You can use dimethyl sulfoxide (DMSO). 

You can just the pH of casein suspension to  alkalinity

Thank you Abu Sayed. How much dimethyl sulfoxide is used to prepare 1liter sample.

Thank you Shenghua He. Can I dissolve casein powder directly into water and then increased the pH around 8.0 by adding NaOH. Is this sample used for curd formation?

You can use 20% DMSO. Actually, I dont know the objectives of your experiment. There are so many techniques to dissolve casein.  You can make 0.1 M NaoH and use this solution to dissolve the casein. In addition, you can add some amount of CaCl2 to dissolve casein completely by keeping it 4-5 hours at room temperature.

It depends on the use you will do to the casein solutions. An alternative to alkalyne sol. and DMSO is the use of ureia solutions, even lower concentrations are effective. 

you can add casein powder to water and raise the pH to 8 and after the solubility of casein powder you can decrease the pH to 6.8 using HCl. Or you can use add urea or SDS to  casein . 

Dear Sultan

The general commercial process to convert casein to soluble sodium caseinate (which is the common form used in dairy/food product applications) is to make up a 10 - 15% solution of casein in water and neutralise to pH6.8 - 7.0 using 5 - 10% NaOH with good agitation at 50 - 60C. NOte that at 15% TS sodium caseinate has a high viscosity (especially at lower temperatures).

I trust this helps.

Rgds