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Factors Affecting The Performance Of Closed Solid State Fermentation

Technical Knowledge 2023-09-08 16:54:37
The fermentation performance of a closed solid-state fermentation system depends largely on factors such as mass transfer phenomena , biological reaction rates , and effective bioreactor system design and operation , and it is very important to precisely control various factors to the appropriate range.


solid-state fermentation system 

1. Factors Affecting the Performance of Closed Solid State Fermentation

1.1 Stirring or mixing condition

Stirring is beneficial to ensure bed temperature, humidity, etc., and can also promote mass and heat transfer in the fermentation system. However, stirring can also break mycelium, affect the growth of microorganisms, and even affect the synthesis of metabolites.

Most filamentous fungi are sensitive to shear force. Therefore, when choosing a closed fermentation system with a stirring device, in addition to considering the frequency of stirring, stirring time, and stirring intensity, it is also necessary to consider whether the stirring will affect the microorganisms or the final product. Yield of

1.2 Particle size vs. porosity

The particle size of the solid-state fermentation substrate is related to the specific surface area and bulk density of the material . In the process of aerobic solid-state fermentation, the growth of microorganisms generally starts from the surface of the particles and gradually penetrates into the interior of the particles. The larger specific surface area is conducive to the growth of microorganisms and the acquisition of nutrients. Particles that are too small will make the material too dense, making oxygen the limiting factor for growth.

In addition, the size of the particles also affects the porosity of the solid-state fermentation substrate , which in turn affects the mass transport. The pores between particles mainly affect the diffusion of gas, and the impact on microorganisms is also more complicated. For example, it affects whether the enzymes produced by microorganisms or external hydrolytic enzymes can penetrate into the interior of the particles and play a role, and also affects whether microorganisms can enter the interior of the particles to grow. .
1.3 Matrix Nutrients

The solid-state fermentation substrate provides microorganisms with essential nutrients such as carbon, nitrogen, phosphorus and trace inorganic elements to maintain the life activities of microorganisms and synthesize extracellular metabolites, which have an important impact on the viability of microorganisms.

The carbon-to-nitrogen ratio is also one of the important factors affecting the growth of microorganisms and the production of metabolites. If the nitrogen content in the solid-state fermentation substrate is too high or too low, it will affect the growth and metabolism of microorganisms. For different types of microorganisms, the required carbon-nitrogen ratio is also different.


Therefore, in the solid-state fermentation substrate used for cultivating microorganisms, the carbon-to-nitrogen ratio should be kept within an appropriate range to ensure that there are sufficient nutrients for their growth and metabolism.
1.4 Temperature

In a closed solid-state fermentation system, a large amount of metabolic heat will be generated as the fermentation proceeds. High temperature has a negative effect on microbial growth and product formation, and low temperature is not conducive to microbial growth and biochemical reactions.

Due to the different heat dissipation efficiency of various fermentation systems, the temperature that can be achieved depends on the complex interaction between the microorganisms and the type of fermentation system and its mode of operation. Therefore, how to control the influence of the temperature of the fermentation system on microorganisms and solve the problem of heat generation and heat dissipation in the matrix bed plays a vital role in improving the production performance of the closed solid-state fermentation system.
1.5 Ventilation

Aeration is a very important parameter in the closed solid-state fermentation system, which can maintain the aerobic conditions in the closed solid-state fermentation system, remove carbon dioxide in the substrate bed, control the temperature in the substrate bed and maintain the humidity of the substrate bed.

However, if unsaturated air is introduced into the closed solid-state fermentation system, it will cause strong evaporation of the substrate bed, aggravate the water loss of the solid-state fermentation substrate, and inhibit the growth and metabolism of microorganisms. Therefore, during the ventilation process, great attention must be paid to this problem.
1.6 Microbial selection

The choice of microorganisms may have the most important impact on the fermentation performance of closed solid-state fermentation systems. This is not only because the choice of microorganism determines the final product of the fermentation, but also because fermentation performance varies with the morphology and growth pattern of the microorganism.

