Seven closed solid-state fermentation systems

Solid state fermentation (SSF) is a fermentation method in which microorganisms ferment on a solid substrate with little or no free water. The substrate not only provides nutrients and growth environment for microorganisms, but also has a continuous gas phase and a small amount of liquid water, making the system stable.

A layer of water film adheres to the surface of the solid matrix, but the liquid phase is not continuous, and most of the gaps are filled by the gas phase. The water in the system is mainly adsorbed between the solid matrix particles. Therefore, in the absence or minimal amount of free water, microorganisms can still utilize sufficient water in the solid matrix to complete growth and metabolism.

A closed solid-state fermentation system is a space that provides a suitable environment and conditions for microorganisms to grow and metabolize on a solid substrate. 

It can prevent the entry of external microorganisms and the leakage of internal substances. It must be non-toxic, vibration resistant, corrosion-resistant, have good stirring, ventilation, heat dissipation, and cooling systems, and be able to perform aseptic operations.

Due to the properties of the substrate and organism used, the mixture in a closed solid-state fermentation system is usually uneven, which is not conducive to heat and mass transfer. Therefore, using different types of closed solid-state fermentation systems can solve the corresponding problems.

The disc type solid-state fermentation system generally consists of multiple trays stacked in a closed culture chamber or incubator, leaving a certain gap between the trays to allow air to circulate between the trays and the surface of the solid-state fermentation substrate. 

There is no stirring device or forced ventilation measures in the system, achieving true static solid-state fermentation.

The fermentation system has a simple structure, easy production, and low cost, but poor heat dissipation. During the fermentation process, the growth and metabolism of microorganisms generate a large amount of heat, and the lack of forced ventilation in the disc solid-state fermentation system can lead to temperature gradient differences, affecting the quality and yield of the final product.

The main body of the packed bed solid-state fermentation system is a vertically placed cylindrical cylinder with a porous plate or sieve at the bottom, used to support the solid-state fermentation substrate and form a column bed of a certain height. There are air inlets and outlets at the upper and lower ends respectively for air circulation and to maintain the normal progress of fermentation.

This fermentation system has high efficiency, easy operation, simple structure, and adopts forced ventilation measures to enhance heat and mass transfer processes, but the mass transfer coefficient and heat transfer coefficient are relatively low. 

As the height of the bed increases, the ventilation rate decreases, and the temperature gradient in the fermentation system gradually increases. When the temperature exceeds a certain value, the growth and metabolism of microorganisms are inhibited, which can lead to damage to the bacterial cells.

In addition, although forced ventilation can promote evaporation and achieve temperature control through air circulation, excessive ventilation can cause the solid fermentation substrate to become dry, and excessive water loss is not conducive to fermentation.

The main body of the rotary drum solid-state fermentation system is a horizontal or inclined cylindrical tube filled with solid fermentation substrate. Oxygen is introduced from the top and circulates inside the fermentation system to meet the metabolic needs of microbial growth. 

Adopting a closed design can prevent the overflow of humid and hot air, ensuring good fermentation hygiene conditions.

With the rotation of the cylindrical cylinder, the solid fermentation substrate can be stirred and mixed evenly, which is conducive to the transfer of oxygen and heat, improves fermentation efficiency, and also prevents the substrate from adhering to the inner wall of the fermentation system. 

But if the rotation rate is too high, the resulting large shear force will hinder microbial growth and metabolism, and even lead to death.

The basic structure and operating parameters of the drum type solid-state fermentation system and the drum type solid-state fermentation system are similar, but the difference is that the main body of the drum type solid-state fermentation system remains stationary, while the bed layer is dynamic.

The fermentation system is equipped with a stirring device on the central axis, which maintains uniform mixing of the solid fermentation substrate through continuous or intermittent stirring, avoiding agglomeration of the solid fermentation substrate, improving heat transfer, ventilation, and uneven water distribution. 

It can promote the growth and metabolism of microorganisms, increase the transfer and utilization of oxygen and nutrients, and improve fermentation efficiency and yield.

The stirred bed solid-state fermentation system is similar to the packed bed solid-state fermentation system, except that a stirring device is embedded in the bed layer of the stirred bed, which can intermittently or continuously stir the solid fermentation substrate.

This design makes the solid fermentation substrate more evenly mixed, and the internal ventilation regulates the humidity inside the fermentation system, achieving a more uniform fermentation effect with internal heat dissipation. Therefore, in the design and operation of a stirred bed, the design and operation of the agitator are also important considerations.