An Overview of Monosodium L-Glutamate: Its Production, Significance in Taste Enhancement, and the Validity of MSG Health Concerns

Abstract
Monosodium L-glutamate (MSG), a food additive present in many processed foods, fast foods, and perhaps most famously Chinese restaurants, has a reputation that precedes itself. First isolated in 1908 by the Japanese chemist Kikunae Ikeda, MSG was discovered as part of an effort to find a cheap way to mass produce nutrition. Since then, it has become an important commercial product that has been at the center of much controversy. While MSG has been venerated for its unique ability to enhance taste, it has also been vilified as a serious safety hazard, accused of having physiological effects ranging from a series of symptoms known as "Chinese Restaurant Syndrome" to those as serious as neurotoxicity. In this paper, we will explore the background of MSG; in particular, how it is produced and why it is so powerful in affecting taste. Furthermore, we will explore several physiological effects of MSG to discover the validity of the health concerns leveled against it and whether it truly deserves the poor reputation it has gained over the past fifty years.
Background of MSG

Structure
Monosodium glutamate (MSG) is the sodium salt of the amino acid glutamic acid, which occurs in nature and is directly encoded for in the universal genetic code. As a salt, it occurs as an odorless white crystalline powder that is soluble in polar compounds. It is stable under ordinary conditions and has a melting point reported as 232 degrees Celsius (1). MSG exists as a pair of enantiomers, but only monosodium-L-glutamate has the ability to enhance taste (2). It is important to note that glutamates and their flavor-enhancing precursors are present in fairly high concentrations in biological systems as building blocks to proteins. These compounds are naturally present in many foods, such as meat, fish, poultry, milk, and vegetables (3).

Discovery
The origin of MSG first began with the isolation of glutamic acid from gluten, a wheat protein, in 1886 (4). It was not until 1907, however, when the Japanese chemist Kikunae Ikeda set out to identify the substance that provided the unique flavor in sea kelp, commonly used in many Japanese broths, that it became an important industrial product. His intention was to find a way to cheaply mass-manufacture nutrition, similar to his German colleague Justus Von Leibig, who fed German armies with his beef extract. Kikunae claimed that while the only basic known tastes at the time were sweet, salty, bitter, and sour, there was a fifth taste, which he named umami, Japanese for "tasty" or "savory" (5). In 1908, Ikeda successfully identified, isolated, and purified monosodium L-glutamate and in 1909 had received a patent and began to look for ways to industrially produce the compound (5).

Methods of Production of MSG

Protein Hydrolysis
There are several methods to producing MSG. The oldest method, the one used by Ikeda, is known as vegetable protein hydrolysis (6). The idea behind this method is to use acid hydrolysis to break the peptide bonds in a vegetable protein to release its individual amino acids, isolate glutamic acid, and then convert glutamic acid into monosodium glutamate (4). This method involves three steps: extraction, isolation, and purification. For his source of glutamate, Ikeda chose to use the wheat protein gluten, because it had the highest content of L-glutamate. He first isolated the gluten from wheat by washing the starch from dough, then heated it with hydrochloric acid for 20 hours. A resulting black residue was then filtered out, and the resulting concentrate was allowed to crystallize for one month. As it was, the simple crystallization process was very effective for isolating L-glutamate because L-glutamate was the only amino acid salt present in the solution with a very low solubility in concentrated hydrochloric acid. Another advantage was that the salt crystal of glutamate was selective against other amino acids because of its unique structure: because L-glutamic acid molecules are arranged along the crystal's a-axis, linking the alpha-amino N-H-Cl and gamma-carboxyl O-H-Cl hydrogen bonds, it is more difficult for other amino acids to insert themselves into the growing crystal. Thus, this process of crystallization is also a process of partial purification. The isolation step involved additional filtrations, and an adjustment of the solution with sodium or potassium hydroxide to a pH of 3.2, the isoelectric point of L-glutamic acid. The significance of this is that there are two polymorphs of the L-glutamic acid crystals - a stable and platelike beta-form, and a metastable, granular alpha-form that forms better in the presence of other amino acids. The alpha-form of the glutamic acid was the dominant one formed at pH 3.2 because the crude solution still contained other amino acids, the fact that the alpha-form grew better meant that other amino acids were not contained in it, further improving purity. In the last purification process, sodium bicarbonate was added to the purified L-glutamic acid alpha-form crystals until the pH was neutral. After this solution was heated and then cooled, the resulting monosodium-L-glutamate (MSG) molecules would precipitate out. Other sources of MSG used in protein hydrolysis are beet sugar molasses and soybean proteins (6).

