Lysine (Figure 1) is an α-amino acid used in the biosynthesis of proteins. The compound contains an amino group and a carboxylic acid group. Under biological conditions, the amino group is protonated (–NH3+) and the carboxylic acid group is deprotonated (–COO–).
The α-carbon of lysine is a chiral center, so two enantiomers of the compound exist. Only L-lysine is biologically active. Lysine is an essential amino acid that must be supplied through the diet. It is commercially produced as L-lysine monohydrochloride (L-lysine·HCl) and L-lysine sulfate. L-lysine·HCl is a yellowish-white, crystalline powder, mainly used as a food and feed supplement. Other uses relate to cosmetics, human medicine, culture media and pharmaceuticals.
Lysine is produced from raw sugar (sucrose) using a conventional fermentation process in which raw sugar is hydrolyzed into glucose and fructose (invert sugars). The invert sugars are then fermented to produce L-lysine, which is recovered via ion-exchange adsorption.
Fermentation-based lysine production from raw sugar is similar to the Archer-Daniels-Midland process, and comprises three major sections: (1) fermentation; (2) product recovery; and (3) product concentration, drying and packaging (Figure 2).
Fermentation. The culture media used in fermentation is prepared by mixing process water, invert sugar and nutrients. The fermentation is performed in fed-batch mode and under aerobic conditions. To start the batch phase, the microorganism seed is fed into the fermenters, which were previously filled with the fermentation batch medium. After glucose exhaustion, the batch phase is finished and the fed-batch phase is started, during which, glucose and nutrients are continuously supplied until the desired L-lysine concentration is achieved. At the end of the fermentation, the broth is sent to a buffer tank to provide a continuous flow in the further process steps.
Product recovery. The fermentation broth is sent to an ultrafiltration system for the removal of cell debris and other suspended solids. Subsequently, the liquor from ultrafiltration is fed to ion-exchange columns, where L-lysine is selectively adsorbed. The adsorbed L-lysine is then eluted from the ion-exchange resins by washing with an aqueous ammonia solution.
Product concentration, drying and packaging. The L-lysine eluate from ion exchange columns is mixed with mother liquor from the product filtration step and concentrated by evaporation. The concentrated lysine solution is acidified with hydrochloric acid, and free L-lysine is converted to L-lysine·HCl. Subsequently, the L-lysine·HCl solution is sent to the crystallizer, and lysine salt is filtered. The mother liquor is recycled to the evaporator and the wet cake is conveyed to dryers. Final dry L-lysine·HCl (98.5 wt.%) is obtained and sent to packaging line before being stored in bags. L-lysine is hygroscopic, so it must be stored in tightly sealed containers tightly in dry, cool conditions.
Pathways and uses
The L-lysine·HCl process involving sugar fermentation is accomplished by modified microorganisms. The sugar used in the bio-based chemical route can be derived from sugarcane, beet or corn. L-lysine can also be produced from cyclohexene via a chemical pathway. However, only a few plants use this alternative (Figure 3).
Editor’s note: The content for this column is developed by Intratec Solutions LLC (Houston; www.intratec.us) and edited by Chemical Engineering. The analyses and models presented are based on publicly available and non-confidential information. The content represents the opinions of Intratec only.
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