Modern Industrial Microbiology and Biotechnology

202 Modern Industrial Microbiology and Biotechnologycheese making, continuous yoghurt starter production and continuous use of lactore inwhey are being vigorously pursued. Medical and veterinary applications include thecontinuous production of vaccines, and cell cultivation.Continuous waste digestion for sewage chemical wastes outside the activated sludgeexist as also do the continuous brewing of beer, the continuous production of wine andthe continuous manufacture of yeasts, vinegar and alcohol.9.5 FED-BATCH CULTIVATIONFed-batch cultivation is a modification of batch cultivation in which the nutrient is addedintermittently to a batch culture. It was developed out of cultivation of yeasts on malt,where it was noticed that too high a malt concentration lead to excessively high yeastgrowth leading to anaerobic conditions and the production of ethanol instead of yeast cells.After its successful introduction in yeast cultivation, the original method ormodifications of it have been used to achieve higher yields or more efficient mediautilization in the production of various antibiotics, amino acids, vitamins, glycerol,acetone, butanol, and lactic acid. Some of the modifications include continuous (ratherthan intermittent) addition of single or multiple media components, withdrawal of aportion of the broth from the growth vessel and immediate dilution of the residue withfresh medium and the use of diffusion capsules. The latter are cylindrical capsules to oneend of which a semi-permeable membrane is fixed. The nutrient diffuses slowly outthrough the membrane into the medium.9.6 DESIGN OF NEW FERMENTORS ON THE BASIS OFPHYSIOLOGY OF THE ORGANISMS: AIR LIFTFERMENTORSThe Stirred Tank Batch Fermentor already described is the most widely used type offermentor. Increasing knowledge of the physiology of industrial microorganisms andbetter instrumentation have provided the bases for more efficient manipulation of theorganisms in the existing batch fermentors:(i) More sophisticated instrumentation is now used to monitor such fermentorparameters as dissolved oxygen and carbon dioxide, redox potential, and controlof the fermentation leading to higher yields.(ii) Different levels of pH, temperature, and phosphate concentration are sometimesneeded during the trophophase and the idiophase for the production of secondarymetabolites. These differences have been exploited in some fermentations forhigher yields.(iii) By careful monitoring using automated sensing devices, it is now possible to addjust enough of the nutrients required by a growing culture so that feedbackinhibition is avoided.The above are a few examples to show that the existing fermentors can be betterutilized when greater knowledge of microbial physiology is harnessed for that purpose.Despite these improvements, needs have arisen for drastic change from the typical stirredtank batch fermentor, and these needs would appear not to be fully met by automation ofbatch fermentors. Some of the needs call for the design of new fermentors based on thefollowing: