A model of the carbohydrate metabolism and the anaplerotic synthesis of oxalacetate in Aspergillus niger, under conditions oc citric acid accumulation, is presented. In this first article we set the stage for subsequent analysis within the framework of the biochemical system theory (BST) : we formulate the model and develop the system representation in power law forms, showing that the steady state is locally stable. In the second article, the control structure of the system is described and a rationale for the optimization of the process is developed,