Why is mass and heat transfer the "life and death switch" of fermentation?
In the future, with the integration of artificial intelligence and sensor technology, real-time monitoring of parameters such as dissolved oxygen, temperature, viscosity, and dynamic control of mass and heat transfer processes will become possible.
1、 Mass transfer
Mass transfer refers to the migration process of various substances in a biological reaction system, including the supply of nutrients, the discharge of metabolites, gas exchange, etc. It is the material basis for microorganisms to maintain their life activities.
Mass transfer requirements vary depending on microbial characteristics in different fermentation systems:
In food fermentation: When making yogurt, lactic acid bacteria are anaerobic microorganisms, and excessive oxygen mass transfer can inhibit their activity.
In the pharmaceutical industry, Penicillin fermentation and Aspergillus flavus production are aerobic bacteria that require enhanced oxygen mass transfer through aeration (1-1.5vvm) and stirring (200-300r/min).
2、 Heat transfer
Heat transfer is a key process in regulating the temperature of fermentation systems, ensuring that microorganisms are in the optimal growth temperature range by removing metabolic heat or supplementing the heat required for reactions.
The heat production intensity varies significantly among different fermentation processes, and the heat transfer system needs to be designed accordingly
High heat release system: During glutamic acid fermentation, the bacterial metabolism intensity is high, and each cubic meter of fermentation broth can generate 1200-1500kJ of heat per hour, equivalent to the power of a 300W electric heater.
Low temperature sensitive system: In vaccine production, CHO cell culture requires strict temperature control, using a "constant temperature water bath circulation" heat transfer system and PID feedback regulation.
3、 The interaction between mass transfer and heat transfer forms a dynamic coupling relationship in the fermentation system, and any process imbalance can trigger a chain reaction.
1） The effect of temperature on mass transfer: High temperature accelerates substance diffusion: during cellulase fermentation, when the temperature rises from 28 ℃ to 32 ℃, the viscosity of the fermentation broth decreases and the diffusion efficiency of glucose increases.
But an increase in temperature will decrease the solubility of oxygen, so it is necessary to increase the ventilation rate to compensate for the lack of oxygen.
Low temperature will slow down the mass transfer process: In L-phenylalanine fermentation, when the temperature drops from 30 ℃ to 25 ℃, the metabolism of the bacterial cells slows down, heat production decreases, but the diffusion rate of the material also decreases, resulting in a decrease in substrate utilization efficiency.
At this point, mass transfer can be enhanced by increasing the stirring speed.
（2） The effect of mass transfer on heat transfer: Concentration changes can affect heat transfer performance: as antibiotics ferment to the later stage, the concentration of mycelium increases, and the thermal conductivity of the fermentation broth decreases.
At this point, it is necessary to lower the cooling water temperature in order to maintain the original heat transfer rate.
Bubble movement can enhance heat transfer efficiency: when the ventilation rate increases, the number of bubbles in the fermentation broth increases, the liquid agitation becomes more intense, and the heat transfer efficiency is improved.
Under the same heat generation conditions, the temperature distribution inside the fermentation tank will be more uniform.
