ABSTRACT
Milk, skim milk, whey, and other milk products can be concentrated, i.e., part of the water can be removed. The main purpose is 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, i.e., high pressure is applied to a solution to pass its water through a suitable membrane (see Section 12.3). Water as well as part (1% to 20%, depending on conditions) of some low-molar-mass substances pass through the membrane. A different method of concentrating is by freezing (see Section 11.2); 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 vaporization of water, usually from concentrated milk.
