Monosodium glutamate (MSG)

 

Monosodium glutamate (MSG), a white crystalline substance is an important ingredient in the cuisines of China and Japan. MSG was first identified as a flavour enhancer in 1908 by Japanese chemist Ikeda Kikunae, who isolated it from seaweed kelp. In 1909 Saburosuke Suzuki, an entrepreneur, and Ikeda began the industrial production of monosodium L-glutamate (MSG) by extraction method in which vegetable proteins were treated with hydrochloric acid to disrupt peptide bonds.

Glutamate is a common amino acid that occurs naturally in a large range of foods including tomatoes, parmesan cheese, dried mushrooms, soy sauce, a host of fruits and vegetables, and human breast milk.  Monosodium glutamate (MSG)/monosodium L-glutamate or sodium glutamate, is the sodium salt of the amino acid glutamic acid.

MSG elicits a unique taste, known as umami, that is different from the other basic tastes (bitter, salty, sour, sweet). It enhances the complex flavours of meat, poultry, seafood, and vegetables. 

MSG-based condiment, Ajinomoto (‘essence of taste’) is now universally popular. MSG comes in many processed foods and snacks.  MSG does not occur naturally in whole foods. Manufacturers are required to state if MSG is included in products on their food content label. It might fall under different titles, including monopotassium glutamate and vegetable protein extract etc.

MSG ingested in large amounts may produce such physical reactions as burning sensations, facial tightness or pressure, and a tingling sensation in some individuals. These hypersensitive reactions, are known as MSG symptom complex—or, more informally, “Chinese restaurant syndrome”. Studies have shown no conclusive link between the syndrome and the consumption of normal levels of MSG, however.

Amino acids are usually produced by protein hydrolysis, chemical synthesis or microbiological (biotechnological) methods. The chemical synthesis of MSG can be carried out on a very large scale but it typically gives a racemic mixture of Land D-forms of amino acids. Extraction of amino acids from protein hydrolysate as a method of obtaining L-amino acids is not suitable for large-scale production of amino acids. Biotechnology methods (fermentation and enzymatic methods) have economic and ecological advantages. The enzymatic method has an advantage of producing optically pure amino acids in higher concentrations with less byproducts. In the industrial production of L-amino acids, the enzymatic method is not as popular as the fermentation method.

FERMENTATION METHOD

Fermentative production of amino acids was first reported by Kinoshita in 1957 where Corynebacterium glutamicum produced considerable amounts of L-glutamine from sugar and ammonia. It is now produced using a bacterial fermentation process with starch or molasses as carbon sources and ammonium salts as nitrogen sources.

The fermentation method is a production process in which a specific amino acid is synthesized in large amounts by a specially selected microorganism in culture. The selected microorganism is cultured with carbohydrates and ammonia and releases the L-form of the amino acid into the culture medium. The cell produces glutamate from 2-oxo-glutarate (2-oxo-pentanedioic acid) by reductive ammonia fixation that uses the enzyme glutamate dehydrogenase, a normal cellular constituent. Purification is by crystallisation and recrystallization of L-glutamic acid crystals  and subsequent conversion to MSG. The mother liquor of the crystallization process is then concentrated and used as a liquid fertilizer (after pH adjustment with ammonia).

The L-glutamate-producing bacterium was reported in 1957. Since then, many bacteria useful in glutamate production have been isolated, including Corynebacterium glutamicum, Brevibacterium lactofermentum, & Brevibacterium flavum. These glutamate-producing bacteria are all Coryneform bacteria, which are gram positive, nonspore-forming, and nonmotile and require biotin for growth.

Glutamate accumulation in the medium occurs only under biotin-limiting conditions. The requirement for biotin limitation prevented the use of standard raw materials such as sugar molasses because they contained biotin. The addition of a surfactant or of penicillin or the use of microorganisms auxotrophic for glycerol or oleate allowed the bacteria to produce large amounts of glutamate without biotin limitation.

The mechanism of excessive glutamate production by such microorganisms is still not understood. Under conditions in which glutamate accumulates in the medium there exists an active transport mechanism in glutamate producing cells that exports the amino acid into the medium. Identification of a glutamate export protein and its gene, yggB supports this mechanism.

 

                   

 

The fermentation method is applied for the industrial production of most L-amino acids, except for a few kinds of amino acids for which high production yields have not been achieved by fermentation. The economy of this method depends mainly on the cost of the carbon source, fermentation yield, purification yield, and productivity in the overall process.

Some problems in the fermentation processes can occur such as contamination of the culture with other microorganisms, bad fermentation reproducibility due to differences in the raw materials, back mutation or loss of genetic material in the production strain, infection of the culture etc.