Fermentation is understood as the whole set of sequential operations from the introduction of seed material into a pre-prepared and heated medium to the required temperature and to the completion of the process of cell growth or biosynthesis of the target product. At the end of fermentation, a complex mixture is formed, consisting of producer cells, a solution of consumed nutrients and biosynthetic products accumulated in the medium. This mixture is called a culture medium.
Among various microbiological processes, two primary categories are distinguished by their technological design: aerobic and anaerobic cultivation. Each of these processes employs distinct types of bioreactors and fermentors tailored to their specific requirements.
Aerobic cultivation necessitates the aeration of the medium, a critical condition for microbiological processes involving aerobic microorganisms. The demand for molecular oxygen by these organisms is influenced by the type of oxidized carbon source utilized, as well as the physiological characteristics and growth activity of the microorganisms themselves. For instance, the biosynthesis of 1 kg of yeast biomass requires approximately 0.74 to 2.6 kg of molecular oxygen. During periods of intensive substrate consumption, regardless of the carbon source, the producer typically assimilates between 0.83 and 4.0 mg of oxygen per liter of medium per minute.
The solubility of oxygen in the fermentation medium is relatively low and is affected by factors such as 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 amount of dissolved oxygen in one liter of distilled water is approximately 7.5 mg. However, in a typical nutrient medium, maximum oxygen solubility ranges from 2 to 5 mg per liter. In practical terms, oxygen reserves in the environment can sustain aerobic producers for only about 0.5 to 2 minutes.
During deep cultivation, the oxygen reserves in the nutrient medium are replenished through aeration with air. The rate at which oxygen is absorbed increases with the intensity of mixing within the medium. Notably, during biomass growth, microorganisms generally consume more oxygen than they do during the supersynthesis of target metabolites.
It is important to consider critical oxygen concentrations that limit cellular respiration. For most aerobic microorganisms growing on sugar-containing substrates, this critical concentration falls between 0.05 and 0.10 mg/l, corresponding to 3-8% of total oxygen saturation in the medium. Interestingly, limitations on cell growth and physiological activity can occur at higher oxygen concentrations; for example, yeast growth in glucose media is restricted when partial oxygen pressure (pO2) reaches about 20-25% of full saturation. Optimal conditions for biomass growth are achieved at an oxygen concentration of 50-60% of full saturation, while a concentration of 10-20% is deemed ideal for the biosynthesis of target metabolites.
To achieve optimal results in aerobic cultivation, it is essential that the design of the bioreactor facilitates the maintenance of the required oxygen concentration. In such bioreactors, several critical factors play a pivotal role, including temperature control, mixing systems, aeration mechanisms, and pH regulation. Each of these elements must be carefully optimized to create an environment conducive to the efficient growth of aerobic microorganisms and the successful production of target metabolites.
Anaerobic biological oxidation processes in heterotrophic microorganisms can be classified into three distinct groups, depending on the final acceptor of hydrogen atoms or electrons: respiration (where oxygen serves as the acceptor), fermentation (where organic matter acts as the acceptor), and anaerobic respiration (involving inorganic matter such as nitrates or sulfates).
In obligate anaerobes, fermentation is the sole means of energy production. Conversely, facultative anaerobes utilize fermentation as the essential first stage of glucose catabolism, which can then be followed by aerobic oxidation of the resultant products if oxygen is present in the environment. A unique intermediate category consists of aerotolerant microorganisms, which derive the energy necessary for their vital functions through anaerobic processes, specifically at the level of substrate phosphorylation. These organisms also possess a respiratory chain that allows them to absorb oxygen from their surroundings, thereby creating favorable anaerobic conditions. This phenomenon is known as the “respiratory protection effect.”
Examples of obligately anaerobic processes include butyric acid and methane fermentations. A common pathway among nearly all microorganisms— with few exceptions— is the catabolism of glucose through glycolysis, resulting in the formation of pyruvate:
Glucose+2ATP+2NAD→2Pyruvate+4ATP+2NADH+2H+
In alcoholic fermentation, yeast decarboxylate pyruvate to form acetaldehyde, which is subsequently reduced to ethanol. In contrast, lactic acid bacteria engaged in homolactic fermentation convert pyruvate into lactic acid. Heterofermentative lactic acid bacteria utilize a slightly different pathway—the pentose phosphate pathway—to ferment glucose, yielding not only lactic acid but also acetic acid, ethanol, and carbon dioxide.
Anaerobic conditions in production settings are achieved by sealing equipment and purging the medium with inert gases, including gaseous byproducts formed during fermentation. The absence of a requirement for medium aeration simplifies the design of bioreactors during anaerobic fermentation and enhances process control.
