SOLUBILIZATION OF ROCK PHOSPHATE BY IMMOBILIZED ASPERGILLUS-NIGER

Aspergillus niger, an acid-producing filamentous fungus, was immobiliz ed on polyurethane foam. Various amounts of foam cubes and spore suspe nsion were tested in order to obtain an efficient immobilization proce ss. The best combination selected for further experiments was 0.2 g po lyurethane foam and 3 ml spore suspension. Immobilized cells were reus ed, with higher levels of acid formation being maintained for longer p eriods (at least 240 h) than for free cells. The highest titratable ac idity of about 315 mmole/l, was reached with 0.5 cm foam cubes after 3 x 48 h batch cultures in a laboratory shake-flask experiment. Rock ph osphate (2.5 g/l and 5.0 g/l) solubilization was carried out in repeat ed 48 h batch fermentation using A. niger immobilized on polyurethane foam cubes. The greatest accumulation of soluble P, approximately 360 mu g/ml, was obtained after the second batch in flasks with 5 g/l rock phosphate, but process efficiency was higher at lower concentration o f the rock phosphate. After 10 days of culture a total production of 1 .2-1.6 mg/ml soluble P was obtained depending on the concentration of rock phosphate in the medium. (C) 1997 Elsevier Science Ltd.