The invention concerns a method for isolation of proteins from whey or soy and for modification of the proteins isolated, according to which method: a) whey or soy or a concentrate thereof is brought into contact with a reagent that forms sulfite ions in order to sulfonate the protein, and optionally b) the sulfonated protein is precipitated at an acid pH, and c) the sulfonated protein or the precipitated and soluble sulfonated protein is recovered and optionally processed further.