Author: Yuan Kristy
Institution: Cell biology and molecular genetics
Date: September 2005
Take a good bite into that meaty chicken leg and savor it. What does it taste like (and yes, tasting like chicken is a given)? But is that tingling flavor in your mouth sweet, salty, sour, or bitter? If you haven't smothered your chicken in barbecue sauce or drenched it in marinade, that savory flavor may not be quite like any of the four traditional tastes. In fact, the main "deliciousness" energizing your taste buds is umami, the fifth taste.
Umami: A Necessary Flavor
Understandably, most people do not know much about umami; after all, most textbooks still do not list umami as a basic taste. Umami is the taste of "savory-ness" (after the Japanese word umai, meaning delicious) with an appetite-boosting effect. So far, no area of the tongue is known to be a specific target for tasting umami, explains Gary Beauchamp, director of the Monell Chemical Senses Center in Philadelphia. That agrees with the phenomenon that no taste is only sensed in one part of the tongue, although some are more strongly identified by the taste receptors in any particular area. The fifth element of taste derives from the palate's ability to detect a specific amino acid, glutamate, if it is unbound to other amino acids.
In 1907, chemistry professor Kikunae Ikeda of the Imperial University of Tokyo was curious to know what made his kombu (a type of seaweed) soup so tasty. He attributed the taste to isolated glutamic acid crystals, from which he created a food seasoning that wasn't not too sour, soluble in water, non-solidifying and unable to absorb humidity. This seasoning, monosodium glutamate (MSG), was born in 1909 and arrived in the United States in 1917.
MSG itself is not savory, but it brings out the delicious umami taste in foods containing glutamate. Some MSG-intolerant people and asthmatics may experience Chinese Restaurant Syndrome, which includes burning sensations, headaches, nausea, and tingling after eating Chinese food. Despite its implication in the syndrome, the glutamate in MSG is metabolized the same way as natural free glutamate molecules and is a "Generally Recognized As Safe" seasoning approved by the Food and Drug Administration (FDA) and the Joint Expert Committee on Food Additives of the United Nations Food and Agriculture Organization. The FDA asserts that "no scientific evidence [proves] that the levels of glutamate in hydrolyzed proteins causes adverse effects or that other manufactured glutamate has effects different from glutamate normally found in foods."
Today, restaurants that hail "no MSG" still use glutamate compounds as a taste enhancer. MSG and other similar compounds have been in popular use in non-American cultures, particularly Asian, in which savoring taste often trumps excess flavor and large food quantity. However, studies suggest that only minimal amounts of MSG should be used to achieve maximum pleasantness in taste (Figure 1).
Glutamate is the most abundant amino acid in nature, and its free form is found in meats, poultry, tofu, cheeses, tomatoes, and various fish sauces (Figure 2). In the average human body, 2 kg (4.4 lbs) of total glutamate and 10 g (0.35 oz) of free glutamate are present in the muscles, brain, blood, kidney, liver, and breast milk, playing important roles in nutrition, metabolism, and neurotransmission. Everyday, 10-20 g (0.35-0.70 oz) of bound glutamate and 1 g (0.04 oz) of natural free glutamate are consumed (Figure 3). Given the prominence of this amino acid, it is sensible that our tongues have a mechanism for detecting it. "Our ability to taste sweet reflects our need for carbohydrates," says Beauchamp. By analogy, the ability to taste umami may have an evolutionary role reflecting our need for glutamate and other amino acids, the building blocks of proteins. Although evolutionary function is difficult to prove, researchers are hard at work on finding receptors and the biochemical pathway of umami taste.
The All-important Taste Receptors
Research on umami did not begin in earnest until the last two decades and only within the last few years have investigators made major progress in identifying umami taste receptors. Receptors are crucial components of the biochemical pathways of taste perception. Their study gives insight into why certain tastes exist (relating to evolutionary development) and their effects on palatability and health from manipulation of chemical compounds. In the search for possible genes governing the ability to taste umami, receptor proteins are inevitable targets for detailed study.
In 2000, Nirupa Chaudhari and colleagues from the University of Miami found an l-glutamate taste receptor, which they named "taste-mGluR4." Since the existence of receptors reflect the detection of a taste, this finding virtually dispelled all doubts about umami existing as a fifth taste. Compared with mGluR4, the receptor for glutamate as a neurotransmitter, taste-mGluR4 is a shortened variant lacking 50% of its cousin's genetic code. As a result, taste-mGluR4 can only detect glutamate at 1,000 times the concentration detectable by plain mGluR4. Nevertheless the response level of taste-mGluR4 needs not be so sensitive when in contact with food, usually harboring more than trace amounts of glutamate.
