Abstract:
Due to its extensive use in the food and pharmaceutical industries, citric acid is an essential organic acid that is in high demand around the world. To meet this increasing demand, an effort has been made to use inexpensive agro-industrial waste products as carbohydrate sources for the production of citric acid using Aspergillus niger. Therefore, the present study was performed to produce citric acid from avocado (Persea americana) peels as a novel substrate through solid state fermentation using A. niger. Isolation of A.niger from soil was carried out on potato dextrose agar. Both the macroscopic and microscopic features were taken into account for their identification. Six A.niger isolates that were tentatively designated as A. niger ASNT-1 to ASNT 4 for those obtained from Tony Farm’s soil and ASND-1 and ASND-2 for those recovered from soil between buildings of Dire Dawa University student’s dormitory were screened qualitatively for citric acid production on Czapek dox agar supplemented with Bromocerol green indicator. Among the fungal isolates, isolate ASNT-4 showed the widest yellow halo zone and was selected for further quantitative estimation studies of citric acid production. The effect of chemical pretreatments was evaluated using the isolate A. niger ASNT-4 and dilute acid pretreated substrates gave high yield of citric acid (16.42 0.28 g/L). Thus, dilute sulphuric acid pretreated substrate was used for further optimization study of the effect of different substrate concentration (10 g, 20 g, 30 g and 40 g), temperature (25 0 c, 30 0 c and 35 0 c), and incubation time (1, 2, 3, 4, 5, 6, 7, 8 and 9 days) on citric acid production using A. niger ASNT-4 by squential one factor at a time (OFAT) design. The maximum amount of citric acid production was reclorded in substrate concentration (30 g) at 30 0 c incubation temperature and incubation time 7 days (45.01±0.80 g/L). Finally, the quality of the product was assessed using its melting point determination and Fourier transform infrared spectroscopy characterization, and it was confirmed that the product was of high purity. Thus, use of inexpensive agro-waste avocado peels which serve as suitable substrate for citric acid production is advocated for its benefits such as revenue generation, reduction of environmental problems caused by its disposal.