Fuente: WIPO "milk" A method of separating / collecting a protein fraction readily reactive with fat / oil from milk by using an ultrafiltration membrane, and a water-soluble fat / oil product using the fraction. In the method, decaseinated butter milk or butter milk powder is ultrafiltrated so that a fraction containing a large amount of proteins having high emulsifying strenght is separated from the decaseinated butter milk or butter milk powder and collected into the reservoir side.