The choice of reactor depends on the concentration of production cells, ventilation and the dispersion of nutrients provided. According to the types of aeration and stirring systems, bioreactors can be divided into the following categories: mechanical stirring bioreactors have large operating range, high mixing degree and wide adaptability, and are widely used in large-scale production. The shear force produced by the stirred tank is large, which is easy to damage cells and directly affects the growth and metabolism of cells, especially the generation of secondary products. The higher the stirring speed, the greater the shear force and the greater the damage to plant cells. For some cells sensitive to shear force, the traditional mechanical stirring tank is not suitable. Therefore, the stirring tank has been improved, including changing the stirring form, impeller structure and type, air distributor and so on. In order to reduce the shear force and meet the requirements of oxygen supply and stirring.
Kaman et al. adopted a stirred tank with1helicalribbonimbeller and three surface baffles, which proved to be suitable for high-density cell culture sensitive to shear stress. Jolicoeur et al. carried out similar research, and obtained the same high concentration biomass as shake flask in the reactor. Zhong Jianjiang and others found that the three-blade propeller reactor (MRP) with microporous wire mesh as air distributor can provide less shear force and good oxygen supply and mixing state, which is superior to the six-blade turboprop reactor, and thought that MRP reactor would show greater superiority when cells were cultured in high concentration. Compared with the cell lift bioreactor, the centrifugal bioreactor has higher lifting capacity, lower shear force, shorter mixing time and higher dissolved oxygen coefficient, which shows that it has great potential in shear force sensitive biological systems. In addition, in the production and research of plant cell culture, different forms of mechanical stirring tanks are used, such as box-type paddle stirring and butterfly turbine stirring. The results show that the order of shear force produced by different impellers is turbine impeller >: flat impeller >: spiral impeller. Lift-streambioreactor uses a rod connected to the perforated plate in the center of the tank to move up and down to achieve the purpose of stirring, which can be used to cultivate shear-sensitive cells. Compared with the traditional stirred reactor, the non-stirred reactor is considered to be suitable for plant cell culture because of its small shear force and simple structure. Its main types are bubble reactor, airlift reactor and drum reactor.
By comparing the bioreactors for cultivating perilla cells, it is found that the bubbling reactor is better than the mechanical stirring reactor. However, due to the low utilization rate of oxygen in bubble reactor, if a large amount of air is used, shear force will damage cells. The research shows that turbulent shear force is an important reason for inhibiting cell growth and damaging cells when injecting large bubbles. Larger bubbles or higher gas velocity lead to higher shear force, which is harmful to plant cells.
Airlift reactors are widely used in the research and production of plant cell culture. Through the study of carrot cell culture, it was found that compared with stirred tank, gas jet tank and airlift reactor with exhaust pipe, airlift reactor could obtain the highest cell concentration and the shortest doubling time. Airlift reactor is used for suspension culture or immobilized cell culture of many plant cells, but its operation flexibility is small and its mixing performance is not good at low gas velocity, especially at high H/D and high density culture. Excessive gas supply and high oxygen concentration will affect cell growth and synthesis of secondary metabolites. The combination of airlift fermentor and slow stirring can make up for the weakness of poor mixing at low gas velocity, and the use of segmented airlift tube is also beneficial to the utilization and mixing of oxygen.
The application of drum reactor in suspension culture of tobacco cells found that the growth rate of drum reactor was higher than that of airlift reactor with ventilation tube under the same conditions, and the damage degree of oxygen transfer and shear force to cells was better than that of airlift reactor. Many plant cells need illumination in the process of culture, so it is often considered to add an illumination system on the basis of ordinary reactors, but there are many problems in practice, such as the installation and protection of light sources, light transmission, and the influence of illumination system on gas supply and mixing in reactors. External illumination is often used in small-scale experiments. There is a transparent illumination area on the surface of the reactor, and the light source is fixed around the outside of the reactor. However, it is difficult to solve the problems such as the setting of light transmission window and the unified acceptance of light by internal culture in mass production, so many people have studied the reactor with internal light source.
In the reactor invented by Mori et al., a plurality of transparent cylinders are installed in a reactor tank in parallel, light sources are placed in the transparent cylinders, and a gas exchanger for supplying CO2 is between the two cylinders in the tank. Ogbonna et al. developed a new type of photobioreactor with internal illumination agitation, which is used to cultivate photosynthetic cells on a large scale. It consists of multiple units, each of which contains a light source. A large photobioreactor is obtained by increasing the number of units. A glass tube is fixed in the center of each unit, and a light source is inserted into it, and mixing is realized through a paddle. The mixing slurry is designed not to contact the glass tube when rotating, and the glass tube is used as a baffle. The reactor still has high mixing degree and low shear force at low rotation speed. Since the luminous body is not mechanically fixed on the reactor or separated from the fermentation broth through the glass tube, the reactor can be autoclaved, and the luminous body can be inserted into the glass tube after cooling. Yamamurak et al. studied the photoreactor for fixing CO2, which is characterized by the luminous function of the agitator. According to the characteristics of plant cells, many new reactors different from traditional microbial reactors are being used in the research and production of plant cells, such as various immobilized plant cell reactors and membrane reactors. Dubuis et al. used a new circulating fluidized bed reactor to cultivate coffee, and measured the kinetic parameters of growth and product synthesis. He believes that the reactor is easy to operate, eliminates the shear force caused by direct gas injection, and is easy to determine the parameters needed for amplification, which is suitable for pilot and industrial production. Gai Na et al. cultivated immobilized tobacco cells in a fixed-bed reactor, and the growth rate was the same as that in a shake flask, and there was no obvious difference between intracellular synthesis and shake flask.
Tyler et al. reported a plant cell surface immobilized culture system, which avoided the problem of fluid flow force or shear force in traditional stirred tank suspension culture, promoted the characteristics of plant cell coagulation, and increased the synthesis and accumulation of secondary metabolites. In addition, the system has simple medium exchange and is easy to extract secondary products. Lang also studied the plant cell membrane reactor, in which the cells are fixed on a 3mm thick membrane, the culture medium circulates in a closed loop under the membrane, the nutrients diffuse to the cell layer through the membrane, and the secondary metabolites are secreted into the culture medium through the membrane.
Humphrey studied the microporous membrane aeration reactor for plant cell culture, and analyzed the oxygen transfer, which provided the design basis for the membrane aeration reactor for plant cell culture with small shear force. The factors to be considered in the design include the length, diameter and membrane thickness of the tube, the composition and pressure of the inlet gas, the cell growth and culture stage, etc.