Environ Exp Biol (2019) 17: 115–122

A comparative study of citric acid production from different agro-industrial wastes by Aspergillus niger isolated from mangrove forest soil

Abstract

Citric acid is an important organic acid having worldwide demand due to its huge application in the food and pharmaceutical industries. To overcome this increasing demand attempt has been made to use cheap agro-industrial waste products as sources of carbohydrate feedstock for citric acid production by fermentation with the fungus Aspergillus niger. In the present study, 12 fungal isolates were isolated from soil samples collected from the Bhitarkanika mangrove swamps and named as Bhitarkanika Mangrove Fungi BMF-1 to BMF-12. Out of the 12 fungal isolates, three fungal isolates were identified as A. niger. These three fungal isolates were screened for their citric acid production ability and found that the fungal isolate BMF-1 showed the widest yellow zone in Czapek-Dox agar medium, and hence was selected for further citric acid production studies. Fruit peels such as banana peel, rice straw, orange peel and sugarcane bagasses were used for the production of citric acid and compared with the control (sucrose) carbohydrate source of the medium. Maximum citric acid production (0.51%) was obtained with banana peel as a substrate after 6 days of incubation followed by sugarcane bagasses (0.46%), orange peel (0.44%) and rice straw (0.28%) with gradual reduction in pH of the fermentation medium. Maximum reduction in pH (3.3) of the fermentation medium (from initial pH 6.0) was observed after 144 h of incubation with maximum citric acid production by the isolate BMF-1 when banana peel was used as a substrate. To obtain higher citric acid production the fungal growth culture was optimized under different concentrations of banana peel, pH, temperature, nitrogen source, inoculum size and methanol concentration. It has been observed that under optimized condition the production of citric acid was increased from 0.51% to 0.62% which is considered assuitable for citric acid production.