General Info

Factors affecting the performance of closed solid-state fermentation

The fermentation performance of closed solid-state fermentation systems depends largely on factors such as mass transfer phenomena, bioreaction rates, and the design and operation of effective bioreactor systems. It is very important to accurately control various factors to the appropriate range.

Factors affecting the performance of closed solid-state fermentation

1. Agitation or mixing degree

Agitation is conducive to ensuring bed temperature, humidity, etc., and can also promote mass and heat transfer in the fermentation system. However, agitation 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 selecting a closed fermentation system with a stirring device, in addition to considering the number of stirring times, stirring time, and stirring intensity, it is also necessary to consider whether agitation will affect the yield of microorganisms or final products.

2. Particle size and porosity

The particle size of the solid-state fermentation matrix is related to the specific surface area and bulk density of the material. In the aerobic solid-state fermentation process, microbial growth generally starts from the surface of the particles and gradually penetrates into the interior of the particles. A 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 a limiting factor for growth.

In addition, the size of the particles will also affect the porosity of the solid-state fermentation matrix, and thus affect the material transfer. The pores between the particles mainly affect the diffusion of gases, and the impact on microorganisms is also relatively complex. For example, it affects whether the enzymes produced by microorganisms or the added hydrolases can penetrate into the particles and play a role, and also affects whether the microorganisms can enter the particles and grow.

3. Matrix nutrients

The solid-state fermentation matrix 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 has an important impact on the survival ability of microorganisms.

The carbon-nitrogen ratio is also one of the important factors affecting microbial growth and metabolite production. If the nitrogen content in the solid-state fermentation matrix is too high or too low, it will affect the growth and metabolism of microorganisms. Different types of microorganisms require different carbon-nitrogen ratios.

Therefore, the carbon-nitrogen ratio in the solid-state fermentation matrix used to cultivate microorganisms should be kept within a suitable range to ensure that there are enough nutrients for their growth and metabolism.

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 impact on microbial growth and product formation, and low temperature is not conducive to microbial growth and biochemical reactions.

Due to the different heat dissipation efficiencies of various fermentation systems, the temperature that can be achieved depends on the complex interaction between the type of microorganism and the fermentation system and its operation mode. 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 of the matrix bed plays a vital role in improving the production performance of the closed solid-state fermentation system.

5. Ventilation

Ventilation is a very important parameter in the closed solid-state fermentation system. It can maintain the aerobic conditions in the closed solid-state fermentation system, remove the 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, this issue must be paid great attention to.

6. Microbial selection

The selection of microorganisms may have the most important influence 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 the fermentation performance varies with the morphology and growth pattern of the microorganism.

For example, some filamentous fungi, such as Rhizopus oryzae, can form a thick mycelial layer that reduces the oxygen and heat transfer between the environment and the substrate. As a result, in the substrate, the consumption of oxygen and the accumulation of metabolic heat make the environment unfavorable for the growth of microorganisms, thereby compromising 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.

7. Water content and water activity

Usually, the water requirement of microorganisms should be defined based on 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 yield of microbial metabolites.

During solid-state fermentation, different microorganisms require different water activity values. If the water activity value is low, it will affect the growth of microorganisms and reduce the yield. On the contrary, too high water activity will lead to the aggregation of solid matrix particles, which will limit the transfer of oxygen and reduce the production of microbial metabolites. Therefore, it is very important to adjust the water activity value to the appropriate range.

8. Design of the fermentation system itself

During the entire fermentation process, except for oxygen, no substance is added to the solid fermentation matrix to ensure that the growth environment of the microorganisms is maintained in an ideal state.

Although the composition and concentration of the solid fermentation matrix usually change due to microbial metabolism, some parameters in the solid fermentation system, such as the transfer of oxygen and metabolic heat, need to be managed by controlling ventilation, stirring, moisture content, temperature and the type of microorganisms and solid fermentation matrix used to ensure the smooth progress of the entire fermentation process.

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

Optimization and Control of Closed Solid-State Fermentation System

The optimal process parameter values can maximize cell growth and metabolite production. Therefore, it is particularly important to optimize the closed solid-state fermentation system.

1. PID (Proportional-Integral-Derivative) Control

In many large-scale closed solid-state fermentation systems, stirring and convection cooling cannot remove more than 50% of the metabolic heat, and the remaining 50% of the 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 become very complicated. Usually, such processes cannot be controlled by PID algorithms alone, and this process takes 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. In order to achieve optimal performance in these complex situations, model-based control methods are necessary.

2. Mathematical modeling optimization

Mathematical modeling is an essential tool for optimizing biological processes. It can not only guide the design and operation of closed solid-state fermentation systems, but also provide insights into how various phenomena within the fermentation system can be combined to control the overall process.

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

At present, mathematical models have reached a mature level. Only by using mathematical models as tools in the design process and optimization operations can the solid-state fermentation system fully realize its potential, thereby maximizing the economic performance of the solid-state fermentation process.