the biosynthesis of glutamic acid includes cross-linking pathway (EMP), hexose phosphate pathway (HMP), tricarboxylic acid cycle (TCA), glyoxylic acid cycle, CO2 fixation reaction and so on. Its synthetic route is roughly as follows: glucose is glycolytic (EMP route) and hexose phosphate branch (HMP route) to generate pyruvate, which is then oxidized to acetyl-CoA, and then enters the tricarboxylic acid cycle to generate α-ketoglutaric acid. α-ketoglutaric acid is catalyzed by glutamate dehydrogenase and produced into glutamic acid in the presence of NH4+. With the progress of fermentation, the bacteria multiply rapidly, resulting in insufficient supply of dissolved oxygen, and the metabolites will be lactic acid or succinic acid, which will reduce the pH value.
The following factors should be paid attention to when producing glutamic acid by fermentation:
1. Oxygen supply concentration
Insufficient oxygen supply: A large amount of lactic acid is accumulated, which makes the pH value of fermentation broth decrease, which is not conducive to the production of glutamic acid. At the same time, glucose is converted into lactic acid, which reduces the yield of products.
2. Ammonium ion concentration
It can affect the pH value of fermentation broth and the formation of products, and it is the raw material for the synthesis of glutamic acid;
3. The phosphate concentration
should be appropriate. Excessive phosphate concentration will promote EMP pathway, break the balance between EMP and TCA, accumulate pyruvate, produce lactic acid and accumulate Val；.
4. The ratio of carbon to nitrogen in the fermentation broth
During the fermentation, the ratio of carbon to nitrogen should be controlled correctly. Generally, the ratio of carbon to nitrogen should be larger (lower nitrogen) in the growth period of bacteria, and smaller (higher nitrogen) in the acid-producing period. When the ratio of carbon source to nitrogen source is 3∶1, Corynebacterium glutamicum will synthesize a large amount of glutamic acid, but when the ratio of carbon source to nitrogen source is 4∶1, Corynebacterium glutamicum will only grow without synthesizing glutamic acid
5, and when biotin < P > is excessive, the cell membrane is dense, which makes it impossible to excrete glutamic acid, and the glutamic acid excreted by strain cells in the fermentation broth can only account for 12% of the total amino acids. When biotin is in a moderate amount, the metabolism of bacteria is out of balance, the permeability of cell membrane is enhanced, and glutamic acid in cells can be excreted in time, which is beneficial to the accumulation of glutamic acid. Therefore, it is controlled in different stages.
6. Fermentation temperature
The optimum temperature for cell growth should be 3 ~ 32℃ in the early stage of fermentation. In the middle and late stage of fermentation, the growth of bacteria basically stopped. In order to accumulate a large amount of glutamic acid, it is necessary to increase the fermentation temperature
7 and the pH of fermentation broth < P > pH, which will affect the growth of cells and the activity of glutamate dehydrogenase. Generally, it is maintained at 7.-7.5
8. Ventilation < P > The essence of ventilation is to provide oxygen and fully mix the bacteria and the culture medium. Glutamate-producing bacteria are facultative aerobic bacteria, which can grow under aerobic and anaerobic conditions, but their metabolites are different. During glutamic acid fermentation, ventilation must be moderate.
9. Foam
Glutamic acid fermentation is aerobic fermentation, and it is normal to produce foam due to ventilation and stirring, but too much foam will bring a series of problems: (1) When foam forms a bubble cap, the gas produced by metabolism cannot be discharged in time, which hinders the respiration of bacteria and affects their normal metabolism; (2) If there is too much foam, the fermentation broth will overflow, resulting in waste and pollution; (3) Too much foam is easy to rush to the top of the tank, causing bacteria contamination.