Protein solubility Proteins can be either positively (cation) or negatively (anion) charged based on pH conditions. When the positive and negative charges on protein are equal, the net charge is zero. The characteristic pH of a solution at which the net charge on protein is zero (positive and negative charges are equal) is defined as the isoelectric point (pH). The isoelectric point of a protein is an important property because it is at this point that the protein is least soluble, and therefore unstable. It should be noted that both below and above the isoelectric point (isoelectric pH) the protein will be soluble. To understand the implication of pH, let us consider an example. Suppose we have a white wine with a pH of 3.30 and a protein fraction with a pH of 3.2. After blending with another wine, the pH of the new blend is changed to 3.2. Notice that the pH of the blend and the pH of the protein are the same. Since the protein is in a pH solution similar to its pH value, it will become insoluble and thus unstable. The blend could therefore be protein unstable even though the wine was stable before