9.6.2.1 Conventional Batch Fermentation

Batch cultures are simple, closed systems. In this system, all the substrates are added at the beginning, before inoculation, and neither anything is added or taken out during the fermentation. A typical growth curve is followed by the organism (Fig. 9.12a) in this type of fermentation. In industrial processes, generally, the actively growing inoculum is added to avoid any lag phase as it leads to the wastage of time (Fig. 9.12b) The batch fermentation has certain limitations like exhausting of nutrients, accumulation of antagonists, product inhibition, etc. which eventually affects the product formation.

The product is recovered at the end of the growth phase. This involves emptying the fermenter out and processing the medium to get the product out. The fermenter has to be cleaned, refilled, resterilized, and then, reinoculated. Such operations are called turn-round and the time it takes to do it is called down time.

Figure 9.13 depicts the batch fermentation equipment layout incorporating heat exchangers and chemical sterilization systems. Most of the currently practiced alcohol fermentations are based on the traditional processes described above. But many advanced methods have been developed in order to increase the produc­tivity, reduce the capital investment, and better utilization of energy. Such advances are the use of continuous fermentations, the increase of yeast population by recycling, and the removal of ethanol during fermentations.

Shared heat exchanger

cooling bank

Fig. 9.13 Batch fermentation equipment layout incorporating teamed heat exchanger and chemical sterilization systems. Source [52]

9.6.2.2 Continuous Fermentation

In continuous fermentation, fresh medium is continuously pumped into the fer­menter and an equal volume of the fermented liquid is continuously pumped out for recovery of ethanol and yeast. This is an open system. The rate at which medium is added or at which the fermenter liquid is withdrawn is expressed as the dilution rate D which is the ratio of withdrawn liquid (F) to the volume of total liquid in the fermenter (V) i. e. D = F/V (Units of D are h-1).

Feed is pumped continuously into the fermenter displacing beer which then overflows from the vessel. The composition of the produced beer is the same as the composition inside the fermenter. Therefore, the fermenter is to run at a relatively slow rate to obtain a higher concentration of alcohol because it will allow complete utilization of sugar and growth of new yeast cells in the fermenter to replace

washed out cells [139]. Stirring is an important factor for successful continuous flow fermentation. The modification of the continuous fermentation process is the Biostil process (Fig. 9.14).