Brent Cornell

    
Batch Culture

• Fermentation is carried out in a closed fermenter, with nothing added or removed during the process (except venting of gas)
• Microorganisms and nutrients are left for a set period of time, during which the nutrient stock is depleted
• The advantage of a batch culture is that the fermenter can be used for different reactions with each separate use
• A disadvantage of a batch culture is that it results in significant periods of idle time between use, resulting in higher costs

Continuous Culture

• Fermentation is carried out in an open fermenter, with nutrients added and product removed at a steady rate throughout
• This results in a continuous reaction with no idle time, reducing labour costs and increasing product yields
• A disadvantage of continuous culture is that there is a higher risk of contamination due to the constant adjustments
• Continuous fermentation is feasible only when the inoculated cells are genetically stable

Batch Culture Versus Continuous Culture

    
The antibiotic penicillin can be mass produced via the use of deep-tank batch fermentation

• Large industrial fermenters are constructed that have the capacity to hold thousands of litres
• Penicillium mold is grown in the deep-tank batch fermenters following the addition of sugars and other key ingredients
• The production process typically lasts 6 – 8 days, with the fermenter drained at the end of the fermentation cycle
• Penicillin is separated from the solution and purified via downstream processing to improve its antibiotic potential

Production of Penicillin via Batch Fermentation

    
Citric acid (citrate) is widely used as a flavour enhancer, a preservative in manufactured foods and an antioxidant

• It is produced as an intermediate of the Krebs cycle under aerobic conditions

Citric acid is mass produced by continuous fermenter systems from cultures of the fungus Aspergillus niger

• Carbohydrates are continuously introduced into the fermenter in order to maintain the citric acid production
• Iron (Fe²⁺ ions) is excluded from the mixture in order to slow the further conversion of citric acid within the Krebs cycle
• As citric acid accumulates it is extracted as part of the medium that is being continuously withdrawn from the fermenter

Production of Citric Acid via Continuous Fermentation