◆Effect of dissolved oxygen on fermentation
Dissolved oxygen is one of the most important parameters for aerobic fermentation control. Since the solubility of oxygen in water is very small, so is the solubility in fermentation broth. Therefore, constant ventilation and stirring are required to meet the oxygen needs of different fermentation processes. The size of dissolved oxygen will have different effects on bacterial growth and product formation and yield. For example, glutamic acid fermentation, when oxygen supply is insufficient, glutamic acid accumulation will be significantly reduced, producing large amounts of lactic acid and succinic acid.
❶Dissolved oxygen has different effects on the growth of microorganisms in various fermentation processes.
Changing the aeration rate causes mycelium to multiply in the early stage of fermentation, the oxygen demand is greater than the oxygen supply, and a low peak of dissolved oxygen appears. During the growth stage and product synthesis stage, oxygen demand decreases and dissolved oxygen is stable, but it is greatly affected by conditions such as feeding and refueling. After supplementing sugar, the oxygen uptake rate will increase, causing the dissolved oxygen concentration to decrease. After a period of time, it will gradually rise again and approach the original dissolved oxygen concentration. If you continue to supplement sugar, it will continue to decline and even cause production to be restricted. In the later stage of fermentation, due to the aging of the bacterial cells, respiration weakens and the dissolved oxygen concentration increases. Once the bacterial cells autolyze, the dissolved oxygen concentration will increase significantly.
❷Effect of dissolved oxygen on fermentation products
For aerobic fermentation, dissolved oxygen is usually both a nutritional factor and an environmental factor. Especially for the production of metabolites with certain redox properties, changes in DO will inevitably affect the redox potential of the strain culture system, and will also affect cell growth and product formation.
In xanthan gum fermentation, although the dissolved oxygen concentration in the fermentation broth has little effect on the bacterial growth rate, it has a significant impact on the length of the bacterial stability period and product quality after the bacterial concentration reaches the maximum.
The activity of aerobic microbial enzymes is highly dependent on oxygen. In glutamic acid fermentation, the activity of lactate dehydrogenase (LDH) under high dissolved oxygen conditions is significantly lower than that under low dissolved oxygen conditions, and the enzyme activity of glutamate dehydrogenase (GDH) decreases in the middle and late stages of acid production. Soon, this may be due to the fact that under high dissolved oxygen conditions, vigorous ventilation and stirring aggravate the death rate of bacteria and the attenuation of fermentation activity.
The level of DO value will also change the microbial metabolic pathway, thereby changing the fermentation environment and even causing the target product to deviate. Studies have shown that the metabolic flux of L-isoleucine is closely related to the concentration of dissolved oxygen. The metabolic flux distribution during the fermentation process can be changed by controlling the dissolved oxygen at different periods, thereby changing the metabolic flux of the synthesis of amino acids such as Ile.
◆Dissolved oxygen control
The purpose of controlling dissolved oxygen is to stably control the dissolved oxygen concentration value within a certain expected value or range. In the process of microbial fermentation, the relationship between dissolved oxygen concentration and other process parameters is extremely complex, and is affected and restricted by a variety of physical, chemical and microbial factors in the bioreactor. It can also be seen from the oxygen transfer rate equation that the control of DO value mainly focuses on the two aspects of oxygen dissolution and transfer.
❶Controlling the amount of dissolved oxygen (C*-CL) is the driving force for oxygen dissolution. The primary factor in controlling the amount of dissolved oxygen is controlling the partial pressure of oxygen (C*). High-density culture often uses pure oxygen to increase the oxygen partial pressure, while anaerobic fermentation uses various methods to control the oxygen partial pressure at a lower level. For example, in the beer fermentation, wort oxygenation and yeast inoculation stages, the oxygen content is generally required to reach 8 to 10PPM; while in the beer fermentation stage, the oxygen content in general beer should not exceed 2PPM.
❷Controlling the Oxygen Transfer Rate The oxygen transfer rate mainly considers the influencing factors of KLa. In a certain sense, the larger the KLa, the better the mass transfer performance of the aerobic bioreactor. The way to control KLa can be divided into three parts: operating variables, physical and chemical properties of the reaction liquid, and the structure of the reactor. Operating variables include temperature, pressure, ventilation volume, rotation speed (stirring power), etc.; the physical and chemical properties of the fermentation broth include the viscosity, surface tension, oxygen solubility, composition of the fermentation broth, flow state of the fermentation broth, fermentation type, etc. ; The structure of the reactor refers to the type of reactor, the proportion of the size of each part of the reactor, the form of the air distributor, etc. Of course some factors are related.
❸Selection of appropriate dissolved oxygen
In aerobic microbial reactions, [DO]>[DO]cri is generally chosen to ensure the normal progress of the reaction. The critical oxygen concentration is the lowest oxygen concentration allowed without affecting bacterial respiration.
•Principles for Selection of Appropriate Dissolved Oxygen If the bacteria are to grow and reproduce rapidly (such as in the early stage of fermentation), the critical oxygen concentration should be reached; if the synthesis of the product is to be promoted, the dissolved oxygen should be controlled at the optimal concentration according to the purpose of production. (Different degrees of satisfaction) For example: Brevibacterium flavum can produce a variety of amino acids, but the required oxygen concentrations may be different, but for the production of phenylalanine, valine and leucine, below the critical oxygen concentration To obtain maximum production capacity, their optimal oxygen concentrations are 0.55, 0.66, and 0.85 of the critical oxygen concentration, respectively.
•In terms of oxygen supply
1) increase the oxygen content in the air to increase the oxygen partial pressure
2) perform oxygen-rich ventilation
3) increase the tank pressure
4) increase the stirring speed
•In terms of oxygen demand
1) adjust the concentration of nutrients
2) adjust and control the temperature: the dissolved oxygen concentration must cooperate with other parameters. In addition, the oxygen saturation will also be affected by temperature, tank pressure, and properties of the fermentation liquid. The aerobic demand of the fermentation process is affected by factors such as bacterial concentration, nutrient matrix concentration, and culture conditions. To maintain the optimal bacterial concentration, the optimal bacterial concentration can be controlled by the nutrient substrate concentration. It can also be controlled by controlling the feeding speed, adjusting the fermentation temperature, liquefying the culture medium, replenishing water in the middle, adding surfactants, etc.
Conclusion
The oxygen content in the fermentation broth has an important impact on bacterial growth and product formation. The control of dissolved oxygen is mainly considered from the two aspects of oxygen dissolution and transfer. With the development of computer and automation technology, the fermentation industry is gradually moving towards an integrated mode of automation and control from measurement of DO to analysis and control. Research on the use of DO as an online control signal for feeding materials will greatly improve the accuracy and accuracy of fermentation regulation. Automation performance.