Surface fermentation is easy to control and to implement. It needs no aeration or agitation of the fermentation broth, so it needs no instrumentation for aeration and agitation. The separation of citric acid from the mycelium is easy because the microorganism is not dispersing into the medium (151). Only the temperature and humidity of the fermentation chamber need controlling. It can be used easily in small plants as well as in third world countries. With surface fermentation, the fermentation broth is concentrated due to a high evaporation rate during fermentation. Thus, expenses and losses during recovery and purification are low (152). However, surface fermentation has the following disadvantages: Building investment costs are high. Personnel expenses are high in developed industrial countries with extremely high wages. Fermentation time is long and therefore productivity is low (152).
Submerged fermentation is favored over surface fermentation for the following reasons: lower total investment costs; higher yields of citric acid; improved process control; reduced fermentation time; reduced floor space requirements; lower labor costs; simpler operations; and easier maintenance of aseptic conditions on an industrial scale (1,4,152). However, submerged fermentation has some disadvantages compared to surface fermentation: expenses for equipment are higher; consumption of electrical energy is higher; and the process is very sensitive to short interruptions or breakdowns in aeration and vulnerable to infections, which result not only in losses of yield, but also in a total breakdown of respective batches.
Shierholt (152) concludes that surface fermentation is theoretically superior to submerged fermentation. Some essential criteria for industrial management of these two processes are ground requirements as well as the number of and costs for assistants to be employed.
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