Fluidity and viscosity. Bingham uses the term fluidity to express the opposite of viscosity. A fluid like water yields readily to any force that tends to change its form, whereas a viscous substance shows some resistance to flow. Viscosity is one of the important properties of colloidal systems. As a general rule, the lyophobic colloids show a viscosity but little greater than that of the dispersion medium, the viscosity increasing only slightly with increasing concentration of the micelles. But the lyophilic colloids may show very high viscosities or even plasticity with very low concentrations of the micelles.

Bingham states that "a mixture of liquids may have an indefinite number of fluidities dependent upon the method of mixing, in other words, upon the structure of the liquid." He also states that colloidal solutions show differences in fluidity due to differences in structure. Thus it is possible that cake or other batters made with the same materials and the same proportion of materials may show differences in the structure of the finished cake on account of different methods of mixing, giving different viscosities to the batter.

Some substances flow readily; others resist flow; and some must have weight applied to start flow. When a substance tends to resist a shearing force it may exhibit a flow that is characterized as viscous, turbulent, or plastic. If the substance entirely regains its original shape, when the shearing stress is removed, it shows perfect elasticity. If the original shape is not entirely regained and the substance is deformed to an extent directly proportional to the shearing force, then the substance is said to show viscosity. This flow that is directly proportional to the shearing force is called linear flow. By this is meant that if a weight of 1 pound produces a definite deformation, a weight of 2 pounds produces twice that deformation. Turbulent flow is the flow obtained when the ratio of the shearing force to the deformation decreases.

A pure liquid at a given temperature and pressure has a definite fluidity. The viscosity of water is approximately six times as great at 0° as at 100°C. The viscosity of sols usually decreases with an increase in ternperature, part of this being due to the effect of temperature upon the intermicellar liquid. Gortner states that in "colloid systems changes due to temperature are influenced not only by the viscosity of the dispersion medium but likewise by the effect of temperature on solvation." Thus gelatin and agar-agar form sols with rather low viscosity at high temperatures when compared to the viscous liquid or plastic gels they form at low temperatures. Starch usually forms a suspension at low temperatures, and its decided increase in viscosity or plasticity comes with rapid hydration at the gelatinization point. Gortner states that heating a starch paste beyond the gelatinization temperature causes a decrease in viscosity or plasticity. Electrolytes added to lyophilic systems, often even in traces, cause great changes in the viscosity of the sol.

The factors affecting the viscosity of lyophilic systems. Gort-ner adds an eleventh factor, that of rate of shear, pointed out by Sharp and Gortner, to the ten given by Ostwald that cause variation in the viscosity of lyophilic systems. They are as follows: (1) concentration, (2) temperature, (3) degree of dispersion, (4) solvation, (5) electrical charge, (6) previous thermal treatment, (7) previous mechanical treatment, (8) the presence or absence of other lyophilic colloids, (9) the age of the lyophilic sol, (10) the presence of both electrolytes and non-electrolytes, and (11) the rate of shear.

Viscosity is closely related to the consistency of the finished product in food preparation. So close is this relation in many cases that the ten factors listed by Ostwald may nearly be taken as ten commandments of food preparation. Thus the consistency of a custard is influenced by the concentration of egg or the protein micelles; the temperature to which it is cooked; the degree of dispersion of the micelles, which is influenced by the reaction and other factors; the degree of hydration, which is influenced by reaction, the kind and concentration of salts present, etc.; the beating of the egg; the use of milk or water; how long the custard has aged in addition to the age of the eggs and milk when used; the kinds and concentration of salts in the egg and milk as well as the addition of sodium chloride and the non-electrolyte sugar.

Since the line of demarcation between sols and gels is not a definite one, fruit jellies, gelatin, milk, cream, as well as egg dishes, may be added to the group of foods in which the consistency of the finished product is related to viscosity. But this does not end the application, for the structure or type of product in baked goods is closely related to the viscosity of the batter or dough, which in turn is influenced by all these factors. Of course these factors or nearly the same ones affect other properties as well as viscosity of food materials. Thus the extensibility of gluten, the heat coagulation of proteins, etc., are influenced by many or all of these factors.

Plasticity. Bingham defines plasticity as "a property of solids in virtue of which they hold their shape permanently under the action of small shearing stresses but they are readily deformed, worked or molded, under somewhat larger stresses. Plasticity is thus a complex property, made up of two independent factors, which we must evaluate separately." Modeling clay is plastic. Plasticity is an important property of fats used for cakes, biscuits, and pastry. A plastic fat has a consistency such that it will form a thin sheet or layer in a batter or it will retain air bubbles when "creamed." The enclosing of these air bubbles in the fat is an aid in leavening cakes and may assist in obtaining a velvety texture, for the enclosing of the air renders the fat more plastic, thus more easily distributed in the batter at lower temperatures.