Study On γ-polyglutamic Acid Production By Microbial Fermentation

y-polyglutamic acid (y-PGA) is a water-soluble polymer of the amino acid glutamic acid (GA) with the peptide bonds between the amino group of GA and the carboxyl group at the end of the GA side chain. As it is edible, biodegradable, and non-toxic to humans and the environment, y-PGA is a potential anionic polymer. The biological polymer materials y-PGA can be used as thickener, gel, filmogen, humectant, retarder, cosolvent, binder and antifreezer. Therefore, it has extensive prospects of application in medicine, food, cosmetics, environmental protection, agriculture and other fields. y-PGA can be produced by chemical synthesis, extraction, enzyme conversion, or microbial fermentation. Among these methods, microbial fermentation, a simple, short cycle and high yield method with appropriate molecular weight distributing production, has attracted great interest all over the world. However, there are many problems in the production process of fermentation. For example, overhigh viscosity affects the mass transfer velocity and dissolved oxygen, which limits the yield of y-PGA.From bean products, a strain of high-yielding y-PGA bacteria was obtained and identified. Then, its fermentation conditions was explored. After optimization, the conditions were as follows:60 g/L sucrose,20 g/L yeast power,80 g/L sodium glutamate,2 g/L K2HPO4·3H2O,0.5 g/L MgSO4,5 g/L NaCl, pH 7.5,30 ml medium in a 250 ml flask, and inoculation quantity 5%(V/V). After 36 h's fermentation, the yield of y-PGA was about 31 g/L. However, in the later stage of the fermentation process, viscosity of the broth became higher and higher, which limited the mass transfer velocity and dissolved oxygen, and bottlenecked the yield.To increase the dissolved oxygen of the high viscosity broth, the vitreoscilla hemoglobin gene (vgb) was transferred to the genome of the bacteria. Then, the method of RSM was used to optimize the condition of y-PGA fermentation of the recombinant strain. After optimization, the the conditions were as follows:76.92 g/L sodium glutamate,27.20 g/L yeast power,0.26 g/L MgSO4,50 g/L sucrose,2 g/L, K2HPO4·3H2O,5 g/L NaCl, inoculation quantity 5%(V/V), pH 7, and 50 mL medium in a 250 ml flask. After 72 h's fermentation, the yield of y-PGA was about 60 g/L.On the basis of the submerged culture condition,10 L fermentor was used to numerous fermentation. The final conditions were as follows:the liquid volume 8 L, the aeration rate 1.5 vvm, the agitation speed 400 r/min, and the temperature 37℃. After 72 h's fermentation, the yield of y-PGA was about 65.7 g/L.