Batch fermentations involve inoculating a growth medium and obtaining the product once all growth has ceased. Growth will terminate when all the nutrients in the growth media have been exhausted.
Continuous fermentation involves maintaining a constant culture density (by measuring the OD), and adding new growth media at the same rate as removing old media. Although continuous fermentation allows more microorganisms to be grown than batch fermentation, the culture density will remain low.
Fed-batch fermentations have nutrients added to the growth media during the fermentation process. This ensures that amino acids and other important molecules never run out during the fermentation process – helping to maintain a high culture density. An inducer molecule can be added when a defined culture density is reached to cause product synthesis, allowing rapid growth before redirecting metabolic flux towards product synthesis. Having the ability to shift the metabolic flux allows for higher product yield, reducing energy wasted on growth.
Product synthesis is highest in fed-batch fermentation, and lowest in batch fermentation. This is due to the inability to change the growth media composition in batch fermentations.
Bacterial growth will be highest in continuous fermentation, as it is possible to maintain log phase growth throughout the fermentation process. This is not possible in batch or fed-batch fermentation.
Due to the ability to change the growth media composition during fed-batch fermentation, it is possible to redirect metabolic flux, increasing the efficiency and yield of the fermentation.
Although these reactions are called fermentations, they can be aerobic if required.