2.4. Assay of proteases activity
Firstly, proteases solutions were prepared at the given concen-tration of 0.5 g/L for alcalase and neutrase, and 0.05 g/L for papain. Then part of proteases solution was treated under different ultra-sonic power and time. Then the activity of proteases before and after ultrasound was determined as follows: casein solutions (1.5%, w/v) as substrates for papain, alcalase and neutrase were prepared in sodium phosphate buffer (pH 7.2) and in lactic acid/-sodium lactate buffer (pH 3.0), respectively. In a reaction vial, 1 mL of the substrate and 1 mL of proteases solution were incu-bated at 37 LC for 10 min and then mixed. After reaction at 37 LC for 60 min, trichloroacetic acid (2 mL, 6.5%, w/v) was added to ter-minate the reaction, and then the mixture was centrifuged at 8000 rpm for 3 min at 4 LC. The supernatant (1 mL) was mixed with sodium carbonate (5 mL, 4%, w/v) and reacted with Folin–Cio-calteu reagent (1 mL, 20%, v/v) at 37 LC for 20 min. Absorbance of the mixture was determined at 660 nm. One unit of protease activ-ity was defined as 100 lg of tyrosine released equivalently by the enzyme [23].