Dissolve it in 50 mM Tris HCL buffer 7.4 pH with continuous stirring over a magnetic stirrer with little heat for 30-40 mins.

use a suitable buffer, depending on the requirement of optimum pH value.The you can also go for the Casein well diffusion method(radial diffusion plate assay) to test the protease activity.

You do not need to dissolve casein at highly alkaline pH. You can solubilize the acidic casein curd with alkali, but manitain the pH at approximately neutral. As the casein dissolves the pH will decline, so you readjust the pH to neutral again until the pH no-longer changes. Alternatively you can dissolve it faster by raising the pH to the highly alkaline side, and when dissolved can carefully readjust the pH to neutral, being careful to avoid local coagulation when you ad the acid.

Thank you all.