simplest approach should be adjusting the pH to the isoelectric point. Adsorbing surfactants should also work.

Thanks for the reply Mr. Titus Sobisch.

But could you please elaborate your answer.

How the particle will become more hydrophobic if we incubate in a solution having pH same as isoelectric pH of the protein?

I would like to avoid the usage of surfactants too...

At the isoelectric point (charge zero) the proteins are loosing polarity considerably.

In other words charged proteins, i.e. dissociated are much more polar then undissociated (more hydrophobic). Similarly ionised carbonic acids (soaps) are water soluble, undissociated are insoluble in water.

The simplest method is the acylation of amino groups of lysine in the protein. For example, using acetic or propionic anhydride at pH 8-9.