The Holding Time in Pure and Mixed Culture Fermentations

Continuous mixed culture fermentations have been studied in the continuous-stirred tank reactor. The residence or holding time, theta, is important in determining which of two mixed organisms shall dominate in numbers. Continuous ethanol and acetic-acid fermentations are known to the brewing industry. The continuous production of ethanol and acetic acid are contingent upon the cells of Saccharomyces and Acetobacter being alive and growing. These are known as growth-associated products. On the other hand, alpha-amylase and glucamylase, or fungal amylase, are known as nongrowth-associated products or secondary metabolites. The organisms that produce the secondary metabolites, for example, penicillin, cephalosporin, streptomycin, and aureomycin, undergo differentiation and growth. These are the higher microbial forms and are produced by batchwise fermentation of molds and actinomycetes. It is submitted that these higher microbial forms can be grown continuously and produce secondary metabolites in amounts so as to make the processes economically viable. It is possible to grow the organisms continuously in plug flow reactors so that the secondary metabolites that have hitherto not been considered for continuous production be so considered.