This thesis examined how dissolved oxygen availability and short aeration interruptions influence citric acid fermentation by Aspergillus niger. Using the NRRL2270 super-producing strain, five submerged shake-flask fermentations were conducted, including a continuously aerated control and four treatments exposed to 30 or 60 minute oxygen interruptions at either 48 or 144 hours. The results showed that uninterrupted aeration produced the highest biomass formation, glucose consumption and citric acid titer, confirming the central role of oxygen in regulating both growth and production. Interruptions at 48 hours, during the exponential phase, caused severe and irreversible reductions in metabolic performance, resulting in low biomass, high residual glucose and minimal citric acid accumulation. Interruptions at 144 hours, during the production phase, produced moderate and largely reversible reductions in efficiency, with the 30 minute stop maintaining high glucose conversion and citric acid yield. These findings demonstrate that oxygen availability is a critical physiological checkpoint and that the timing of oxygen limitation is more influential than its duration in determining fermentation success.