Fermentation, an ancient technique imbued with the power of modern science and technology, is fundamentally driven by the intricate metabolic processes taking place inside microbial cells. It is not simply a process of “decay” or “spoilage,” but rather a controlled and highly efficient biological conversion system. This article takes you into the microscopic world to explore the basic principles of fermentation processes and reveal the metabolic magic behind everything from a spoonful of sugar to a bottle of alcohol, and from a gram of microorganisms to a pharmaceutical product.
I. The Essence of Fermentation: Microbial “Life Activities” and “Industrial Production”
From a biological perspective, fermentation is a way for microorganisms to obtain energy by breaking down organic matter under anaerobic conditions. However, in industrial fermentation, the definition is much broader: it refers to the process of utilizing the growth, reproduction, and metabolic activities of microorganisms—including bacteria, yeasts, and fungi—to produce target products on a large scale under controlled conditions.
The central challenge lies in the fact that the natural metabolism of microorganisms is aimed at their own survival and reproduction. They pursue “economic efficiency” and balanced growth rather than excessive accumulation of any particular substance. The objective of industrial fermentation is precisely the opposite—we need to “trick” or genetically and metabolically engineer microorganisms, disrupting their normal metabolic regulation and forcing them to allocate large amounts of resources and energy toward the overproduction of specific target products, such as alcohols, antibiotics, or amino acids.
II. The Cellular Metabolic Network: A Highly Sophisticated “Chemical Factory”
We can imagine a microbial cell as a highly automated, miniature chemical factory with complex production processes. The “production lines” of this factory are the metabolic pathways, while the “workers” are the thousands of different enzymes.
✔ Raw Materials and Energy (Substrates): These are typically carbohydrates such as glucose, starch, or molasses. They serve as the “fuel” and “raw materials” of the factory.
✔ Central Production Lines (Central Metabolism):These mainly include:
1. Glycolysis (EMP Pathway): One molecule of glucose is broken down into two molecules of pyruvate, generating a small amount of ATP—the cellular “energy currency”—as well as reducing power in the form of NADH.
2. Tricarboxylic Acid Cycle (TCA Cycle): Under aerobic conditions, pyruvate is further oxidized, generating large amounts of reducing power and precursor metabolites.
3. Pentose Phosphate Pathway (PPP): This pathway provides precursors required for the synthesis of nucleotides and aromatic amino acids, as well as reducing power in the form of NADPH.
✔ Product Packaging Lines (Branched Metabolism and Biosynthesis): Starting from intermediates of central metabolic pathways, such as pyruvate and α-ketoglutarate, a series of enzymatic reactions leads to the synthesis of various primary metabolites, such as amino acids and nucleotides, and secondary metabolites, such as antibiotics and pigments.
III. Metabolic Regulation: The “Control Center” and “Valves” of the Factory
Microbial cells possess sophisticated regulatory systems to manage this “factory” and ensure efficient utilization of resources. The key to industrial fermentation lies in how this regulatory system can be deliberately manipulated.
✔ 1. Regulation of Enzyme Activity — The Instantaneous “On/Off Switch” of the “Valves”
Feedback Inhibition: When the end product of a production pathway, such as a particular amino acid, accumulates excessively, it acts as a “negative feedback signal” to inhibit the activity of the first key enzyme in that pathway, thereby shutting down the entire production line. This is a classic mechanism used by cells to avoid wasting resources.
Industrial Strategy: Select or engineer feedback-resistant mutant strains. For example, in lysine-producing strains, the key enzyme can be modified so that it is no longer inhibited by lysine, allowing the microorganism to continuously synthesize and accumulate large amounts of lysine.
✔ 2. Regulation of Enzyme Synthesis — “Hiring and Firing” the “Workers”
Catabolite Repression: When a readily metabolized carbon source such as glucose is available, cells preferentially utilize it and repress the synthesis of enzyme systems required for the utilization of other carbon sources, such as lactose. This phenomenon is known as the “glucose effect.”
Industrial Strategy: Use fed-batch fermentation to slowly feed glucose into the bioreactor, maintaining its concentration at a relatively low level. This relieves catabolite repression and allows metabolism to continue.
Induction and Repression: Some enzymes are induced only when their substrates are present, whereas the synthesis of certain enzymes can be repressed when the end product accumulates excessively.
Industrial Strategy: Add specific inducers to the culture medium or control nutritional conditions to prevent premature accumulation of end products.
✔ 3. Regulation of Cell Membrane Permeability — The “Gate” for “Product Release”
For products such as glutamate, high production requires not only strong biosynthetic capacity but also efficient secretion of the product into the extracellular environment. Glutamate-producing strains are typically biotin-limited.