Chemical Synthesis Method
Several alternative methods were developed in the 1950s, most notably the chemical synthesis method and the fermentation method. The chemical synthesis method is particularly advantageous because it can be carried out on a large scale, but the most significant disadvantage is that it produces a racemic mixture of glutamic acid (4). This requires additional steps to resolve the mixture into the two enantiomers. A common starting material used in chemical synthesis was acrylonitrile because of its easy attainability, due to the launching of the polyacrylic fiber industry in Japan in the 1950s. In this process, the starting gas, a 2:1 mixture of hydrogen and carbon monoxide gas was combined with acrylonitrile to yield 4-oxobutylo-nitrile. Ammonium cyanide was then added to this to yield 2-amino-pentane-dinitrile, and this was hydrolized by sodium hydroxide to yield DL-disodium glutamate. In order to separate the D- and L-isomers, a method called the optical resolution method was created in which D- and L-glutamic acid seed crystals were separately planted in racemic mixtures. Since each seed crystal only allows for additional crystallization of its own optical isomer, each isomer can be grown and isolated separately. The L-glutamic acid crystals were then converted into MSG using existing methods, while the D-glutamic acid crystals were reheated to create another racemic mixture, which would then undergo another optical resolution process (4).

Fermentation Method
A third alternative for producing MSG is by using selectively chosen bacteria such as Corynebacterium glutamicum, Brevibacterium flavum, and Brevibacterium lactofermentum to convert carbohydrates into amino acids(4). Sources of carbon were usually cane or beet molasses, raw sugar, or hydrolysate. Ammonia provides a source of nitrogen, while oxygen is present in the air passed into the fermenting mixture. Glutamic acid is then isolated and then converted to MSG via partial neutralization. By optimizing the temperature, pH, time of fermentation, and the amount of medium constituents (glucose,urea, and biotin), yields as high as 37.1 kg/m^3 can be obtained (7). Due to production costs and environmental effects, most glutamate manufacturers have turned to the fermentation method (6).

The Effect of MSG on Taste

Today, MSG production by fermentation worldwide is estimated to be about 200 million tons per year. Although it is most commonly associated with East Asian cuisines, MSG is widespread in many Western foods as well. According to a government study performed in 2003, a typical Chinese meal contains between 10 and 1500 mg of MSG per 100 g, while Parmesan cheese contains 1200 mg, and packaged seasonings contain from 90-1200 mg (13). So why is MSG, a simple food flavoring, so vastly produced and how is it different than any other food additive? What, exactly, is so unique about the way it affects taste?

What is Umami?
It has previously been mentioned that the particular taste attributed to MSG, named umami, does not belong to any of the basic four taste categories many are familiar with. It is not sweet, salty, bitter, or sour, but belongs to a fifth taste dimension mostly referred to as "savory." It has also been described as "sweet saline," accompanied with tactile sensations that include "mouth satisfaction" and a "mouth-watering sensation" (3) A study reported in the journal Chemical Senses in 1996 examined whether MSG and another compound, polycose, really have a taste distinct from the four basic tastes (9). Taste was determined by human subjects using whole-mouth tests with their noses open or clamped to reduce smell input. The subjects were ask to classify the tastes they detected into one of the seven following categories: "salty," "sweet or sour," "bitter," "sulfurous," "soapy or metallic," "other," and "no taste." Since polycose had a prominent "other" taste quality with the nose-open group that was eliminated with the nose-clamped group, the distinctiveness of the taste of polycose was discredited and attributed to its smell. MSG, however, had a prominent "other" taste quality that did not differ in both the nose-open group and nose-clamped group. Since the taste of MSG could not be classified into any other the other categories and did not differ under the condition of smell, it was determined that MSG did indeed have a taste distinct from the four basic tastes. The study further suggested that since L-MSG activates taste much more than D-MSG, there may be a chiral receptor site for the unique umami taste (9).

A Comparison of MSG and Table Salt
Without having any distinct characteristics that makes it instantly recognizable, the recognized main function of MSG is to enhance other flavors. For example, Sake, which has a high glutamate content, is believed by the Japanese to compliment and enhance a meal (13). Most foods that contain MSG taste salty, which is an unsurprising fact because it contains 12.3% sodium. The detection level of MSG is 6.26x10^-4 mol/L, about equal to that of salty tastes, higher than the detection level of bitter and sour tastes and lower than that of sweet tastes. Since it has a similar effect to salt and an equal detection level, MSG is an appealing alternative for reducing sodium consumption (7). Since it only has a third of the sodium content that table salt has, yet can produce an equal perception of saltiness, using MSG can reduce sodium consumption by about 30-40% (3).

