Experimental cookery

Freundlich states that hydrophobic colloids and also solids may be peptized by suitable electrolytes, but the process does not take place spon- taneously. It is necessary to divide up mechanically the liquid or solid mass very finely, in order that the charging action of the peptizing ion may be effective. In some liquids it is often suflScient to divide them by energetic stirring. "Salting-out" of hydrophilic colloids occurs at high concentrations. But 22 RELATION OF COOKERY TO COLLOID CHEMISTRY at lower concentrations electrolytes frequently bring about peptization. Freundlich states that this has been investigated particularly in some pro- teins or mixtures of proteins, it being found clearly in the case of globulins. Some globulins remain in solution only in the range of their isoelectric point because of the peptizing effect of electrolytes. Both the concentration of the electrolyte and its valence affect the extent of peptization. In general, the peptizing action is increased with increasing valence. For example, there is little peptization with the chlorine ion, the sulfate ion peptizes in higher concentrations, and the citrate ion brings about peptization with low concentrations. Bound and Free Water 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 press.ure 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 quan- tities 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 orienta- tion 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.