How to resolve insulin powder?

Recently I've prepared to add insulin solution to the cell culture, but I find insulin is difficult to resolve. Below is my protocol to resolve insulin, could you give me some advise? My protocol stock solution of insulin was prepared in 0.5M Tris-HCl, pH7.0, by suspending 50mg of insulin in 2.5 mL of buffer and adjusting pH to about 3 with 1M HCl to dissolve the insulin. Then the solution was back titrated to pH 7.0 with 1M NaOH (note: the solution becomes cloudy and precipitation appears again, why ?)