Bioconversion of coffee pulp residues into citric acid using fungi isolated from Tanzania

Coffee pulp residues (CPR) are among the most abundant waste produced during coffee processing thus posing an environmental problem. The objective of this study was to develop a process for bioconversion of Coffee pulp residues into citric acid using fungi isolated locally from the coffee growing areas in Tanzania. The study involved isolation of fungi from coffee processing environment followed by screening for the citric acid production by the isolated fungi using CPR as a substrate. The fungal isolates were identified by amplification and sequencing of the ITS 1- 5.8- ITS 2 region of ribosomal DNA. A total of 9 isolates were found to produce citric acid where the highest amount of citric acid was 6.64 mg/ml. Of the 9 citric acid producing isolates, 5 were identified as Aspergillus niger, 1 as Fusarium sp. and the remaining 3 belonged to other Aspergillus species. Different parameters including pH, nutritional supplements and trace metals controlling substances were analyzed for their potential in citric acid production. Methanol supplementation at varying concentrations improved citric acid production from 6.6 g/l (0% methanol) to 35.5 g/l (3% methanol) whereas addition 50 ppm of Potassium ferrocyanide in the presence of other supplements further enhanced citric acid up to 43.6 mg/l indicating that these compounds play an important role in citric acid production. The overall citric acid production from CPR by the locally isolated fungus Aspergillus niger was greatly enhanced up to 7 fold through optimization of growth conditions. The study pointed out the potential of coffee pulp residues in biotechnology for production of citric acid by using fungi isolated from Tanzania and thus contributing in solving the environmental problem being caused by these wastes while producing a valuable product.