Production of citric asid from renewable oil palm empty fruit bunches by solid state bioconversion using locally isolated apergillus niger

Abstract

The study for the production of citric acid from oil palm empty fruit bunches (EFB) by solid state bioconversion using locally isolated Aspergillus niger was conducted by several approaches. The main objective of the study was to produce citric acid from EFB by solid state bioconversion using locally isolated Aspergillus niger. A total of 29 fungal strains were isolated from two different sources: orange and lemon, among which 21 strains belong to the genera of Aspergillus, five strains to Penicillium and three strains to Rhyzopus. Three strains, i.e. A. niger IBO-103MNB, A. niger IBO- 109MNB and A. niger IBO-114MNB were found to be potential in terms of citric acid production, growth, consumption of sugar and evolution of pH during the bioconversion through the screening test among the 26 strains of A. niger including five strains from laboratory stock. None of the mixed cultures of these three potential strains were found to be compatible in terms of mutual intermingling growth as well as production of citric acid, growth, consumption of sugar, lignocellulolytic enzymes secretion and evolution of pH. The strain A. niger IBO-103MNB was found to be potential for citric acid production by comparing the results of the single culture and their mixed cultures. Particle size of EFB and its distribution showed the important role on improvement of citric acid production. Well-graded particle size distribution of 0.5 mm down graded particle was found to be suitable for the improvement of citric acid production. On the other hand, non-treated EFB was found to be better than the sample treated by H2SO4, NaOH and boiling in water. The optimization results showed that the media constituents and the inoculum dose highly influenced the production of citric acid from EFB. The sucrose content of 6.4% (w/w), mineral content of 9% (v/w) (Zn 8.1, Cu 9, Mn 36 and Mg 450 mg/kg-EFB), methanol concentration of 2% (v/w) and inoculum concentration of 15.5% (v/w) were found to be the optimum conditions for the maximum production of 337.94 g/kg-EFB citric acid. The optimum process conditions: moisture content of 71.70% (v/w), initial pH of 6.0 and incubation temperature of 32.5oC significantly affected the bioconversion process for citric acid production of 370.6 g/kg-EFB. The highest production of citric acid of 417.95±5.33 g/kg-EFB with the productivity of 59.7±0.8 g/kg-EFB-day was obtained after seven days of bioconversion with optimum media constituents and process conditions in 250 ml Erlenmeyer flask. The highest yield of citric acid production based on sugar consumption (YP/S) was 0.774 g/g. It was observed that about 61.6% of cellulose and 33% of hemicellulose were degraded into total sugar of 394.27±4.2 g/kg-EFB which is 39.4% of the total EFB. The highest citric acid of 419.45±6.4 g/kg-EFB can be extracted from the fermented substrate with the optimum agitation of 160 rpm, solvent ratio of 9.6 and agitation duration of 56 minutes. The optimum media and process conditions obtained from flask experiment were employed to optimize the operating conditions of semi-pilot scale production in a horizontal drum bioreactor. The highest production of citric acid of 327.67±3.7 g/kg- EFB was obtained in horizontal drum bioreactor with the continuous supply of saturated air of 8.4 L/min (2.8 L/kg/min) and the agitation of 90 rotation/day (3 times per day with 3 rpm for 10 minutes. The findings of the present study might contribute to the economic development by encouraging for proper management of oil palm industry biomass as well as other lignocellulosic residues.