In February 2002, Greg Nelson, along with Howard Hughes Medical Institute investigator Charles Zuker, published in Nature the discovery of a "broadly tuned" amino acid receptor. Nelson experimented with cell cultures containing different combinations of T1R proteins, which are also related to glutamate receptors in the brain. The scientists found that taste buds expressing combined T1R1 and T1R3 (known as T1R1+3) showed the highest nerve fiber activity when bombarded with amino acids. T1R1+3 was also highly stimulated by l-oriented amino acids (of which free glutamate is a member), which are naturally found as biosynthetic precursors to small molecules. Although tantalizing for speculation, it is still unknown whether the taste-mGluR4 receptor and the T1R1+3 receptors are involved in the same or similar biochemical pathways of taste perception. These receptors provide evidence for the evolutionary importance of detecting amino acids for nutritional survival. Although we have receptors for many amino acids, perhaps we can only taste glutamate because of its large quantity in nature and its popular presence in food additives.
Glutamate is not the only substance exuding umami; other food additives such as IMP (disodium 5'-inosine monophosphate) and GMP (disodium 5'-guanosine monophosphate) undergo a synergistic reaction with glutamate to embolden umami taste in foods with low concentration of the amino acid. However, says Beauchamp, "if I knew how the synergism worked, I'd be famous." Taste-mGluR4 does not detect the compounds, but T1R1+3 expression in receptor cells is greatly up-regulated when in contact with IMP, suggesting that complex pathways are involved with umami taste. In the meantime, IMP and GMP are being used along with MSG in taste experiments to identify new components of glutamate taste pathways.
Other recent scientific findings report common pathways between sweet and umami tastes. Sweet taste is known to involve T1R2+3 receptors, which differ slightly from the T1R1+3 amino acid receptor. Beauchamp speculates that some rats may even perceive umami as sweet, suggested by the rats' avoidance of sugar when given too much glutamate. That sweet and umami taste ability may share a positive evolutionary history is both a promising and novel idea to investigate. Zuker explains that finding common receptors "provides a powerful platform to help decode the interplay between the various taste modalities and the link between events at the periphery (taste receptor cells) and the central nervous system (perception and behavior)."
Data regarding umami taste receptors are on the rise, but much work is still needed to make connections between findings for a solid story on the science behind the fifth taste.
Inspirations from Umami
Umami has become a big hit worldwide as a palatability enhancer. Mature, well-fermented multi-dimensional wines are said to bring out more umami from a meal, allowing the tongue to enjoy many sensations from food. MSG and lower sodium containing IMP and GMP can be added to foods like vegetables to allow individuals whose sharpness of taste sensation has been dampened by age or drug use to continue healthy eating. Companies such as Senomyx, Inc., of La Jolla, Calif., have sprung up to exploit the use of glutamate to create new food flavoring releasing that umami mouth-watering taste.
Umami has even inspired the hunt for other taste enhancers. A group of German scientists reported this year in Chemical Senses the discovery of a new compound, alapyridaine, that augments salt, sweet, and umami flavors in food. Alapyridaine is isolated from beef stock, and like umami, is itself tasteless. While you can't add umami to chocolate to make it sweeter, you can add this substance to do the trick, providing support for sweet and umami sharing taste pathways. As a possible general taste enhancer, alapyridaine also relies on GMP synergism with other umami-flavored food to strengthen taste. Alapyridaine is definitely a substance that deserves special attention as research advances.
The ideas from umami research have sparked other taste studies, with the ultimate goal of uncovering taste receptors, defining signaling pathways, creating a possible topographical map of taste sensations, and understanding the neuronal processes that underlie taste perception. Science may be humbled by the idea that such hard work and enthusiasm are geared toward explaining so simple an element of life. But umami is important; you can't get enough of that subtle taste. So supply your tongue with that chicken and miso soup and really savor the glutamate.
International Glutamate Organization
"Isolating the fifth element of taste"
"The umami way of food and wine matching"
Charles Zuker, HHMI Investigator profile
"Monosodium Glutamate" from Chemical and Engineering News
FDA statement on Monosodium Glutamate
Chaudhari N et al. (2000) A metabotrophic glutamate receptor variant functions as a taste receptor. Nature Neuroscience. 3(2):113-119.
Nelson G et al. (2002) An amino acid taste receptor. Nature. 416(6877):199-202.
Li X et al. (2002) Human receptors for sweet and umami taste. PNAS. 99(7):4692-6.
Soldo T et al. (2003) (+)-(S)-alapyridaine A general taste enhancer? Chemical Senses. 28(5): 371-9.