Isolation of citric acid-producing fungi and optimization of citric acid production by selected isolates

The aim of the present research work was to screen for fungal isolates with potential for citric acid production. Submerged fermentation was carried out using 1.0%w/v soluble starch in a 250 ml Erlenmeyer flask during the screening. A total of thirty three strains of fungi were isolated. Aspergillus specie EGN006 exhibited the highest citric acid yield of 3.456g/L followed by Aspergillus specie EGN004 with 2.432g/L and Aspergillus specie EGN003 with 2.304g/L after 96 hours incubation period on 1.0%w/v soluble starch. Based on the result of the screening investigations, Aspergillus species EGN003, EGN004 and EGN006 were selected for optimization studies. The citric acid productivity was strongly affected by fermentation conditions. The optimal starch concentration, temperature, pH and fermentation period were 60g/L, 30OC, 5.5 and 144 hours respectively. Under optimal culture conditions, the maximum productivity and yield of citric acid produced by Aspergillus specie EGN006 were 23.261 1.447g/L and 65% respectively. The productivities and yields of Aspergillus species EGN003 and EGN004 were respectively 15.998 2.343g/L, 50% and 19.072 1.327g/L, 54%. Under the same optimum culture conditions, citric acid production by Aspergillus specie EGN006 using cassava starch and fedbatch fermentation were 24.712 2.430g/L and 23.444 1.379g/L respectively. The factors such as energy source, incubation period, initial pH, aeration, agitation and temperature strongly affected citric acid production. Moreover, the results suggest that cassava starch could be potentially utilized in citric acid production. This would stimulate cassava production, increase earnings from it and provide employment opportunities for the teeming unemployed youths