Production of Citric Acid by Local Strains of Aspergillus niger using Pineapple Peels as Substrate

This study was conducted to screen for citric acid production by local strains of Aspergillus niger isolated from soils of four locations within Ahmadu Bello University, Zaria, Nigeria namely: botanical garden, refuse dumpsite, flower bed and sheep pen sites. Proximate compositions of the pineapple peel were determined using standard procedures described by the Association of Official Analytical Chemists (AOAC). A total of sixteen (16) soil samples were collected from the different locations and stock suspensions were prepared before being separately diluted serially from 10-1 to 104. Aliquots of each suspension were separately inoculated onto Potato Dextrose Agar (PDA) and incubated at room temperature (25°C) for seven (7) days for the isolation of Aspergillus niger. Colonies suspected to be Aspergillus niger were characterized macroscopically and then microscopically using lactophenol cotton blue-staining preparations. The seven isolates identified were then screened on a Czapek-Dox agar medium for potential citric acid production. The isolates were further subjected to citric acid production by submerged fermentation using pineapple peels as the substrate. The isolates confirmed to be Aspergillus niger had percentage occurrences of 25%, 100% and 50% from sheep pen, flower bed and botanical garden sites respectively. No Aspergillus niger was isolated from refuse dumpsite soil. Isolate BGS3 (from botanical garden soil) produced the highest yellow zone of citric acid production (78.5mm) during screening, whereas, isolate SPS (from sheep pen soil) showed the lowest (41.5mm) potential. During production, an overall yield of 0.76g/100ml was obtained using pineapple peel as substrate. Aspergillus niger can be easily isolated from various soil types with highest frequency in soils from sheep pen. The research revealed the potential of various Aspergillus niger isolates from different soil to produced citric acid using pineapple peels (agricultural waste) as substrate.