Glutamic acid-producing bacteria include Corynebacterium glutamicum, Brevibacterium lactose-fermenting, Brevibacterium sporadicum, Brevibacterium flavum, Brevibacterium ammoniaphagogenes, etc. Commonly used strains in my country include Corynebacterium pekingensis, Corynebacterium purulentum, etc.
The biosynthetic pathway of glutamate is roughly as follows: glucose generates pyruvate through glycolysis (EMP pathway) and hexose phosphate branch (HMP pathway), and then oxidizes to acetyl coenzyme A (acetyl COA). Then it enters the tricarboxylic acid cycle to generate α-ketoglutaric acid. α-Ketoglutarate is catalyzed by glutamate dehydrogenase and in the presence of NH4 to generate glutamate. When there is a lack of biotin, the bacterial growth is very slow; when there is an excess of biotin, lactic acid fermentation occurs. Therefore, it is generally necessary to control biotin under sub-optimal conditions to obtain high yields of glutamic acid.
In glutamic acid fermentation, if the permeability of the cell membrane can be changed so that glutamic acid can be continuously discharged out of the cell, a large amount of glutamic acid will be produced. Research shows that the main factor affecting cell membrane permeability is the phospholipid content in the cell membrane. Therefore, the breeding of glutamate-producing bacteria often starts with controlling the synthesis of phospholipids or damaging the cell membrane, such as the breeding of biotin-deficient strains. Biotin is a coenzyme of acetyl-CoA required in the synthesis of unsaturated fatty acids. Biotin-deficient strains cannot synthesize biotin, thereby inhibiting the synthesis of unsaturated fatty acids. Unsaturated fatty acids are one of the components of phospholipids. Therefore, the synthesis of phospholipids is also reduced accordingly, which will lead to an incomplete cell membrane structure and increase the permeability of the cell membrane to glutamate.
During the fermentation process, the regulation and control of oxygen, temperature, pH and phosphate are as follows: ① Oxygen. Glutamic acid-producing bacteria are aerobic bacteria. Ventilation and stirring will not only affect the utilization rate of nitrogen and carbon sources by the bacteria, but also affect the fermentation cycle and the synthesis of glutamic acid. Especially in the later stages of fermentation, increasing aeration is beneficial to the synthesis of glutamic acid. ②Temperature. The optimal temperature for bacterial growth is 30 to 32°C. When the bacterial cells grow to a stable stage, appropriately increasing the temperature is beneficial to acid production. Therefore, in the later stage of fermentation, the temperature can be increased to 34-37°C. ③pH. The optimal pH for fermentation by glutamic acid-producing bacteria is 7.0 to 8.0. However, during the fermentation process, with the utilization of nutrients and the accumulation of metabolites, the pH of the culture solution will continue to change. For example, as the nitrogen source is utilized, ammonia is released, and the pH will rise; when sugar is utilized to generate organic acids, the pH will fall. ④Phosphate. It is necessary in the glutamic acid fermentation process, but the concentration cannot be too high, otherwise it will turn to valine fermentation. After fermentation, extraction is usually carried out using ion exchange resin method