5 Factors Affecting Fermentation Process

During the fermentation process, to be able to control the production process is necessary. It is also necessary to regularly sample and determine the relevant process parameters or to conduct continuous measurement.

The parameters that reflect changes in the fermentation process can be divided into two categories: One is the parameters that can be directly detected by a specific sensor. They include parameters that reflect changes in the physical environment and chemical environment, such as temperature, pressure, stirring power, speed, foam, fermentation broth viscosity, turbidity, pH, ion concentration, dissolved oxygen, matrix concentration, etc., which are called direct parameters.

The other is the parameters that are still difficult to detect with sensors, including cell growth rate, product synthesis rate, and respiration. These parameters need to be calculated with the help of computer calculations and specific mathematical models based on some directly detected parameters. Therefore such parameters are called indirect parameters. Among the above parameters, temperature, pH, dissolved oxygen concentration, etc. have a greater influence on the fermentation process.

The Main Factors Affecting Fermentation Process are as follows

1. Temperature

  • The effect of temperature on microorganisms is numerous. 
  • Temperature can affect the enzyme activity. In the optimum temperature range, as the temperature increases the growth and metabolism of the bacteria accelerate, and the rate of the fermentation reaction increases. 
  • When the temperature exceeds the optimal temperature range as the temperature rises the enzymes are quickly inactivated, the bacteria die the fermentation cycle is shortened and the output is fermentation reduced. 
  • Temperature can also affect the pathway of biosynthesis. For example, when the temperature of Streptomyces aureus is below 30°C, the ability to synthesize chlortetracycline is stronger, but when the temperature exceeds 35°C, it only synthesizes tetracycline but not chlortetracycline. 
  • In addition, the temperature will also affect the physical properties of the fermentation broth and the decomposition and absorption of nutrients by the bacteria. 
  • Therefore, to ensure the normal fermentation process, it is necessary to maintain the optimum temperature. 
  • However, the optimum temperature required for bacterial growth and product synthesis is not necessarily the same. 
  • For example, the optimum growth temperature of Streptomyces griseus is 37℃, but the optimum temperature for antibiotic production is 28℃. Generally, experiments must be conducted to determine the optimum temperature for each fermentation stage of different strains, and segmented control is adopted.

Read: Sterilization in Fermentation Process 

2. pH  

  • pH can affect the activity of enzymes and the charge status of cell membranes. 
  • If the charge status of the cell membrane changes, the permeability of the membrane will also change, which may affect the absorption of nutrients and the secretion of metabolites by microorganisms. 
  • In addition, pH will also affect the decomposition of nutrients in the medium. Therefore, the pH of the fermentation broth should be controlled. 
  • However, the optimum pH of different strains in the growth stage and the synthetic product stage is often different and needs to be controlled separately.
  • During the fermentation process, with the utilization of nutrients and the accumulation of metabolites by the bacteria, the pH of the fermentation broth will inevitably change. For example, when urea is decomposed, the concentration of NH+4 in the fermentation broth will rise, and the pH will also rise accordingly. 
  • In industrial production, it is often used to add a buffer system to maintain pH in the fermentation broth, or to control pH by adding ammonia, urea, ammonium carbonate, or calcium carbonate in the middle. 
  • At present, domestic pH electrodes have been developed to detect the fermentation process, which is used to continuously measure and record pH changes, and the pH controller adjusts the amount of acid and alkali added.

Read: What are Bioreactors 

3. Dissolved oxygen Concentration

  • The supply of oxygen is a key factor for aerobic fermentation. 
  • From the perspective of oxygen demand for glucose oxidation, 1 mol of glucose is completely oxidized and decomposed and 6 mol of oxygen is required; when sugar is used to synthesize metabolites, 1 mol of glucose requires about 1.9 mol of oxygen. 
  • Therefore, aerobic microorganisms require a large amount of oxygen, but the bacteria can only use the dissolved oxygen in the fermentation broth during the fermentation process, but the oxygen is difficult to dissolve in water. 
  • At 101.32 kPa and 25°C, the solubility of oxygen in water is 0.26 mmol/L. 
  • Under the same conditions, the solubility of oxygen in the fermentation broth is only 0.20 mmol/L. And as the temperature increases, the solubility will decrease. 
  • Therefore, a large amount of oxygen must be continuously added to the fermentation broth, and stirring can increase the solubility of oxygen in the fermentation broth.

Read: What is Culture Media: Definition, Types, Uses, Examples

4. Foam Concentration

  • During the fermentation process, aeration and stirring, the metabolic process of microorganisms, and the decomposition of certain components in the culture medium, etc., may produce foam. 
  • It is normal to produce a certain amount of foam during the fermentation process, but too much persistent foam is detrimental to the fermentation. 
  • Because the foam will occupy the volume of the fermentation tank affect the normal operation of aeration and stirring and even cause abnormal metabolism, it is necessary to eliminate the foam. 
  • There are two types of commonly used defoaming measures: one is to install a defoaming baffle to promote the collapse of the foam through strong mechanical oscillation; the other is to use a defoaming agent.

Read: Ethanol Production By Fermentation

5. Concentration of nutrients

  • The concentration of various nutrients in the fermentation broth, especially the ratio of carbon to nitrogen, inorganic salts, and vitamins, will directly affect the growth of bacteria and the accumulation of metabolites. 
  • For example, in the fermentation of glutamate, the change of NH+4 concentration will affect the metabolic pathway. Therefore, during the fermentation process, a sufficient amount of nutrients are added for fermentation.
The control of various parameters in the fermentation process is very important. At present, the direction of fermentation process control is to turn to automatic control. Therefore, it is hoped that more and more effective sensors can be developed for the detection of process parameters. In addition, the judgment of the fermentation endpoint is equally important. Production cannot simply pursue high productivity without taking into account the cost of the product.
The two must be combined. The reasonable set-up time is determined by experiment, that is, the productivity and product cost of the fermenter is calculated according to the product output obtained from different fermentation times, and the time with high productivity and low cost is used as the set-up time.
The indicators to determine the tank release are product output and filtration speed. The content of amino nitrogen, mycelial morphology, and pH value. The appearance and viscosity of the fermentation broth, etc. The determination of the fermentation endpoint requires comprehensive consideration of these factors.
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