Fermentation can be done using three different methods:
(i) Batch culture : In the batch culture, the conditions are set up and not changed from outside once the fermentation starts; for example, no more nutrients are added. This is why the process is described as a closed system. The process is stopped once sufficient product has been formed. The contents of the fermenter are then removed, the product isolated, the microorganism discarded and the fermenter is cleaned and set up for a fresh batch.
(ii) Continuous culture : Continuous culture involves continuous long-term operation over many weeks, during which nutrient medium is added as fast as it is used, and the overflow is harvested. Although this is closer to the situation in natural environments, such as the gut, continuous culture has found only limited application. However, it is used for the production of single cell protein where a large biomass of cells is required.
(iii) Fed-batch culture : This is a compromise between the two systems. With fed-batch culture more control of the process is involved than with batch culture. The ** of growth is extended by adding nutrients at low concentrations during the fermentation, and not adding all the nutrients at the beginning. One advantage of this is that the rate of growth can be regulated to keep pace with the rate at which oxygen can be supplied. The process is commonly used to produce yeast cells for the baking industry
