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The production of modern monosodium glutamate (MSG), whose main component is L-glutamic acid sodium, is dominated by microbial fermentation (accounting for over 90% globally). This process, which has been optimized over decades, offers the advantages of high efficiency, low cost, and high purity. The entire process can be broken down into four core stages, each with clear technical goals and operational details:

1. Raw material pretreatment: Converting "starch" into "sugar" that microorganisms can consume

Microorganisms, such as Corynebacterium glutamicum, cannot directly utilize starch and need to convert it into monosaccharides (glucose), which is the fundamental step of fermentation. The main raw material is corn starch (widely available and low-cost; rice, sugarcane, etc. can also be used):

Slurry preparation and liquefaction: Mix corn starch with water in a ratio of approximately 1:3 to form a "starch slurry" with a concentration of 30%-35%. Add α-amylase (a biological enzyme preparation) and stir at 80-100°C for 30-60 minutes to break down the long starch chains into short-chain polysaccharides (dextrin), obtaining a "liquefied liquid" (at this point, the liquid becomes clear from turbid);

Saccharification: Cool the liquefied liquid to 55-60°C and add saccharifying enzymes, continuing the reaction for 12-24 hours to completely decompose dextrin into glucose (monosaccharide), forming a "saccharified liquid";

Refinement: Remove undigested starch residues through plate and frame filtration, then use activated carbon to adsorb pigments and ion exchange resin to remove impurity ions (such as calcium and magnesium ions), ultimately obtaining "refined glucose liquid" (with a glucose concentration of 15%-20%, high purity, and no interference from impurities).

2. Strain cultivation and main fermentation: Let microorganisms "produce" glutamic acid

This is the core stage of MSG production - using selected and cultivated "high-yield Corynebacterium glutamicum" (a safe strain with no pathogenicity) to metabolize glucose into glutamic acid, with precise control of environmental conditions:

Strain activation and expansion culture:

First, take frozen Corynebacterium glutamicum from the strain library and inoculate it onto a "slant medium" (containing glucose, peptone, etc. for nutrition) and cultivate it in a constant temperature incubator at 30-32°C for 24-48 hours to restore the strain's activity;

Then transfer the activated strain to a "seed tank" (a small fermentation tank), introduce sterile air, maintain 30-32°C, and cultivate for 8-12 hours to increase the bacterial count to 10⁸-10⁹ cells per milliliter, forming a "seed liquid" (ensuring sufficient "production bacteria" for subsequent fermentation);

Main fermentation:

Transfer the seed liquid into a large fermentation tank (with a capacity of tens of cubic meters), and add refined glucose liquid, urea (for nitrogen source), phosphate (to regulate metabolism), and other nutrient solutions to the tank;

Control key parameters throughout the process: temperature 30-34°C (the optimal growth temperature for bacteria), pH 6.5-7.0 (regulated with urea to ensure glutamic acid synthesis), and continuously introduce sterile air (to avoid oxygen deficiency leading to metabolic disorders and no production of glutamic acid);

After 30-40 hours of fermentation, the glutamic acid concentration in the fermentation liquid can reach 100-150g/L. At this point, terminate the fermentation to obtain the "fermentation liquid" containing glutamic acid.

3. Purification and neutralization: From glutamic acid to monosodium glutamate

The fermentation liquid contains not only glutamic acid but also bacterial cells, residual sugar, and impurity ions, which need to be purified through multiple steps and then reacted with alkali to generate monosodium glutamate (the effective component of MSG):

Pre-treatment of fermentation liquid: Heat the fermentation liquid to 80-90°C to denature and solidify the bacterial cells, then remove the cell residues through centrifugation or plate and frame filtration to obtain "glutamic acid clear liquid"; Glutamic acid extraction (isoelectric point crystallization): Add sulfuric acid (or hydrochloric acid) to the clear liquid to adjust the pH value to 3.22 (the isoelectric point of glutamic acid, at which the solubility of glutamic acid is the lowest), then cool it to 5-10℃. Glutamic acid will precipitate as white crystals, which can be separated by centrifugation to obtain "crude glutamic acid crystals". Dissolve the crude crystals in hot water and repeat the isoelectric point crystallization 1-2 times to obtain "high-purity glutamic acid" with a purity of ≥98%.

Neutralization reaction: Dissolve the high-purity glutamic acid in hot water and slowly add sodium hydroxide (or sodium carbonate) solution at 60-70℃ to adjust the pH value to 6.5-7.0 (neutral), and the reaction occurs: glutamic acid + sodium hydroxide → sodium glutamate + water. After the reaction, "sodium glutamate solution" is obtained, which is then decolorized with activated carbon and desalted with ion exchange resin to further remove trace impurities.

4.Concentration, crystallization and drying: Production of finished monosodium glutamate

Finally, the sodium glutamate solution is converted into solid crystals through physical means and processed into commercial form:

Vacuum concentration: The refined sodium glutamate solution is sent to a vacuum concentration tank and heated at 60-70℃ to evaporate water (to avoid high temperature destroying the components), until the solution concentration reaches 30-35°Bé (Baumé degree, an indicator of concentration).

Crystallization: Transfer the concentrated solution to a crystallization tank and slowly cool it to 20-30℃. Sodium glutamate will gradually precipitate as regular crystals, which can be separated by centrifugation to obtain "wet monosodium glutamate crystals" (with a moisture content of about 10%).

Drying and packaging: Send the wet crystals to a fluidized bed dryer and dry them at 60-80℃ until the moisture content is ≤0.5% to obtain "dry monosodium glutamate crystals". Then, control the particle size through sieving (such as common 8-12 mesh, 20-40 mesh), and finally perform aseptic packaging to produce bagged or bottled monosodium glutamate available on the market.

Supplement: Traditional process that has been phased out

Before the popularization of microbial fermentation (1909-1960s), monosodium glutamate was mainly produced by hydrolyzing plant proteins: hydrolyzing wheat gluten and soy protein with concentrated hydrochloric acid to extract glutamic acid. However, this process was costly (plant proteins were expensive), highly polluting (difficult to treat the waste acid liquid), and had low purity (only 80%-90%). Therefore, it was gradually replaced by the fermentation method after the 1960s.