Fundamentals of Fermentation
Fermentation encompasses a series of sequential operations, beginning with the introduction of seed material into a pre-prepared and thermally regulated medium, followed by the progression of cell growth or biosynthesis of the desired product. Upon completion of the fermentation process, a complex mixture is formed, comprising producer cells, residual nutrients, and biosynthetic products accumulated within the medium. This mixture is commonly referred to as the culture medium.
Technological Aspects of Fermentation Processes
Microbiological processes are classified based on their technological design, with two primary categories being aerobic and anaerobic cultivation.
Aerobic Cultivation
Aeration is an essential requirement for microbiological processes involving aerobic microorganisms. The oxygen demand of these microorganisms is influenced by the oxidized carbon source, their physiological properties, and their growth activity. For instance, the biosynthesis of 1 kg of yeast biomass necessitates approximately 0.74–2.6 kg of molecular oxygen. Under conditions of intensive substrate consumption, irrespective of the carbon source, the microorganism assimilates between 0.83–4.0 mg of oxygen per liter of medium per minute.
The solubility of oxygen in the medium is inherently low and is contingent on temperature, pressure, and the concentration of dissolved, emulsified, and dispersed components. At a pressure of 0.1 MPa and a temperature of 30°C, the maximum solubility of oxygen in 1 liter of distilled water is approximately 7.5 mg. However, in real nutrient media, this solubility typically ranges between 2 and 5 mg/L. Oxygen availability in the medium sustains aerobic microbial activity for only 0.5–2 minutes.
In deep cultivation, oxygen levels are replenished through continuous aeration and enhanced by increased mixing intensity. During biomass growth, microorganisms typically exhibit higher oxygen consumption compared to the phase of metabolite supersynthesis. A critical oxygen concentration is defined as the threshold below which cell respiration becomes restricted. For most aerobic microorganisms growing in sugar-containing substrates, this critical concentration ranges from 0.05 to 0.10 mg/L, equivalent to 3–8% of the total oxygen saturation of the medium. Cell growth and physiological activity begin to decline at oxygen concentrations of approximately 20–25% of full saturation in glucose-based media.
The optimal oxygen concentration for biomass growth is generally considered to be 50–60% of full saturation, whereas for the biosynthesis of target metabolites, the optimal concentration is 10–20%.
Anaerobic Cultivation
Anaerobic microbial processes, classified based on the final acceptor of hydrogen atoms or electrons, are divided into three groups:
- Respiration (oxygen as an acceptor),
- Fermentation (organic compounds as acceptors), and
- Anaerobic respiration (inorganic compounds such as nitrates and sulfates as acceptors).
For obligate anaerobes, fermentation represents the sole method of energy production. In contrast, facultative anaerobes undergo an initial fermentation stage in glucose catabolism, followed by aerobic oxidation of intermediate products if oxygen is present. A distinct intermediate group includes aerotolerant microorganisms, which derive energy through anaerobic processes (substrate phosphorylation) while possessing a respiratory chain that facilitates oxygen absorption. This mechanism is termed the “respiratory protection effect.”
Examples of obligate anaerobic fermentation include butyric acid and methane fermentations. A nearly universal metabolic pathway among microorganisms is glucose catabolism via glycolysis, leading to pyruvate formation:
Glucose++ 2АТР + 2 NAD = 2 Pyruvate + 4АТР + 2NADH + 2Н+
In alcoholic fermentation, yeasts decarboxylate pyruvate to form acetaldehyde, which is subsequently reduced to ethanol. Homolactic fermentation, characteristic of lactic acid bacteria, reduces pyruvate directly to lactic acid, whereas heterolactic fermentation follows a pentose phosphate pathway, producing lactic acid, acetic acid, ethanol, and carbon dioxide.
Anaerobic conditions in industrial applications are maintained through sealed fermentation equipment, purging the medium with inert gases, or utilizing fermentation-derived gaseous byproducts. The absence of aeration simplifies the bioreactor’s structural design and facilitates process control during anaerobic fermentation.
| Quantity of supplied air, m3/(m3*min) | Rotation speed of the mixer, min-1 | ||||
| 0 | 500 | 800 | 1000 | 2000 | |
| 0,35 | 1,3 | 4,0 | 7,5 | 14,5 | 15,1 |
| 0,65 | 3,5 | 7,3 | 12,1 | 19,1 | 22,1 |
| 1,00 | 6,0 | 10,0 | 15,0 | 23,0 | 24,0 |
| 1,30 | 7,5 | 13,9 | 18,0 | 26,0 | 28,0 |
| 1,60 | 11,0 | 15,5 | 20,0 | 27,0 | 29,0 |
