Interfacial tension is caused by the repulsion between water and oil. Surface tension corresponds to the tension between water and air. Proteins and other surfactants reduce the tension between these phases because of their afficinity with both water and oil (amphiphilic nature). The reduction of interfacial and surface tension is measured with a tensiometer. Surfactants are used in food systems (emulsions, foams) and no food sytems (detergents, soaps).