The design of continuous sterilisation networks involves the selection of design variables, such as the sterilisation temperature, heat exchanger sizes, etc., to-satisfy the prime objective of destroying sufficient micro-organisms in the fermentation medium. Secondary objectives include minimising nutrient degradation and minimising annual costs. Common design practices for continuous sterilisation networks rely on simple heuristics which typically do not account for any suspended solids and do not guarantee cost optimality. In this paper, an optimal design procedure is formulated which guarantees sterility both in the liquid and solid phases, whilst avoiding excessive degradation of any labile nutrients present, and takes account of capital and operating costs. The results of four optimal design problems are presented and compared with a typical. heuristic design.