Decreasing the Non-Fermentable Sugar Concentration in Aspergillus niger by Destroying the α-Glucosidase Genes

Aspergillus niger is the main microbial cell factory for industrial citric acid production, but 2–3% of residual sugar typically remains unutilized in the fermentation broth. To reduce carbon loss caused by residual sugars, seven α-glucosidase genes potentially involved in non-fermentable sugar formation were investigated. In this study, seven α-glucosidase gene-disrupted strains were constructed and were designated as An214233, An119858, An52452, An50927, An50055, An49940, and An40261. Disruption of each gene reduced isomaltose accumulation and increased citric acid production; at the same time, some morphological characteristics of the strain also changed. Among them, the isomaltose content in the fermentation broth of An214233, An50927, An50055, and An40261 decreased by more than 30%, indicating that these genes play important roles in isomaltose formation. However, the reduction in isomaltose content was not directly proportional to the increase in citric acid production. An49940 exhibited the greatest increase in citric acid production despite a relatively moderate reduction of 13.62% in isomaltose content. This strain also exhibited favorable overall performance in total and reducing sugar utilization, together with smaller mycelial pellets and thicker hyphae. These results indicate that citric acid production is influenced not only by non-fermentable sugar formation but also by sugar utilization and mycelial morphology, thereby providing a theoretical basis and potential genetic targets for reducing carbon source loss, improving citric acid production, and decreasing residual sugars in industrial fermentation.