For example, some filamentous fungi, such as Rhizopus oryzae, can form thick hyphal layers that reduce oxygen and heat transfer between the environment and the substrate. As a result, the consumption of oxygen and the accumulation of metabolic heat in the matrix make the environment unfavorable for the growth of microorganisms, thereby damaging the performance of the fermentation.

Therefore, the optimal microbial selection will depend on the type of solid-state fermentation substrate, growth requirements, and target end products.
1.7 Moisture content and water activity

Usually the water requirement of microorganisms should be defined in terms of water activity (Aw) rather than the water content of the solid substrate. Water activity directly affects the type and number of microorganisms that can grow during solid-state fermentation, thereby affecting the final output of microbial metabolites.

In the solid-state fermentation process, different microorganisms require different water activity values. If the water activity value is low, the growth of microorganisms will be affected and the yield will be reduced. On the contrary, if it is too high, it will lead to the aggregation of solid matrix particles, which will limit the transfer of oxygen and lead to a decrease in the production of microbial metabolites. Therefore, it is very important to adjust the water activity value to the appropriate range.
1.8 Self-design of fermentation system

During the entire fermentation process, except for oxygen, nothing is added to the solid-state fermentation substrate to ensure that the growth environment of microorganisms is maintained in an ideal state.

Although the composition and concentration of solid-state fermentation substrates are usually changed by microbial metabolism, some parameters in solid-state fermentation systems, such as oxygen and metabolic heat transfer, need to be adjusted by controlling aeration, agitation, moisture content, temperature and the microorganisms and nutrients used. The type of solid-state fermentation substrate is managed to ensure the smooth progress of the entire fermentation process.

Therefore, each specific fermentation process requires a specific design and setting of appropriate fermentation parameters to ensure the effectiveness and reliability of the closed solid-state fermentation system.

2. Optimal Regulation of Closed Solid State Fermentation System

Optimal process parameter values can maximize cell growth and metabolite production. Therefore, it is particularly important to optimize and regulate closed solid-state fermentation systems.
2.1 PID (proportional-integral-derivative) control
  In many large-scale closed solid-state fermentation systems, stirring and convective cooling cannot remove more than 50% of the metabolic heat, and the remaining 50% of heat can only be removed by other means. Therefore, evaporative cooling is the most effective way to remove metabolic heat.
  When large-scale closed solid-state fermentation systems use evaporative cooling, the dynamic response and control configuration of the process will become very complex. Usually, such a process cannot be controlled by the PID algorithm alone, and this process requires a long time to respond to changes in operating variables, which brings great difficulties to PID tuning.
  In addition, the dynamic response of the system is nonlinear, and the response of the fermentation system is not consistent throughout the fermentation time. This situation will cause the PID tuning parameters to be only applicable for a period of time, so the PID parameter settings need to be changed frequently. To achieve optimal performance in these complex situations, model-based control methods are necessary.
2.2 Mathematical modeling optimization

Mathematical modeling is an essential tool for optimizing biological processes, not only guiding the design and operation of closed solid-state fermentation systems, but also providing insights into how various phenomena within fermentation systems combine to control the overall process.

Some researchers have simulated the oxygen consumption, heat production and cell growth in the solid-state fermentation system through mathematical models, which will help to better understand the migration process of solid-state fermentation, and thus contribute to the optimal design of closed solid-state fermentation systems.

At present, the mathematical model has reached a mature level, and only by using the mathematical model as a tool in the design process and optimization operation, the solid-state fermentation system can fully realize its potential, thereby maximizing the economic performance of the solid-state fermentation process.
3 Epilogue

With the continuous advancement of modern biotechnology and monitoring methods, closed solid-state fermentation systems will become more automated and intelligent, monitoring tools and automatic control systems will be further optimized, and fermentation control will become more precise.

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