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How to control the PH value in the fermentation process?

Data: 2020-09-14


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How to control the PH value in the fermentation process?

Because microorganisms continuously absorb and assimilate nutrients and discharge metabolites, the pH value of the fermentation broth is always changing during the fermentation process. 

In order to enable microorganisms to grow, reproduce, and synthesize target metabolites within the optimum pH range, the pH value of the fermentation process must be strictly controlled.

In microbial growth and product production, there are four situations in which the relationship among the optimum pH value and the specific growth rate (p) and the product specific production rate (Qp).

The first case is that the optimum pH of the specific growth rate (p) of the bacterial cell and the specific production rate of the product (Qp) are in a similar and wide range, and this fermentation process is easy to control; the second case is the optimum pH of p The range is very wide, and the optimal pH range of Qp is narrow. 

The third case is that p and Qp are both sensitive to changes in pH, and their optimal pH is the same; the second and third modes of fermentation pH The value should be strictly controlled; the fourth situation is more complicated, p and Qp have their own optimal pH values, at this time, the respective optimal pH should be strictly controlled to optimize the fermentation process.

After determining the optimum pH required for each stage of fermentation, various methods need to be used to control the fermentation process within the predetermined pH range. 

To control the pH within the appropriate range, it is necessary to control the pH according to the characteristics of different microorganism, not only to control the appropriate pH value in the original medium, but also to check the change of pH value at any time during the whole fermentation process, and carry out corresponding adjustments.

First of all, considering the formula of the basic medium, the pH value of the culture solution can be stabilized by balancing the ratio of salts and carbon sources, or a buffer (such as phosphate) can be added to adjust the initial pH value of the medium to a suitable range and make it Strong buffering capacity. In batch fermentation, CaCO 3 is often added to neutralize keto acids to control pH changes.

Paragon thinks, during the fermentation process, the pH value will fluctuate greatly as the substrate is consumed and the product is produced. Therefore, the corresponding pH adjustment and control methods should also be adopted during the fermentation process, mainly including the following methods:

①  Add acid and alkali substances directly, such as h2S04, NaOh, etc. When the pH value does not deviate much, the use of strong acid-base substances will easily damage the buffer system and cause the hydrolysis of the components of the culture solution, so this method is rarely used at present.

②  Control the pH value by adjusting the ventilation volume. This method is mainly used in individual cases where more defoamers are added. 

     Increasing the air flow can accelerate the oxidation of lipoic acid to reduce the pH drop caused by the accumulation of fatty acids.

③  Add physiological acidic or alkaline salt substrates, such as ammonia, urea, (Nh4)2SO4, etc., to adjust the pH value through metabolism. 

     The addition of physiological acid-base substances not only regulates the pH value of the fermentation broth, but also supplements nutrients and reduces the deterrent effect. 

     The method of supplementation depends on the actual production situation, and can be direct addition, flow addition, multiple flow addition, etc.

④   Adopt the feeding method to adjust the pH value. For example, when the pH value rises above the optimum value, it means that the bacteria are in a hungry state and can be adjusted by adding sugar. 

      Excess sugar  will reduce the ph. The feeding method can achieve several purposes at the same time, such as supplementing nutrients, extending the cycle, adjusting the pH and changing the properties of the culture   medium (such as viscosity), especially those nutrients that have a deterrent effect on the synthesis of the product. Can avoid their inhibitory effect on product synthesis

Be cautious when using ammonia and organic acids to adjust pH during fermentation. Excessive ammonia can poison microorganisms, causing their breathing intensity to drop rapidly. 

In the fermentationprocess that requires the use of ammonia gas to adjust pH or supplement the nitrogen source, the changes in the concentration of dissolved oxygen can be monitored to prevent excessive ammonia poisoning of the bacteria.

In the actual production process, one or more of these methods can generally be selected and combined with the online detection of pH to effectively control the pH to ensure that the pH value is in the appropriate range for a long time.