The present paper aims to formalize the use of a macro-approach to describe the kinetics of complex (bio)reactions. The reasons for a holistic approach are outlined in order to propose the fuzzy modelling as a mathematical framework able to handle the holistic parameters in bioreactor description. Two examples are discussed to highlight the peculiarities and the power of the methodology. Particular attention is devoted to a comparison of the traditional approach and the fuzzy one to describe the kinetic behaviour of a biocatalyst in a bioreactor.