Synergism with GMP and IMP
It is significant to note that MSG is often used in the presence of two other food enhancers, GMP (guanosine 5'-phosphate) and IMP (inosine 5'-monophosphate). GMP and IMP have a unique synergistic relationship with MSG, meaning that the combination of the substances can have a greater effect than the sum of the effects of the individual substances. It has been reported that the presence of just 0.9 grams of a 50:50 mixture of GMP and IMP can reduce the content of MSG in a food from 100 grams to 17 grams without affecting the quality of its taste (3). A study that examined thirty-six women and twenty-four men and their preference for different foods when eaten with MSG and IMP found results consistent to this. The study found that when levels of IMP were raised, the subjects' detection threshold for was MSG lowered. In addition, the subjects' liking for the food increased with the presence of MSG and IMP, most significantly when it came to high-protein foods (10). There are several theories concerning how GMP and IMP have an effect on MSG, most suggesting combined allosteric interactions between these flavor enhancing molecules and receptor proteins. One theory proposes that the function of GMP is to uncover the binding site for L-glutamate so it becomes more available to bind to glutamate (8).

The Plasticity of MSG Perception
Despite research relating MSG levels to people's preference for a food, part of the mystery of MSG and the somewhat indistinct definition of umami is that different people may have different reactions to the food additive. In a study by Scinska-Bienkowska, it was found that people with different endogenous glutamate levels in their saliva also experience of taste of MSG differently (11). The study compared those with high glutamate saliva levels (HG) and low glutamate saliva levels (LG), reporting that while the intensity of MSG perceived did not change between the two groups, the LG group rated the higher MSG concentration as more unpleasant (11). Another study that reinforced this idea of MSG perception as being more plastic by analyzing human subjects to MSG in food for 10 days (12). Those who were exposed to MSG were able to identify the MSG taste significantly better than those who were not. In addition, subjects who were previously exposed but were no longer exposed to MSG for another ten days performed significantly worse than they did ten days before. This indicates a certain flexibility when it comes to MSG perception and also show that the ability to perceive MSG may be quickly reversed if MSG exposure is not continued (12).

Considering the Physiological Effects of MSG

MSG and Chinese Restaurant Syndrome

If MSG is so effective in taste enhancement and reduction of sodium consumption, what is the controversy? In American culture today, the most legendary effect attributed to MSG is what is known as the "Chinese Restaurant Syndrome" (CRS). The symptoms of Chinese Restaurant Syndrome are various and many, including: numbness in the back, arms, and neck, palpitations, flushing, tearing, dizziness, and facial pressure (13). The concern over MSG first began in 1968, when Dr. Robert Ho Man Kwok wrote a letter to the New England Journal of Medicine, citing several symptoms of weakness and numbness whenever he ate in a Chinese restaurant (16). He cited several culprits, which included cooking wine, high sodium content, and lastly, MSG. Many people responded to this report with similar stories and were most suspect of MSG, since it was the least familiar food additive of the three (16).

MSG became a more widespread concern when a study by Reif-Lehrer in 1977 at Harvard Medical School reported that perhaps 25% of the population experiences CRS (13). The study, however, was only a simple cross-section survey, and only suggested a correlation between eating Chinese food and having certain symptoms instead of a direct causality. Many of the studies done in the 1970s that claimed CRS as a result of MSG intake contained serious methodological flaws. In his review, Freedom deconstructs several of these studies: some studies used a small sample size (less than 20), others directly asked subjects "Do you think you get Chinese Restaurant Syndrome?", while in another study, fasting subjects who were given 200 g. of parmesan cheese on an empty stomach reported feeling weak - less likely a symptom of CRS than an effect of eating an abnormally large amount of cheese on an empty stomach (13). Despite their flaws, these studies have contributed to the stigmatism of MSG as a safety hazard in the public eye.

Although there have been several case reports linking CRS to MSG, there have been no further clinical studies that consistently show that MSG intake is directly related to these symptoms (15). In 1970, Morselli and Garattini found no statistical difference between the symptoms of subjects who had been administered 3 g. MSG and those who had been administered a placebo. in 1972, Kenney and Tidball administered tomato juice dosed with 5 g MSG and studied if subjects experienced the symptoms of tingling and warmth. Although some reported symptoms and others did not, results showed that there was no statistical difference between the plasma glutamate between the two groups (14). In 1979, a further study by Kenney studied those "reactors" to MSG. When the subjects were given up to 6 g of MSG in tomato juice, they reported symptoms of warmth, weakness, tightness and palpitation, although their temperature, blood pressure, and EKG were all being monitored yet showed no changes (14). An additional rigorous double-blind study by Tarasoff performed in 1993 showed that there was no linkage of MSG or CRS (15). Although it may be possible that an individual is hypersensitive to MSG, there is no clinical data to support to claim that MSG content leads to the Chinese Restaurant Syndrome.

