From a technological perspective, fermentation processes can be classified into the following microbiological methods: aerobic and anaerobic cultivation; solid-phase, surface, and deep cultivation; as well as batch and continuous cultivation. This article will cover surface and deep cultivation, as well as batch and continuous cultivation.
Surface Fermentation on Liquid Substrates. Surface fermentation is conducted in cuvettes placed within air-ventilated chambers, where the culture of microorganisms forms biomass as a film or solid layer on the surface of a liquid medium. Oxygen is directly absorbed from the gaseous phase (air), and mass transfer under these conditions is relatively low in intensity.
Deep Cultivation of Microorganisms. Deep cultivation occurs throughout the entire volume of a liquid nutrient medium containing a dissolved substrate. The fermenter must facilitate microbial growth and development within the liquid phase by supplying nutrients to microbial cells, removing metabolic by-products, and dissipating heat generated by cellular activity.
Batch and Continuous Cultivation
- Batch Cultivation. In batch cultivation, the entire volume of the nutrient medium is introduced into the fermenter along with the inoculum. Microbial growth occurs under optimal conditions for a specified duration, after which the process is halted, and the contents of the fermenter are harvested for product isolation. The microbial growth cycle consists of the following phases: lag phase, exponential phase, deceleration phase, stationary phase, and death phase.
Fed-batch cultivation is a widely employed variation, wherein fresh medium is periodically added without removing culture fluid. Additionally, semi-continuous cultivation involves intermittent removal of part of the culture volume, which is replaced with an equivalent amount of fresh medium.
- Continuous Cultivation. In continuous cultivation, fresh nutrient medium is continuously supplied to the bioreactor while culture medium, containing microbial biomass, is simultaneously removed. This method maintains microorganisms in a steady exponential growth phase, ensuring equilibrium between biomass production through cell division and its removal due to dilution with fresh medium.
The most extensively studied continuous fermentation method is submerged fermentation, which can be classified as either homogeneous or heterogeneous continuous fermentation:
- Homogeneous Continuous Fermentation: Conducted in a well-mixed bioreactor where all parameters remain constant over time.
- Heterogeneous Continuous Fermentation: Involves multiple fermenters connected in series, with nutrient medium entering the first unit and the final culture fluid exiting the last unit.
To prevent microbial washout, continuous cultivation requires maintaining a stable cell concentration. Under sterile conditions, continuous flow-through fermentation ensures prolonged physiological activity of the microbial culture.
Based on the method used to sustain dynamic equilibrium, continuous cultivation follows either the turbidostatic or chemostatic principle:
- Turbidostat: The nutrient flow rate is regulated to maintain a constant biomass concentration.
- Chemostat: Microbial growth is limited by a specific nutrient (e.g., carbon, oxygen, or essential vitamins) while other nutrients remain in excess. Growth can also be regulated through pH control (pH-stat) or oxygen availability (oxystat).
Optimization of Fermentation Processes. The choice of fermentation method depends on the production objective—whether the target is biomass or metabolic products. If biomass production is the goal, fermentation aims to achieve the highest possible cell concentration. Conversely, for metabolite production, the accumulation of target compounds occurs over time, with peak production phases differing between biomass and metabolites. As a result, fermentation duration varies:
- Biomass production in batch processes: ≤24 hours
- Primary metabolite biosynthesis: 48–72 hours
- Secondary metabolite biosynthesis: 72–144 hours
The cultivation conditions for different microorganisms vary, with operating temperatures ranging between 25–60°C, pH values typically around 2.9, and air consumption in aerobic processes ranging from 0.15 to 2.5 m³ per m³ of medium per minute.
