Isolation of citric acid-producing Aspergillus niger from environmental sources usingCorn starch as a substrate

This study focuses on producing citric acid through fermentation using Aspergillus niger and corn starch as a substrate. Indigenous A. niger strains were isolated from environmental sources, including soil, onion and raw coconut by using Sabouraud’s dextrose agar medium. For this isolation and identification, plating, pure culturing, incubating, staining, biochemical inoculating such techniques were used. The mycelia and cytoplasm were stained using Lactophenol and cotton blue. 21 isolates in total were collected, with 3 (NB07, NB15, NB19) displaying morphological characteristics similar to the reference isolate, A. niger. All isolates demonstrated the ability to produce citric acid, though their production efficiencies varied based on their respective citric acid production indices. Onion sample NB19 proved to be the most effective sampling source for isolating citric acid-producing A. niger strains. Pure cultures of the isolated A. niger were inoculated into flasks containing corn starch at various concentrations and incubated at 28°C for a duration of 144 hours to optimize citric acid production. Qualitative and Quantitative analyses were then performed via addition of bromocresol green and titration to determine citric acid concentration respectively.