MSG and Neurotoxicity

A more recent concern and perhaps a more serious potential effect of MSG intake is the effect it can have on neurons in the brain. Glutamic acid is a excitotoxin, which at high levels can cause damage to parts of the brain unprotected by the blood-brain barrier. Given this neurotoxicity, a host of chronic diseases could potentially transpire (20). Cooper reviews several studies that have examined the possibility that MSG intake can have certain detrimental effects on the nervous system, particularly on bodies that are still developing (17): A study that treated newborn rats with MSG in doses of either 1.25, 2.5, or 5 g/kg/day over a five day period found that rats that had received the higher dose had more behavioral problems and reduced motor activity. Another study by Johnston showed that newborn rats who were injected with MSG and other similar compounds such as aspartate and methylaspartate experienced convulsions similar to those experienced by felines injected with the same compounds. A third study by Olney showed that glutamic, aspartic, and cysteic acids contained in casein hydrolysates given to neonatal mice led to acute degeneration of neurons in the developing hypothalamus, with similar effects occurring whether the dosages were given orally or parenterally. However, Arthur found that weanling mice were able to metabolize large quantities of these amino acids with no significant changes to their enzymes, while newborn mice could not - responding to these amino acids with a two to three times increase in enzymatic activity in the brain and liver. A study by Wen. et al showed that large amounts of MSG have no detrimental effect on infant squirrel monkeys, weanling rats, and sucking mice when consumed with a normal diet. While there is little evidence to show that MSG taken orally can give rise to neurotoxic disorders, more research should be done on how parenteral nutrients often injected into newborn infants have an effect on the central nervous system (17).

There are several theories for the way MSG may damage the nervous system. Because glutamate is absorbed very quickly in the GI tract, ingestion of MSG could spike blood plasma levels very quickly, then causing damage to parts of the brain unprotected by the blood brain barrier, as previously mentioned. Bizzi analyzes the kinetics of MSG in relation to its neurotoxity by measuring glutamic acid plasma levels in laboratory animals after they are orally administered MSG (19). It was found that newborn mice and rats showed a great area under the curve (AUC) than their adult counterparts. Studies supporting this show that loss of cell viability is glutamate concentration- and time-dependent (20). In 2008, Kondoh searched for a pathway for MSG to affect the nervous system, and suggested that there were L-glutamate receptors in the gut and the oral cavity, which when stimulated would them stimulate the vagus nerve in the brain (18). It was discovered that rats who had ingested MSG had reduced weight gain, fat deposition, and plasma leptin levels, suggesting that umami substances are significant in digestion, absorption, and metabolism through activation of the brain.

The most recent report, published in 2010, studied the difference in metabotropic glutamate receptors (mGluR) in neuronal and glial cells. Results showed that when exposed to glutamate, total mGluR levels in neuronal cells were significantly reduced. However, in glial cells, these receptors would be regulated in two phases: they would decrease after 6 hours, and then increase after 24 or 48 hours of treatment. This ability to regulate glutamate exposure could potentially create a way for neurons to be less vulnerable to excitotoxicity (20).

Conclusion

MSG, despite all the controversy that surrounds it, continues to be a very important industrial product today. It is produced most efficiently using the fermentation method and is used widely around the world, in both Eastern and Western cuisines. As a food additive, it has a unique ability to enhance the flavor of other foods, particularly salty and savory foods, and by doing so, provides an alternative to sodium consumption. It has a unique umami taste which may be perceived differently from individual to individual. Although MSG is most commonly known for having side effects collectively known as the "Chinese Restaurant Syndrome," clinical studies show that there is no evidence that MSG is the cause of any of these symptoms. A more recent concern about MSG is its role in neurotoxicity, a concern brought about by reports of lab animals having increased behavioral problems and reduced motor activity when administered MSG. It appears that MSG has a greater effect on infant animals instead of their adult counterparts, and this continues to be an important topic of research, with the most recent reports proposing methods of regulating glutamate receptors as a way to reduce neuron damage.

