Abstract: The fungal strain Aspergillus niger HU53 is capable of producing an acid protease that can be used as a feed additive.In the present study, solid state fermentation conditions for the production of the protease by the fungal strain were optimizedand its enzymological properties were investigated in laboratory. An ideal composition of the solid substrate for fermentationincluded 38.8% wheat bran, 9.7% soybean meal, 1.5% NH4NO3 and 50.0% water with an initial pH of 5.5. On this substrateinoculated with 2.0% (V/W) of a suspension containing 107 conidia/ml, 50h fermentation at 28℃ resulted in a specific activityof 93.8 U/g for the acid protease, a maximum among the various combinations of fermentation conditions. The optimal pH andtemperature for the activity of the acid protease were determined as 3.0 and 50℃, respectively. The Km of the enzyme on caseinwas 2.8016 mg/ml. In the assay of its heat stability, the relative activity of the acid protease remained 93.5% after 180min incubationat 40℃. It was found that the activity of the acid protease could be highly enhanced to 171.9% and 121.1% by 2.0 mmol/L ofCu2+ and Mn2+ but significantly reduced to 62.5% and 44.6% by Fe3+ and Ca2+ at the same concentration.

Key words: Aspergillus niger, acid protease, solid state fermentation