Milk, skim milk, whey, and other milk products can be concentrated, Le., part of the water can be removed. This is applied to diminish the volume and to enhance the keeping quality. Water can be removed from milk by evaporation. In addition to water, volatile substances, especially dissolved gases, are removed as well. Evaporation is usually done under reduced pressure-hence, decreased temperature-to prevent damage caused by heating. Water can also be removed by reverse osmosis, Le., high pressure is applied to pass milk through a suitable membrane. Water as well as part (some 1% to 20%, depending on conditions) of some low molar mass substances pass the membrane. A different way of concentrating is by freezing. The more ice crystals are formed, the higher the dry matter content in the remaining liquid. Removal of water to such a low level that the product becomes solid-like is called drying. Drying is achieved by evaporation of water, usually from concentrated milk.