References
(1) Source (accessed Dec 2, 2010)
(2) Source (accessed Dec 2, 2010)
(3) Loliger, J. Function and Importance of Glutamate for Savory Foods. The Journal of Nutrition. [Online] 2000, 130, S915 Source (accessed Nov 20, 2010)
(4) Kauffman, G.B. The Monosodium Glutamate Story: The Commercial Production of MSG and Other Amino Acids. Journal of American Education. [Online] 2004, 81, 347-355 Source (accessed Nov 25, 2010)
(5) Sand, J. A Short History of MSG: Good Science, Bad Science, and Taste Cultures. Gastronomica. [Online] 2005, 5, 38-49 DOI (accessed Nov 20, 2010)
(6) Sano, C. History of glutamate production. The American Journal of Clinical Nutrition. [Online] 2009, 90, 728S-732S DOI (accessed Nov 25, 2010)
(7) Sunitha, I; Subba Rao M.V; Ayyanna C. Optimization of medium constituents and fermentation conditions for the production of L-glutamic acid by the coimmobilized whole cells of Micrococcus glutamicus and Pseudomonas reptilivora. Bioprocess and Biosystems Engineering. [Online] 2004, 18, 353-359 DOI (accessed Dec 1, 2010)
(8) Cairoli, P; Pieraccini, S; Sironi, M; Morelli, C.F; Speranza, G; Manitto, P. Studies on Umami Taste. Synthesis of New Guanosine 5'-Phosphate Derivatives and Their Synergistic Effect with Monosodium Glutamate. Journal of Agricultural and Food Chemistry. [Online] 2008, 56, 1043 DOI (accessed Nov 20, 2010)
(9) Hettinger, T.P; Frank, M; Myers, W. Are the Tastes of Polycose and Monosodium Glutamate Unique? Chemical Senses. [Online] 1996, 21, 341-347 DOI (accessed Nov 28, 2010)
(10) Luscombe, M; Smeets, A; Westerterp-Plantenga M.S. Taste sensitivity for monosodium glutamate and an increased liking of dietary protein. The British Journal of Nutrition. [Online] 2008, 99, 904-909 DOI (accessed Nov 30, 2010)
(11) Scinska-Bienkowska, A; Wrobel, E; Turzynska, D; Bidzinski, A; Jezewska, E; Siensiewicz-Jarosz, H; Golembiowska, K; Kostowski, W; Kukwa, A; Plaznik, A; Bienkowski, P. Glutamate concentration in whole saliva and taste responses to monosodium glutamate in humans. Nutritional Neuroscience. [Online] 2006, 9, 25 DOI (accessed Nov 24, 2010)
(12) Kobayashi, C; Kennedy, L.M; Halpern, B.P. Experience-Induced Changes in Taste Identification of Monosodium Glutamate (MSG) Are Reversible. Chemical Senses. [Online] 2006, 31, 301-306 DOI (accessed Dec 2, 2010)
(13) Freeman, M. Reconsidering the Effects of Monosodium Glutamate: A Literature Review. Journal of the American Academy of Nurse Practioners. [Online] 2006, 18, 482-186. DOI (accessed Nov 20, 2010)
(14) Taliaferro, P.J. Monosodium glutamate and the Chinese Restaurant Syndrome: A Review of Food Additive Safety. Journal of Environmental Health. [Online] 1995, 57, 8 Source(accessed Dec 10, 2010)
(15) Tarasoff, L; Kelly, M.F. Monosodium L-Glutamate: A Double-Blind Study and Review. Food and Chemical Toxicology. [Online] 1993, 31, 1019-1035 DOI (accessed Dec 2, 2010)
(16) Kenney, R.A. The Chinese Restaurant Syndrome: An Anecdote Revisited. Food and Chemical Toxicology. [Online] 1986, 24, 351-354 DOI (accessed Dec 2, 2010)
(17) Cooper, P. The winding monosodium glutamate trail. Food and Cosmetics Toxicology. [Online] 1975, 13, 124-126 DOI (accessed Nov 20, 1010)
(18)Kondoh, T; Torii, K. Brain Activation by Umami Substances via Gustatory and Visceral Signaling Pathways, and Physiological Significance. Biological and Pharmaceutical Bulletin. [Online] 2008, 31, 1827-1832 DOI (accessed Nov 28, 2010)
(19) Bizzi, A; Veneroni, E; Salmona, M; Garattini, S. Kinetics of monosodium glutamate in relation to its neurotoxicity. Toxicology Letters. [Online] 1977, 1, 123-130 DOI (accessed Dec 2, 2010)
(20) Castillo, C.A; Leon, D.A; Ballesteros-Yanez, I; Iglesias, I; Martin, M; Albasanz, J.L. Glutamate Differently Modulates Metabotropic Glutamate Receptors in Neuronal and Glial Cells. Neurochemical Research. [Online] 2010, 35, 1050 DOI(accessed Dec 3, 2010)

Note: There seems to be a problem with the Drexel library server, so some of the DOIs which are correct (particularly the Wiley ones) don't seem to work.