Submerged Fermentation: Principles, Technologies, and Applications Across Industries

Submerged Fermentation (SmF) Definition

Submerged fermentation (SmF), also known as submerged liquid fermentation and liquid state fermentation, has been widely used to produce a range of metabolites. In this particular fermentation process, microorganisms are submerged in a solution containing nutrients required for growth. The choice of carbon and nitrogen sources includes monosaccharides, disaccharides, polysaccharides, cereals, legumes, bran, peptone, etc. Submerged fermenters can be classified as aerated or anaerobic and batch or continuous. The most common type of submerged fermentation involves agitation-aeration.

It is well known that submerged fermentation has been widely used for enzyme production. Since most industrial enzymes are secreted by cells into the external environment and remain in fermentation broth after the removal of biomass, thus enzyme purification can be carried out in a simple manner. The product is obtained directly from the fermenter without the need for cell disruption operations, and recovery of the desired molecules is achieved by filtration, centrifugation, or chromatography purification of the product.

Submerged fermentation Types

Classification by Mode of Operation

Batch Fermentation: Nutrients and microorganisms are added to the fermenter all at once for cultivation. No material exchange occurs with the external environment during fermentation, except for air input and exhaust gas output. Its main features include simple operation, continuous depletion of nutrients, continuous microbial growth, and a non-steady-state environment. The four stages of microbial growth are clearly defined, making this method widely applicable.

Continuous Fermentation: Fresh culture medium is continuously added to the fermenter at a constant rate, while the same volume of culture broth is simultaneously removed, keeping the liquid volume inside the fermenter constant. Microorganisms grow under stable conditions. This method can effectively prolong the logarithmic growth phase observed in batch fermentation, offers operational stability, facilitates automation, improves equipment utilization, reduces sterilization frequency, and supports process optimization. However, it is more susceptible to contamination and microbial mutation, has limited adaptability for different product types, and requires high standards for equipment and accessories.

Fed-Batch Fermentation: Also known as semi-continuous fermentation, it lies between batch and continuous fermentation. During batch fermentation, specific materials are intermittently added to the culture system. This approach prevents nutrient depletion during the later stages of microbial growth caused by excessive initial feeding, and avoids early entry into the decline phase due to exhaustion of nutrients. It effectively extends the fermentation cycle and improves product yield.

Classification by Aeration Conditions

Aerobic Submerged Fermentation: Oxygen is continuously supplied to the fermentation system to support the growth, reproduction, and metabolic activity of aerobic microorganisms in an oxygen-rich environment. This method is commonly used in the production of penicillin, citric acid, and similar products.

Anaerobic Submerged Fermentation: Fermentation is carried out in the absence of air using facultative or obligate anaerobic microorganisms. Biochemical degradation occurs under oxygen-free conditions, leading to the production of desired products. Examples include the production of acetone and butanol by Clostridium species, and biogas (methane) fermentation.

Facultative Anaerobic Submerged Fermentation: Fermentation is carried out using facultative anaerobic microorganisms, which can grow in both aerobic and anaerobic environments. Fermentation conditions can be flexibly adjusted according to the oxygen supply and the physiological characteristics of the microorganisms to produce different products.

Classification by Fermentation Equipment

Mechanically Agitated and Aerated Fermenter (Submerged Fermentation): Mechanical stirring ensures thorough mixing of the fermentation broth and effective aeration, suitable for the cultivation of most microorganisms and the production of metabolic products. It enhances oxygen transfer and mixing efficiency, ensuring a uniform microbial growth environment during fermentation. However, the equipment is complex and energy consumption is relatively high, making it more suitable for large-scale industrial production.

Air-Lift and Agitation Fermenter (Submerged Fermentation): Relying on the rising force and agitation generated by the injected air in the fermentation broth, this method promotes liquid circulation and facilitates oxygen transfer and mixing. It features a simpler structure, easier operation, and lower energy consumption compared to mechanical agitation fermenters. However, its mixing and oxygen transfer efficiency are slightly lower, making it suitable for fermentation processes with lower requirements for mixing and oxygen supply.

Methods of Submerged Fermentation (SmF)

Batch Submerged Fermentation: Microorganisms are inoculated into a fermenter containing liquid culture medium, and aeration and agitation are carried out under specific conditions. After a certain period, the entire fermentation broth is withdrawn at once to extract the product. This method is commonly used for processes such as alcohol fermentation and acetic acid fermentation.

