Each colloidal solution as well as each true solution has its own peculiar properties. These depend upon the nature of the particles in solution and the dispersing medium. But a large group of colloidal systems may have similar properties, and for convenience they are classified in a group or subdivision. The classification of colloidal systems into groups is not always satisfactory, for there is no distinct line of demarcation between the different subdivisions. Ostwald, Freundlich, Gortner, and Buchanan and Fulmer give excellent discussions of the properties of colloidal systems which are of interest to those concerned with food preparation.

Suspensoids and emulsoids. One basis for classification of colloidal systems is the nature of the dispersed phase. In a suspensoid the dispersed particles are in a solid state. In an emulsoid the dispersed particles are in a liquid state. Many authorities classify suspensoids and lyophobes, emulsoids and lyophiles, as being coextensive, but Freundlich states that this is in-correct, for there are many emulsoids with lyophobic properties.

Reversible and irreversible colloids. If after a colloidal solution is evaporated, a sol is reformed upon the addition of water, the colloid is classified as a reversible colloid. Gelatin and dried egg white are examples of this type of colloid. An irreversible colloid does not spontaneously form a sol with the addition of water, after water has been evaporated. Reversible and hydrophilic colloids are coextensive; irreversible and hydrophobic colloids belong to similar groups.

Sols and gels. Colloidal solutions are also classified upon the basis of their consistency. Those which are apparently solutions are called sols. Those with a jelly-like consistency are called gels. The consistency of fruit jelly or a gelatin dessert is that of a typical gel. There is no distinct line of separation between sols and gels, nor on the other hand, according to Jordan Lloyd, between gels and curds. The classification of gels on the basis of consistency includes many gels that do not have similar properties. Some gels are formed from sols by coagulation. Custard is an example of this type. A starch gel is formed by gelatinization of the starch during cooking. Gelatin, agar-agar, and soaps form sols above certain temperatures and gels at lower temperatures. Such gels are called reversible. The sol-gel transformation is brought about gradually, there being no definite melting or setting temperature. The change from a sol to a gel by cooling is termed gelation and is a distinct process from that of coagulation. Temperature, time, concentration, and the presence of electrolytes or non-electrolytes are factors in gel formation. There are various theories regarding gel structure, but space does not permit considering them here. It may be added that there is a similarity between crystallization and gelation. Jordan Lloyd states that "gelation is a process closely parallel to crystallization and is accompanied in most cases by evolution of heat." One other factor of similarity is that with rapid cooling a finer structure of the micelles is obtained.

Swelling of colloidal gels. Freundlich designates the gels that may imbibe a liquid and give it up as turgescible, and those which do not swell as non-turgescible. The imbibition of water is known as turgescence; the giving up of water is designated as deturgescence. The term hydration is used to indicate imbibition of water by the micelles; solvation is the general term used for all liquid dispersion mediums. The lyophilic colloids are characterized by their affinity for their dispersion mediums. Whether the micelles of gelatin, agar-agar, etc., actually act as a solvent for the inter-micellar liquid, or whether the liquid is attracted and bound, probably by electrical forces, to the surface of the micelle in the form of a shell is still a disputed question. The amount of water that can be held by many of the micelles in the form of a shell around the particle is relatively enormous. As the concentration of the micelles is increased, the viscosity of the solution increases, owing to a larger portion of the dispersion liquid being bound. A concentration of 0.75 per cent of pectin micelles gives a fruit jelly of good texture. Increasing their concentration gives a stirrer jelly. The molecules of the dispersion medium are probably oriented in a definite manner around the micelle, for the volume of a gelatin gel is less than the combined volume of the dry gelatin and the water.

The ability of the micelles to take up and hold water is important in food preparation. The thickening of a cup of milk by an egg in a custard is due to the ability of the egg proteins to bind the liquid. The thickening power of starches is due to the swelling of the starch granules during heating. The ability of proteins, starches, etc., to imbibe water is very great. A pressure of 2500 atmospheres is required to prevent the swelling of starch when it is heated in water. Gortner defines imbibition pressure as "the pressure against which such a colloid will imbibe a liquid, or conversely the pressure which is required to force the dispersions medium out of a gel. Imbibition pressures should not be confused with osmotic pressure, and in many instances they assume values greatly in excess of values obtainable by osmotic pressure. If a sheet of dried gelatin is placed in a saturated solution of sodium chloride, water will be withdrawn by imbibition forces against the osmotic pressure of the sodium chloride solution, and sodium chloride will crystallize out in the solution."

The micelles possess either a positive or a negative charge. When the electrical forces holding the bound water to the micelle are neutralized, as may happen when electrolytes are added under suitable conditions, the bound water is released and the viscosity of the system decreases. When a custard is curdled the bound water is set free. The ability of muscle fibers to hold water during cooking prevents drying of the meat to a great extent.