Extraction Method

From 1909, when MSG production started, until 1965, when the extraction process ended, wheat gluten and defatted soybeans were used as the main raw materials, containing as much as 15 to 25% L-glutamaic acid. Crude raw materials were hydrolyzed by heating with hydrochloric acid. After concentration under reduced pressure, the hy-drolyzate was cooled to crystallize L-glutamic acid hydrochloride. L-Glutamic acid can be easily separated from other amino acids in the form of its hydrochloride because of its low solubility in concentrated hydrochloric acid. The crude hydrochloride is dissolved in hot water and filtered to separate the humic substances formed in the hydroly-zation process. The pH was adjusted with caustic soda to 3.2, the isoelectric point of L-glutamic acid needed to crystallize L-glutamic acid crystals at a yield of more than 90%. The crude L-glutamic acid crystals are suspended in water and neutralized with sodium hydroxide. The solution is decolonized with activated carbon and crystallized MSG (monosodium L-glutamate monohydrate) under reduced pressure. Commercially preferable crystals are grown with a small amount of amino acids (31). The crystals are separated by centrifuge and then dried for packaging.

Beet, sugarcane molasses or Stefan's waste (which contained 2-pyrrolidone carboxylic acid) has been used as raw material for MSG production in the United States and Europe. The pyrrolidone carboxylic acid was hydrolyzed at 85 °C at pH 10.5 to 11.5 for about 2 h to avoid racemization (32).

Industrial Fermentation Method

For industrial production of MSG, molasses or starch hy-drolyzate is generally used as raw material. Sugar molasses usually contains excess amounts of biotin, so when used, it is necessary to repress the activity of biotin by adding penicillin or certain surface-active substances at an early stage of the production to retard microorganism propagation. Furthermore, sugar concentration in culture media at below about 10% is necessary so as not to retard propagation. Additional sugar was fed during fermentation to obtain a higher concentration of excreted L-glutamate. This batch-fed fermentation process made possible a concentration of more than 120 g/L of L-glutamic acid in media.

Microorganisms used for L-glutamate fermentation are usually preserved under lyophilization below —80 °C or, for a short period, by keeping the stock culture below 10 to 15 °C. To refresh the microorganisms, stocked in either form, they are inoculated on an agar medium composed of 1% yeast extract and polypeptone, 0.5% sodium chloride, and 2% agar, at an optimum temperature of microorganisms. The refreshed microorganisms are then cultivated in liquid medium, shaken vigorously, and transferred to a small fermenter to allow them to propagate to about several kiloliters for seed culture. Industrial-scale fermenters are pressure tight, stainless steel containers, built to hold up to several hundred kiloliters of cultivating medium. They are equipped with aeration and stirring devices, as well as other automatic controls. Fermentation takes from 30 to 45 h.

Gaseous ammonia or a solution of urea and ammonium salts are convenient nitrogen sources for fermentation, not only as the initial medium, but also to maintain the pH of the medium at 7 to 7.5 for microbial growth and product formation. The culture medium becomes acidic because of assimilation of ammonium ions and the formation of L-glutamate. Gaseous ammonia can be used advantageously to maintain neutral pH and avoid dilution of culture medium, resulting in the high accumulation of L-glutamate in the fermentation broth, because it does not contain OH — ions or water.

Microorganisms required several minerals, such as ferrous and potassium ions, which play important roles in L-glutamate fermentation. Other important conditions include regulating the aeration stirring. The biosynthesis of glutamate is performed under regulated aerobic conditions. When oxygen is not sufficiently dissolved, lactic acid and succinic acids accumulate and reduce the accumulation of glutamate. On the other hand, an excess of dissolved oxygen results in the formation of a-ketoglutaric acid. The optimal oxygen transfer rate was determined by measuring the rate ofconsumption ofsodium sulfite, which is considered to depend on the characteristics of microorganisms used (33). The pressure of dissolved oxygen was usually kept above 1 kPa (0.01 atm) by aeration and agitation in the fermenter. The sterilized air is fed through a filter. Carbon dioxide is formed during fermentation and causes the medium to become heavy. Mechanical and chemical defoaming systems are provided.

The optimum temperature for fermentation is dependant on the character of microorganisms used. Regulation of the temperature using heat exchangers installed inside of the fermenter is indispensable. Fermentation is an exothermic reaction, and the temperature critically affects not only the propagation of microorganisms, but also the formation of glutamate.

In large-scale production the formation of trehalose very often reduces the product yield of glutamate. Trehalose consists of two a-1,1-bond glucose molecules and is excreted by the bacteria as an osmoprotectant. A process has been developed and successfully industrialized in which trehalose formation is controlled and decreased by culturing the overproducing mutant in media with inverted sucrose from molasses (34).

When fermentation is complete and after sterilization of the broth, the glutamate-recovery process is performed.

Separation Process

The separation process consists mainly of the following two processes (35):

1. Recovery of L-glutamic acid crystals from the fermentation broth

2. The purification of L-monosodium glutamate monohydrate from crude L-glutamic acid crystals

The typical outline of a production flow sheet of l-MSG from the fermentation broth is shown in Figure 7.

Hydrochloric acid at 35%% is added to the fermentation broth with agitation to crystallize the L-glutamic acid crystals at pH 3.2, the isoelectric point of L-glutamic acid. The crude L-glutamic acid crystals, retaining some mother liquor when removed from the slurry, are dissolved in water with equimolar sodium hydroxide; then the solution is applied to the activated carbon column to obtain the decolonized l-MSG solution, on which most of the color-related and hydrophobic substances are adsorbed. This purified l-MSG solution is fed continuously to a vacuum crystal-lizer to crystallize l-MSG crystals. l-MSG crystals separated by centrifugation are dried, sieved, and finally packaged to deliver to the consumers.

L-Glutamate broth

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