In the process of biological reaction production, the DO value is generally used as the parameter of its dissolved oxygen, and the DO value is one of the most critical parameters in the entire fermentation process. It can not only directly reflect the dynamic changes of the entire fermentation process, but also indirectly reduce the production time and energy consumption of fermentation by increasing the DO value, and improve the yield and quality of the final product.

Dissolved oxygen is one of the most important parameters for the control of aerobic fermentation. Because the solubility of oxygen in water is very small, and the solubility in fermentation broth is even smaller, therefore, it is necessary to constantly adjust ventilation and stirring to meet the oxygen demand of different fermentation processes. The size of dissolved oxygen will have different effects on the growth of bacteria and the formation and yield of products.

Here, we will introduce the changes in dissolved oxygen concentration in different periods, the reasons for the abnormal drop/rise in dissolved oxygen, and the aspects of oxygen supply and consumption.

Changes in dissolved oxygen concentration

During the fermentation process, under certain fermentation conditions, the change of dissolved oxygen concentration of each product fermentation has its own rules. Generally, the change of dissolved oxygen concentration is divided into three stages. In the logarithmic growth phase, the DO value drops significantly, and the growth of the bacteria can be roughly estimated from the rate of decline; at the end of the logarithmic growth phase, there will be a trough in dissolved oxygen; in the late fermentation period, due to the aging of the bacteria, the respiration intensity weakens, The dissolved oxygen concentration will also gradually increase. Once the bacteria are autolyzed, the dissolved oxygen concentration will rise more obviously.

Causes of abnormal decrease/increase in dissolved oxygen

During the fermentation process, sometimes there will be abnormal changes in which the dissolved oxygen concentration decreases or increases significantly. There are many reasons, but they are essentially caused by the imbalance of oxygen supply and demand caused by changes in oxygen consumption or oxygen supply. The abnormal drop in dissolved oxygen during fermentation may have the following reasons:

① Pollution of aerobic bacteria, a large amount of dissolved oxygen is consumed, so that the dissolved oxygen drops to near zero in a short period of time

②An abnormal phenomenon occurs in the metabolism of bacteria, and the demand for oxygen increases, so that the dissolved oxygen decreases

③ Faults or changes in equipment or process controls that affect oxygen supply, causing a drop in dissolved oxygen. If the stirring power consumption becomes smaller or the stirring speed becomes slower, it will affect the oxygen supply capacity and reduce the dissolved oxygen.

The main reason for the abnormal increase of dissolved oxygen is the significant reduction of oxygen consumption. For example, the pollution of strong bacteriophage will inhibit the respiration of the production bacteria, and the dissolved oxygen will rise. When the bacteria burst, they will completely lose their ability to breathe, and the dissolved oxygen will rise in a straight line.

Any change in the DO value in the fermentation broth is the result of an imbalance between the supply and demand of oxygen. When the oxygen supply is greater than the oxygen consumption during the fermentation process, the dissolved oxygen concentration will increase; otherwise, it will decrease. Therefore, to control the dissolved oxygen concentration in the fermentation broth, it is necessary to start from the two aspects of oxygen supply and oxygen consumption.

Oxygen supply

 
According to the oxygen transfer rate equation OTR=KLα(c*-cL), any factor that can increase KLα and c* can improve the oxygen supply for fermentation.

Therefore, it is mainly to try to improve the driving force of oxygen transfer and the volumetric oxygen transfer coefficient KLα. The oxygen saturation concentration c* in the fermentation broth is mainly affected by temperature, tank pressure and properties of the fermentation broth. And these parameters are difficult to change under the optimized process conditions. Therefore, in actual production, the oxygen supply capacity of the equipment is usually improved by increasing the volumetric oxygen transfer coefficient KLα of oxygen.

In addition to increasing the ventilation, it is generally to improve the stirring conditions. Increase the KLα value by increasing the stirring speed or ventilation flow rate, reducing the viscosity of the fermentation broth, etc., thereby increasing the oxygen supply capacity. Changing the stirrer diameter or rotational speed can increase the power output and thus the alpha value.

In addition, changing the number and position of the baffles can change the flow state of the fermentation broth during stirring, which can also increase the α value. In recent years, the method of increasing the oxygen transfer coefficient for biological applications by adding an oxygen transfer intermediate has attracted extensive attention. Oxygen transfer intermediates include hemoglobin, paraffin, etc.

Oxygen consumption

The oxygen consumption in the fermentation process is affected by factors such as cell concentration, type and concentration of nutrient substrate, and culture conditions, among which the effect of cell concentration is the most obvious.

The specific growth rate of bacteria is controlled by the concentration of nutrient substrate, so that it can be fermented at a level slightly higher than the critical oxygen concentration to achieve the optimum bacterial concentration. This is an important method to control the optimum dissolved oxygen concentration. For example, penicillin fermentation is to control the concentration of bacteria by controlling the rate of adding glucose, thereby controlling the concentration of dissolved oxygen.

At present, the utilization rate of oxygen in the fermentation industry is still very low, only 40% to 60%, and the antibiotic fermentation industry is even lower, only 2% to 8%. The growth and metabolic activities of aerobic microorganisms need to consume oxygen, and they can only complete biological oxidation in the presence of oxygen molecules. Therefore, oxygen supply is essential for aerobic microorganisms.