The manufacture of virtually all milk and dairy products involves heat treatment. Such treatment is mainly aimed at killing of microorganisms and inactivation of enzymes, or to achieving some other, mainly chemical, changes. The results greatly depend on the intensity of the treatment, Le., the combination of temperature and duration of heating. It is also useful to distinguish between irreversible and reversible changes. The latter are often involved when milk is brought to a high temperature to facilitate some reaction or process, such as renneting of cheese milk, growth of starter organisms, efficiency of water evaporation or centrifugal separation, etc.