Solid-state bioprocessing is a microbial process on a support material in the absence of free water. Among its principal application are found the production of metabolites as enzymes, colorants, bioactive compounds. In the process, it can use a great variety of supports, emphasizing the use of agroindustrial wastes. In the present study, the fungal growth was evaluated by respirometry and the percentage of solids reduction was determined. Also, the recovery of fermented extracts with antimicrobial activity was analyzed. The fermentation was carried out with grapefruit wastes and Aspergillus niger GH1, and the measure of CO₂ production was made by gas chromatography. Results showed a reduction of 40 and 56% of solids for systems of 50 and 70% of humidity, respectively, at the end of the fermentation; besides, ethanolic extracts showed antimicrobial activity.