This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Water plays a very important role in both plant and animal life as a solvent for sugars, electrolytes, etc., and thus in the translocation of food material and metabolism products. But in addition to being a solvent water forms part of the inmost structural portion of the cell. For example, from muscle tissue, although it is composed of more than 65 per cent of water, even with considerable pressure only a few drops of liquid can be pressed. Part of this water is free water, for it contains the dissolved salts, proteins, and other materials. But as the period after death increases, changes occur in the tissue, and greater amounts of liquid can be obtained with pressure. This water, held by the colloidal micelles so that it forms an intimate part of the material, is designated as bound water.
Not only cells of plants and animal tissues, but starches, proteins of flour, gelatin, eggs, and other complex compounds such as lecithin have the capacity to bind water, giving the product certain characteristics. The free water is designated as that portion of the water in which solutes such as sucrose and salt can be dissolved. The bound water is that portion which is held so tightly that not even sucrose will dissolve in it. The density of bound water is so great that some investigators state it is equivalent to having a pressure of 10 thousand atmospheres on it. From this and other properties bound water is often considered as solid water. Bound water has a very low dielectric constant. Burns states, "All the physiological colloids have the property of taking in relatively large quantities of water even against enormous pressures, and of holding this water against even strenuous methods of removal. This 'bound' water stored in the micropores is under considerable compression, so much so that its density and all its physical properties are altered."
The compression of the bound water in bulk is probably due to orientation and packing of the water molecules around the micelles. It has no appreciable vapor pressure and freezes with difficulty or forms such small ice crystals that the biological structure is not injured.
Bound water often requires the application of heat and suction to drive it off. Burns says, "An alumina gel cannot be dried by heating it for 2 or 3 days at 500°C, yet some relationship does exist between the 'free' and 'bound' water. Under certain conditions as yet undefined, bound water may become free again, and the reverse." The greater power of some samples of meat to retain moisture and yet appear firm and less juicy than other samples cooked under the same conditions is known. Ostwald states that pork can be distinguished from other meats by the fact that its water-holding capacity suffers the least change when cooked or dried. In his work with angel cake Barmore speaks of the crumb of cake baked at higher oven temperatures as containing as much moisture but apparently binding the water more securely for it appears more dry.