by a four-year grant Chaudhari recently received from the National Institutes of Health.

  "We think we may have one of the players," says Roper. "If this holds up with further tests, it will be the first taste receptor actually found."

Beyond the limits of taste

  Such advances appear promising, but other scientists believe that more flavor-enhancing issues need to be explored. For example, some experiments have determined that flavor-enhancing ingredients actually contribute chemical feeling factors.

  In the early 1990s, researchers at what is now Integrated Ingredients, Bartlesville, OK, did a flavor continuum study that examined a cross-section of flavor ingredients to look at the intensities of basic flavor components -- taste, aromatics and feeling factors. The hypothesis was that flavors would be characterized by contributing aromatics or taste, and that enhancers would contribute chemical feeling factors.

  The researchers tested flavoring substances such as hydrolyzed vegetable proteins, beef extracts, yeast-based flavor ingredients, etc., and what have commonly been recognized as enhancers -- the nucleotides and MSG -- through descriptive analysis. When the results were charted, the researchers found that the substances with high aromatic contributions tended to group together and consisted of the flavoring substances. At the same time, the flavor-enhancing substances scored low in aromatics, yet formed a group of their own because of their high contribution of feeling factors.

  "So it was pretty easy to look at the group and divide them into flavors and enhancers," says Frannie Hildabrand, manager of technical services for Integrated Ingredients. "The flavors are characterized primarily by contributing aromatics, and enhancers are characterized by contributing primarily chemical feeling factors."

  Other recent testing has confirmed these observations.

  "We'll take a basic beef broth and add MSG and have the panel focus on the changes," says Kathleen Rutledge, president, 21st Sensory Inc., Bartlesville, OK. "To them the broth has a more substantial feel -- more weight and thickness in the mouth."

  Further support for chemical feeling factor contributions by flavor enhancers is found in the fact that the sensation is not limited to the tongue.

  "You can actually feel it on your lips and on your cheek as opposed to just on your tongue," says Gail Vance Civille, president, Sensory Spectrum. "To me, this makes it more like heat, astringency or metallic. You also get that sensation from umami ingredients."

  Other enhancing ingredients also may be contributing to flavor in ways other than taste. Initially investigated as a sweetness potentiator, maltols have not received anywhere near the scrutiny of other flavor-enhancing ingredients. Consequently, little is known about how they work. Cultor Food Science researchers are now taking the time to make more fundamental studies. Using descriptive analysis, researchers there have been looking at specific flavor profile changes the maltols will make in a system.

  What has been learned is that maltols perform more than the narrow role often assigned to them, which has usually been limited to enhancing fruit and berry flavors. The ingredients appear to have potential applications in dairy and savory products. A particular surprise in this research is the effect maltols have on aromatics.

  "They really seem to either potentiate or modify the impact of the aromatics," says Stagnitti. "Sometimes the amount of fingering flavor is associated with some of the aromatic character."

  To other researchers, however, that maltols affect aromatics comes as no surprise because they believe maltols don't actually enhance taste at all. Instead, they believe that the aromatics maltols contribute are responsible for observed enhancement effects. To show that aromatics are responsible for reported flavor profile changes, test subjects were administered samples while their noses were pinched. This eliminated any enhancing effects. Furthermore, the enhancing effect was minimized when test subjects were trained to distinguish odor experiences from taste. (Bingjan, A.F.; Birch, G.G.; de Graaf, C.; Behan, J.M.; and Perring, K.D., "Sensory Studies with Sucrose Maltol Mixtures," Chemical Senses, 15, 447-456,1990.).

  Another point that supports the aromatic nature of maltols is their low use level -- in the parts-per-million range. This is far below the threshold levels for other flavor-enhancing ingredients. Olfactory thresholds, however, often are lower than taste thresholds are.

The outer limits of taste

  So now, it seems, flavor enhancers do it all. They contribute taste, aroma and feeling factors, and they potentiate and suppress other flavor qualities. Does this mean that enhancers are indeed the miracle ingredients they were touted to be in the 1950s, or does it simply mean that our grasp of the concept of taste and flavor is woefully inadequate to explain it? This is a question that U.C.-Davis' O'Mahony seeks to answer.

  "The concept of basic taste is kind of daft," says O'Mahony. "When people talk about the four basic tastes, the simple question is, 'What do you mean by basic?'... No one really knows how many flavor receptor types there are or how many brain areas there are dealing with taste. We don't know how many ways the taste receptors interact and send messages to the brain. It's odd to say 'four basic tastes' when we really don't know."

  O'Mahony further eschews the idea that all tastes can be described as combinations of the four basic tastes. "If you taste things like potassium chloride, sodium benzoate or sodium bicarbonate, they all have very different tastes and, perhaps, the best way to describe them is 'potassium chloride taste,' 'sodium benzoate taste,' and 'sodium bicarbonate taste'," he says. "We haven't invented many words to describe taste because we don't talk about it much. I think we use sweet, sour, salty and bitter because those are important to our culinary habits. It's like the Eskimos have many words to describe snow because it's more important for them to do so."

  O'Mahony believes that the limited language of the basic taste concept is the root of the confusion about flavor and flavor enhancement. The solution he offers, however, may strike some as extreme.

  "I'd like to say enhancement exists, but the answer isn't one way or another," he says. "There is a lot said, but not a lot of experimentation to really find out. People are making a lot of assumptions and it's best to ignore them and start anew."

  No one said figuring out flavor enhancers would be easy. At the same time, no one expected that the obstacles would include the fact that the whole concept of taste perception is a moving target. It's also frustrating that most research into enhancer function only generates more questions than it answers. Still, research -- using existing concepts or creating new ones -- is the only way to get answers. In the meantime, food product designers will just have to wait.

What's in a Name

  The subject of flavor enhancement generates more than its share of confusion. Part of the problem is a language barrier. Some researchers theorize that the whole concept of taste potentiation is based on misinterpreting the meaning of the word "enhance."

  "There are two main meanings of the word," says Michael O'Mahony, professor of food science and technology, University of California, Davis. "I can enhance something by making it more pleasant. I can enhance a painting by putting a frame around it. I can enhance a room by decorating it. I can enhance a food by changing it so people like it more.

  "The other definition of enhance is to intensify," he continues. "You can enhance a radio signal by making it stronger. We can enhance a flavor by making it stronger. These are very different meanings and they get confused."

  Others agree that this confusion not only occurs, but is the root of much controversy about the role flavor enhancers play. Sarah Kemp, Ph.D., of Givaudan-Roure Corp., Clifton, NJ, and Gary Beauchamp, Ph.D., director of the Monell Chemical Senses Center, Philadelphia, suggest the following limited definitions to avoid confusion: