The rate of biomass production is optimised for a predefined feed exhaustion using the residue ratio as a degree of freedom. Three modes of operation are considered: continuous, repeated batch, and repeated fed-batch operation. By means of the Production Curve, the transition points of the optimal modes of operation are derived. The analytical expressions of these transitions for variable bioreaction kinetic parameters are determined. The key measures "degree of difficulty of conversion" and "degree of exhaustion" are introduced to define the optimal modes in more general terms. The "degree of difficulty" describes the effect of the kinetic parameters and the feed substrate concentration on the conversion; the "degree of exhaustion" describes the desired final condition. In fed-batch operation, the proposed constant feed policy approximates the optimal feed policy closely.