Protease activity was determined by the casein digestion method of Drapeau (1976). The reaction mixture consisted of 2.5 mL of 1% (w/v) casein prepared in 0.01 N NAOH, 0.05M tris phosphate buffer (pH 7.8) and 0.1 mL tissue homogenate. The reaction mixture was incubated at 37 °C for 15min. The reaction was stopped by adding 2.5mL, 10% trichloroacetic acid (TCA) and the whole content were filtered. The reagent blank was made by adding tissue homogenate just before stopping the reaction and without incubation. The absorbance was recorded at 280 nm. The protease activity was determined from the tyrosine standard curve and expressed as micromole of tyrosine released mg−1 protein min−1 at 37 °C.