Prediction of costs in continuous fermentations

From a development of the theory of the cost of continuous fermentations, the unit cost of any given product (e.g. biomass, primary or secondary metabolites) as a function of time can be predicted. For any given system there is a particular value of the dilution rate at which the unit cost of product will not change with fermentation time. At dilution rates higher than the critical one the unit cost of growth-linked products (e.g. biomass) will decrease with time, while that of non-growth-linked products (e.g. penicillin) will increase with time. The converse will happen at lower dilution rates. In all cases the unit cost will tend towards a finite, fixed value. Two numerical examples are presented and the relevance of these findings to the problem of estimating a priori the convenience of a continuous fermentation process is discussed.