To optimise the performance of a recombinant fermentation under realistic conditions, it is important to understand how it is affected by non-idealities. This study proposes a fractal approach. From the time-domain profiles of key concentrations in a feed-batch fermentation, fractal dimensions have been computed. They serve as indicators of the effects of imperfect mixing and disturbances. Fractal dimensions are applicable to both intra- and extra-cellular variables, and include information on the internal metabolism and its interactions with the extra-cellular environment.