Prior to presenting the principles of fed-batch operations, it is instructive to review quickly various forms of idealized reactors: batch, continuous, and semi-batch reactors. This will help in understanding the simplest form of fed-batch operation as equivalent to a continuous-stirred tank reactor (CSTR) followed by a batch reactor (BR). It becomes simple and easy to understand the operation of a fed-batch bioreactor as mimicking a dynamic CSTR followed by a BR to maximize the reaction rate or product yield. In other words, for a single reaction, the sufficient condition for superior performance of fed-batch operation is that the reaction rate and/or product yield show a maximum or decrease with the substrate concentration.

Basically, a fed-batch operation is preferred when the main or a side reaction rate exhibits a maximum or if the side reaction is more sensitive than the main reaction to reactant concentrations. Fed-batch operations can take advantage of the maximum rate or the sensitivity of side reactions (yield) by manipulating the substrate (reactant) concentration in the reactor.