Continuous Submerged Fermentation: The fermentation broth continuously flows through the fermenter while fresh culture medium is constantly replenished, allowing microorganisms to maintain a stable growth state and achieve consistent fermentation yields. This method can be used in the production of certain amino acids.

Fed-Batch Submerged Fermentation: A small amount of culture medium is initially added to the fermenter. As the microorganisms grow, fresh medium is gradually added, ensuring that the microorganisms receive sufficient nutrients throughout the fermentation process and maintain prolonged growth and metabolism.

Advantages of Submerged Fermentation (SmF)

Wide range of substrates: Submerged fermentation can use for industrial starch, soybean meal, bran, and other low-cost industrial and agricultural products, as well as corn deep processing wastewater and starch wastewater as substrates.

Short production cycle: It is able to achieve optimal culture conditions by controlling aeration and agitation, temperature, pH, and medium fed in the bioreactor and executing the production of bioactive secondary metabolites in a shorter time period.

Various fermented products: In the process of submerged fermentation, a variety of bioactive molecules can be produced from the fermentation broth, including peptides, nucleic acids, amino acids, polysaccharides, enzymes, terpenoids, and sterols.

Disadvantages of Submerged Fermentation (SmF)

High Equipment Investment: Requires complex equipment such as fermenters, aeration systems, agitation systems, and temperature control systems, resulting in high capital costs.

High Energy Consumption: The fermentation process demands continuous aeration, agitation, and maintenance of optimal temperature and pressure, leading to significant energy use.

Strict Sterility Requirements: The entire fermentation must be carried out under sterile conditions. High sterility standards are required for media, equipment, and operating environments. Any contamination can lead to batch failure, increasing production risks and costs.

Complex Downstream Processing: After fermentation, the separation, purification, and waste treatment processes are relatively complicated and costly.

High Personnel Requirements: Operation and management require skilled professionals with specialized technical expertise and training.

Foam Generation: Fermentation may produce excessive foam, necessitating the use of defoaming agents or additional measures, which adds to production costs and operational complexity.

Applications of Submerged Fermentation (SmF)

Submerged fermentation process can be used for industrial-scale manufacturing of products such as citric acid, glycerol, or other valuable metabolites that can be processed into nutritional products, food additives, beverages, etc. Submerged fermentation has been widely used for the production of industrial enzymes. It commonly applies bacteria and fungi to produce protein biomass of single-cell proteins.

Pharmaceutical Industry

Antibiotic Production: Antibiotics such as penicillin and streptomycin are produced using submerged fermentation technology, which enables large-scale cultivation of antibiotic-producing microorganisms and improves yield.

Vitamin Production: The production of certain vitamins, such as vitamin B12, also relies on submerged fermentation techniques.

Amino Acid Production: Amino acids like glutamic acid are synthesized by microorganisms through submerged fermentation.

Enzyme Production: Enzymes such as amylase and protease are produced by microbial synthesis and secretion through submerged fermentation.

Food Industry

Alcoholic Beverage Production: The production of beer, wine, and other alcoholic beverages involves submerged fermentation by yeast to convert sugars into ethanol.

Organic Acid Production: Organic acids such as citric acid and lactic acid are synthesized by microorganisms through submerged fermentation.

Soy Sauce and MSG Production: In soy sauce production, microorganisms like Aspergillus oryzae decompose soybean proteins via submerged fermentation. In MSG (monosodium glutamate) production, glutamic acid is synthesized by microbes through submerged fermentation.

Agriculture and Environmental Protection

Biofertilizer Production: Biofertilizers like nitrogen-fixing agents and phosphate-solubilizing bacteria are produced by culturing microorganisms via submerged fermentation to create fertilizers with specific effects.

Biopesticide Production: Microbial pesticides such as Bacillus thuringiensis are produced through submerged fermentation, allowing large-scale propagation for pest and disease control.

Organic Waste Treatment: Submerged fermentation technology is used to treat organic waste such as livestock manure and crop straw, producing biogas and biofertilizers, promoting waste resource utilization, and reducing environmental pollution.

Energy Sector

Bioethanol Production: Raw materials like corn and sugarcane are fermented by microorganisms through submerged fermentation to convert sugars or starches into ethanol for use as biofuel.

Biohydrogen Production: Certain microorganisms can decompose organic matter under specific conditions through submerged fermentation to produce hydrogen gas, a clean energy source.

Biodiesel Production: While biodiesel is mainly produced via esterification of fats and alcohols, some microorganisms can synthesize lipids through submerged fermentation to serve as raw materials for biodiesel production.

What is the Difference Between Submerged Fermentation and Solid State Fermentation?

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