SELF-CYCLING FERMENTATION IN A STIRRED-TANK REACTOR

Self-cycling fermentations (SCFs) were conducted in a stirred tank app aratus using Bacillus subtilis and Acinetobacter calcoaceticus. The sy stems were very stable and the experiments lasted through many cycles. The variation of parameters such as biomass and doubling time from cy cle to cycle was small. The stirred tank reactor (STR) allowed a much better control of the working volume in the fermentor from cycle to cy cle, compared to the cyclone column, and it was not necessary to make periodic corrections. The production of surfactin from B. subtilis was achieved without extending the cycle time. The harvested broth at the end of each cycle was allowed to remain in a secondary vessel, at amb ient temperature, before being collected. It is exhaustion of the limi ting nutrient which causes an increase in dissolved oxygen (DO). At th is point, the computer, which constantly monitors the DO, triggered th e harvesting sequence to end the cycle. Thus, the mature culture in th e secondary vessel experienced appropriate conditions for the producti on of the secondary metabolite. Meanwhile, the next batch of cells was being grown in the primary reactor. The response of a gas analyzer on the effluent paralleled that of the DO measurements in the fermentor. These data for oxygen and carbon dioxide exhibited less noise than th e DO readings. Either would be a more reliable parameter for feedback control of the SCF because the problem of fouling of the DO probe afte r extended runs of many cycles would be eliminated. (C) 1993 John Wile y & Sons, Inc.