Aerobic Fermentation Technology: Principles, Applications, and Future Trends

What is Aerobic Fermentation?

Aerobic fermentation refers to a metabolic process in which microorganisms convert organic compounds, such as carbohydrates, into valuable products in the presence of oxygen. Unlike anaerobic fermentation, which occurs in the absence of oxygen, aerobic fermentation relies on the supply of oxygen to sustain the growth of microorganisms.

Aerobic Fermentation Principle

Aerobic fermentation is a metabolic process in which microorganisms such as bacteria or yeast use oxygen to convert organic substances like sugars into energy, producing carbon dioxide, water, and a large amount of ATP (adenosine triphosphate) as metabolic products. This process mainly occurs in the mitochondria of cells. For example, the reaction C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 30 (32)ATP represents the aerobic respiration of glucose by yeast under aerobic conditions, generating carbon dioxide and water while releasing a large amount of energy.

Aerobic Fermentation Process

Microorganisms must receive adequate oxygen during aerobic fermentation which is achieved by performing the process in well-aerated fermenters. The initial stage of raw material processing includes pretreatment procedures like crushing and sterilization before they are combined with appropriate microbial strains for fermenter loading. The fermentation process requires ongoing delivery of sterile air and precise control of temperature and pH levels within the fermenter to sustain microbial activity and fermentation performance. When oxygen levels are adequate microorganisms break down the organic substances in raw materials transforming them into target products and heat energy. The final product becomes available after applying separation and purification steps to the fermented material.

Aerobic fermentation mainly involves the following steps:

1. Pretreatment

Raw Material Selection and Pretreatment: Choose proper raw materials that match fermentation objectives by selecting starchy or sugary substrates. Crushing or grinding raw materials increases their surface area which supports better dissolution, absorption, and microbial metabolism. Corn flour requires a crushing process before it can be utilized in alcohol fermentation.

Sterilization: The pretreated raw materials and fermentation equipment require sterilization to remove any contaminating microorganisms that might compete with fermenting microbes for nutrients or generate harmful substances. The typical method of sterilization involves high-pressure steam treatment where both the fermenter and culture medium reach 121°C for a duration of 15 to 30 minutes.

2. Fermentation Process

Inoculation: Add the chosen high-quality cultured microbial strains to the sterilized fermentation medium. The volume of inoculum added to the fermentation process needs to fall within the range of 5% to 20% for optimal fermentation initiation. Lactic acid bacteria serve as inoculants during the production of yogurt.

Aeration and Agitation: The fermentation process requires sterile air to be pumped into the fermenter which supplies the necessary oxygen for microbial growth and metabolic functions. Agitation facilitates the complete mixing of microorganisms, nutrients, and oxygen within the fermentation broth which results in uniform and effective fermentation. The optimization of the aeration rate and agitation speed requires consideration of the specific features of fermenting microorganisms and the current stage of fermentation progress.

Fermentation Condition Control: Maintain precise regulation of temperature and pH throughout the fermentation procedure. Each microorganism requires unique parameters to thrive in fermentation. Yeast produces ethanol most effectively within a temperature span of 28°C to 32°C while maintaining a pH level between 4 and 6. Adjustments to the conditions must be made promptly by following the microbial growth curve and metabolic characteristics.

3. Post-Processing

Fermentation Termination: Terminate fermentation immediately once it achieves its predetermined endpoint through reaching the target product concentration or experiencing microbial metabolic activity decline. Fermentation can be terminated by utilizing heating procedures, cooling methods, or altering the pH levels.

Separation and Purification: The fermentation product undergoes separation from the fermentation broth followed by purification. The initial removal of microbial cells and impurities through filtration or centrifugation precedes purification processes like distillation, extraction, and chromatography that produce high-quality target compounds.

Fermentation Condition Control

Dissolved Oxygen Concentration: During the fermentation process, the dissolved oxygen concentration must be regulated based on the type and metabolic characteristics of the microorganism. This is achieved by adjusting the aeration rate and stirring speed. For example, during glutamic acid fermentation, sufficient oxygen must be provided to meet the respiratory needs of the cells during the growth phase, while even higher oxygen levels are required during the production phase to support the cells' maximum respiratory intensity.

Temperature Control: Different microorganisms have distinct optimal temperature ranges for growth. For instance, the optimal temperature for the growth of penicillin-producing mycelia is 24°C–26°C, while that for erythromycin-producing mycelia is around 28°C. Throughout the fermentation process, temperature monitoring equipment is used to continuously track temperature changes, and heating or cooling systems are employed to maintain the desired temperature.

pH Control: Microbial growth and metabolism are highly sensitive to environmental pH. Different microorganisms and fermentation stages require different pH levels. For example, the optimal pH for mycelial growth in penicillin fermentation is 6.0–6.5, while for erythromycin fermentation it is 7.0–7.5. pH can be adjusted by adding acidic or alkaline substances or by using a buffering system to stabilize the fermentation broth's pH.

Aeration Control: The aeration rate affects both the dissolved oxygen concentration in the fermentation broth and the respiration of the microorganisms. Aeration should be adjusted according to the microorganism's oxygen requirements and the progress of fermentation to ensure a sufficient oxygen supply. Generally, during periods of vigorous metabolic activity, the aeration rate needs to be increased.

Stirring Speed Control: Stirring promotes the even distribution of microorganisms, nutrients, and oxygen in the fermentation broth, thereby enhancing mass and heat transfer. However, excessively high stirring speeds may generate shear forces that damage the microorganisms, while too low speeds can result in poor mixing. Therefore, the stirring speed and the type of impeller should be selected according to the characteristics of the fermentation system and the microorganism's tolerance.

Commonly Used Equipment

Fermenter (Bioreactor): The fermenter is the core equipment for aerobic fermentation, consisting of the vessel, stirring system, aeration system, temperature control system, and monitoring and control system. The vessel is typically made of stainless steel or fiberglass, offering good corrosion resistance and sealing performance, and capable of withstanding pressure and temperature variations. The stirring system ensures thorough mixing of the fermentation broth; the aeration system supplies oxygen to the microorganisms; the temperature control system maintains a stable fermentation temperature; and the monitoring and control system tracks and regulates various fermentation parameters in real time.

Aeration Equipment: This includes air compressors, filters, flow meters, etc., and is used to introduce sterile air into the fermenter to meet the oxygen demand of the microorganisms. The dissolved oxygen concentration can be regulated by adjusting the aeration rate.

Agitation Equipment: Common types of impellers include turbine-type and propeller-type. Their shape and design can be selected based on the properties of the fermentation broth and the requirements of the fermentation process. Stirring speed can be adjusted via the control system to meet the needs of different fermentation stages.

Temperature Control Equipment: This includes steam heating systems, electric heating systems, jacketed cooling systems, and coil-type cooling systems, all used to maintain suitable fermentation temperatures. Temperature sensors continuously monitor the fermentation broth temperature, and the control system automatically adjusts the heating or cooling power as needed.

Monitoring and Control System: This system can continuously monitor parameters such as temperature, pH, dissolved oxygen, stirring speed, and pressure during fermentation. It can automatically adjust fermentation conditions according to preset programs, ensuring a stable fermentation process and improving efficiency and product quality.

Aerobic Fermentation Products

Aerobic fermentation yields a wide variety of products and is widely applied across multiple fields. Below are some common products derived from aerobic fermentation:

Food Industry

Flavor Enhancers: Monosodium glutamate (MSG), produced via glutamic acid fermentation, is one of the most common flavor enhancers. Citric acid, produced through fermentation by Aspergillus niger, is used as a souring agent and chelating agent in beverages, candies, and other foods.

Alcoholic Beverages: During beer brewing, yeast undergoes aerobic fermentation first to rapidly multiply, preparing for subsequent anaerobic fermentation to produce alcohol.

Bread: In bread-making, yeast performs aerobic respiration in the dough, producing carbon dioxide that causes the dough to rise and improves the bread's flavor and texture.

Pharmaceutical Industry

Antibiotics: Antibiotics such as streptomycin and azithromycin are produced via fermentation using various actinomycetes.

Vitamins: Aerobic fermentation is used to produce various vitamins, including B vitamins and vitamin C, for pharmaceutical and nutraceutical applications.

Bioactive Compounds: Enzymes, hormones, and other bioactive substances extracted via solid-state aerobic fermentation can be used in the production of health supplements.

Chemical Industry

Organic Acids: Besides citric acid, other organic acids like lactic acid and malic acid can also be produced. These are used in food, cosmetics, and pharmaceuticals.

Amino Acids: Amino acids such as lysine and glutamic acid are key ingredients in food additives and are also widely used in pharmaceuticals and animal feed.

Polysaccharides: Hyaluronic acid, produced by microbial aerobic fermentation, is a crucial cosmetic ingredient with excellent moisturizing properties and is also used in medical applications.

Energy Industry

Biofuels: Certain microorganisms can convert biomass into bioethanol and other biofuels through aerobic fermentation.

Environmental Protection

Organic Fertilizers: Aerobic fermentation of organic waste, such as livestock manure and crop straw, can produce nutrient-rich organic fertilizers that improve soil fertility and reduce environmental pollution.

Cosmetic Industry

Natural Active Ingredients: Collagen, hyaluronic acid, and similar compounds can be extracted via solid-state aerobic fermentation for use in skincare and makeup products, offering excellent biocompatibility and bioactivity.

What is the Difference Between Aerobic and Anaerobic Fermentation?

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