The knowledge acquisition is considered a critical stage in the process of knowledge based systems development. In this paper, we present the knowledge acquisition approach used in the Nutri-Fuzzy-ORIXÁS. This project aim to study, acquire and manage knowledge and development systems in the nutrients bioavailability area. The process of knowledge acquisition used was proposed in ORIXÁS, a software environment development for knowledge based systems and we introduce an ontology knowledge representation. The ontology has been developed in the Nutri-Fuzzy-ORIXÁS to organize the knowledge and to guide the development of several systems specially the knowledge based system.