The development of a biochemical reactor is a highly demanding task. Bioreactor features are the result of several non-linear interactions insight and between cellular aggregates which depend on the strength of exchange from the abiotic phase to the biotic one. The present paper is devoted to highlight the significance of the macro-approach focusing the attention on the so called “macro-parameters”, but mainly it deals with the application of fuzzy inferences reasoning in order to handle the macro-parameters with modelling purposes. The basis of fuzzy reasoning are put in evidence via a model of a very complex fermentation: citric acid production by Aspergillus niger. Particular attention is devoted to some critical fermentation parameters namely : rheological index, to take into account the rheology of the broth, morphological index to consider the mold`s morphology, and Kolmogorov number as an index of the shear stress effects upon the microorganisms. The reader is gradually introduced to fuzzy reasoning and the above macro-parameters are inferred as antecedents in a fuzzy model to predict the specific productivity of the mold considered as consequent.