Principle: Biotin is an important cofactor involved in the synthesis of membrane phospholipids. When the biotin supply is maintained at a suboptimal level, phospholipid synthesis becomes insufficient, resulting in changes in membrane structure and increased permeability. This facilitates the “leakage” and accumulation of glutamate outside the cell.
Industrial Control: During glutamate fermentation, the biotin concentration in the culture medium can be precisely controlled. Alternatively, surfactants such as Tween 60 or penicillin can be added during the middle or late stages of fermentation to artificially regulate cell membrane permeability and promote glutamate production.
IV. Examples of Typical Fermentation Metabolic Processes
Case 1: Yeast Alcoholic Fermentation — Growth-Coupled Fermentation
Pathway: Glucose is converted into pyruvate through the EMP pathway. Under anaerobic conditions, pyruvate is converted into ethanol and CO₂ through reactions catalyzed by pyruvate decarboxylase and alcohol dehydrogenase.
Characteristics: The formation of ethanol is directly coupled with microbial growth and energy metabolism. The higher the specific growth rate (μ), the faster the rate of ethanol production.
Case 2: Glutamate Fermentation — Partially Growth-Coupled Fermentation
Pathway: Glucose is metabolized through the EMP pathway and TCA cycle to generate the key intermediate α-ketoglutarate. Catalyzed by glutamate dehydrogenase, α-ketoglutarate reacts with ammonia and undergoes reductive amination to form glutamate.
Key Regulatory Strategies:
1. Block the Side Pathway:
Select strains with weak α-ketoglutarate dehydrogenase activity to prevent further oxidation of α-ketoglutarate, effectively “blocking” it at the glutamate biosynthesis node.
2. Strengthen the Main Pathway:
Ensure high glutamate dehydrogenase activity.
3. Promote Product Secretion:
As described above, maintain biotin at a suboptimal level or add surfactants to alter membrane permeability and promote glutamate secretion.
Characteristics: Cell growth and product synthesis are partially coupled. During the early stage of fermentation, conditions primarily support biomass growth. During the middle and later stages, process conditions—such as increasing dissolved oxygen and adjusting pH—are controlled to redirect metabolic flux toward the high-level synthesis of glutamate.
Case 3: Penicillin Fermentation — Non-Growth-Coupled Fermentation
Characteristics: The synthesis of secondary metabolites such as antibiotics is not directly associated with biomass growth. Significant production generally begins during the stationary phase, after microbial growth has largely ceased.
Process Control: The fermentation process is clearly divided into a “biomass growth phase” and a “product synthesis phase,” with different temperatures, pH values, and feeding strategies used during the two stages. For example, the temperature may be slightly higher during the growth phase (~30°C) and lower during the antibiotic production phase (~25°C). Slow feeding of the precursor phenylacetic acid is also required.
V. Process Control: Creating the Optimal Environment for the “Metabolic Factory”
Once the underlying metabolic mechanisms are understood, external process control has clear objectives:
Temperature: Temperature affects enzyme activity and metabolic pathways. For example, during the early stage of glutamate fermentation, 30–32°C is favorable for microbial growth, while 32–36°C during the later stage favors the activity of enzymes involved in acid production.
pH: pH affects membrane properties, enzyme activity, and substrate uptake. Different stages require different pH conditions. For example, the optimal pH for penicillin synthesis is approximately 6.2–6.8.
Dissolved Oxygen (DO): DO is the lifeline of aerobic fermentation. Aeration and agitation must be adjusted in real time according to biomass concentration and metabolic activity to ensure that DO remains above the critical oxygen concentration.
Feeding Strategy: This is a core process-control method. By continuously feeding carbon sources, nitrogen sources, precursors, and other nutrients, substrate inhibition or repression can be avoided, while metabolic flux is continuously “directed” toward product synthesis.
Conclusion: From the “Realm of Necessity” to the “Realm of Freedom”
The principles of fermentation represent a fascinating interplay with the intrinsic metabolic regulation of microbial cells. From the initial passive utilization of natural fermentation, to the active regulation of primary metabolism, and then to the precise manipulation of secondary metabolism and the utilization of genetically engineered microorganisms, human understanding and control of fermentation metabolism have continued to deepen.
Modern fermentation engineering is now integrating systems biology, metabolic engineering, and Process Analytical Technology (PAT). It not only enables real-time monitoring of metabolic flux—for example, through Oxygen Uptake Rate (OUR) and Carbon Dioxide Evolution Rate (CER)—but also uses model-based prediction and intelligent control to dynamically optimize metabolic pathways. In this way, the microbial “cell factory” can produce the products we need with maximum efficiency and highly specific metabolic direction.
This marks the transformation of the fermentation industry from an experience-dependent “realm of necessity” toward a precisely designed “realm of freedom.”
Post time: Sep-10-2026