The early lines of food work cannot be expressed better than in the words of Ostwald (1922). "Scientific study of the field still contents itself with chapters on analysis and the recognition of adulterants, but chapters dealing with the preparation of food are hardly started. Much as every one would like to obtain better food for less money, study of such questions is regarded as menial and best left to the cook. A scientific study of the preparation of food is considered as only amusing even in scientific circles." Later Ostwald remarks that the chapters that are missing in food preparation are the ones to which colloid chemistry may be applied.

Since so many phases of food preparation are based on colloid chemistry it seems necessary to include in this book a chapter on its relation to cookery. It is a short and brief outline. As such, it can be used as an introduction or as a summary or for both introduction and summary of the work on food preparation, although the subsequent chapters have been written with the idea that at least portions of this chapter will be used in connection with each of them.

Thomas Graham's contributions constitute the foundations of colloid chemistry. His most important results were published between 1861 and 1864. But it is only since the publications of von Weimarn and of Ostwald in 1906 and 1907 that rapid development has been made in colloid chemistry, and it is an even shorter time since the most extensive applications of it have been made to food preparation. Graham used the terms crystalloids and colloids to apply to definite materials. Crystalloids were dialyzable through parchment membrane, whereas colloids were not dialyz-able. The terms are now misleading, for we know that any crystalloid can, by a definite treatment and the selection of the right medium, be brought into the colloidal state. Many of the so-called colloids can be crystallized. The term colloidal state indicates that the material is dispersed in another substance, so that it is preferable to speak of colloidal systems rather than colloids. Colloidal systems differ from molecular ones in the size of the dispersed particles. This difference in size of the particles gives different physical properties to the system.

Bancroft states that "adopting the very flexible definition that a phase is called colloidal when it is sufficiently finely divided, colloid chemistry is the chemistry of bubbles, drops, grains, filaments, and films, because in each of these cases at least one dimension of the phase is very small. This is not a truly scientific classification because a bubble has a film round it, and a film may be considered as made up of coalescing drops or grains." A knowledge of colloid chemistry is necessary to have a real understanding of processes and methods used in a large number of industries and occupations. Bancroft gives a list of 60 such occupations, which include: "cream, butter, cheese, and casein products; cooking and washing."

Classification of Substances Based upon the Degree of Dispersion in Solution

Solutions and suspensions. When a solid substance is added to a liquid one of three types of mixtures may be formed. (1) A true solution, which is a homogeneous mixture. (2) A colloidal solution, which appears to be homogeneous. The dispersed particles have a size of 1 mu to 0.1u and can be separated by a sufficiently fine filter. (3) A suspension in which the dispersed particles are greater than 0.1u. The dispersed particles can be separated from the liquid by filtration or sedimentation. Gortner states, "Inasmuch as fine suspensions possess, to a large degree, certain characteristic properties of colloidal systems, there has been a general tendency of recent years to raise the upper limit to 0.5u," instead of 0.1u. It should be understood that the above classification is an arbitrary one and that in nature there are no abrupt transitions. There can be no definite division made between true and colloidal solutions for the transition is gradual. There are some colloidal solutions known in which the dispersed particles are less than 1 mu. There is also no line of demarcation between colloidal solutions and suspensions.

In true solutions the dispersed phase consists of particles of molecular or ionic size, whereas the colloidal solutions contain particles of larger size, and the suspension contains particles large enough for mechanical separation. The smaller the particles with a definite quantity of material the more dispersed the substance; the larger the particles the less dispersed. A true solution can be reproduced if the temperature, pressure, and concentration are known, since the degree of dispersion is constant. But in colloidal solutions it is necessary to know the degree of dispersion as well as the temperature, pressure, and concentration to reproduce the system. The properties of solutions like gelatin also depend upon the method of preparation and their previous history; the properties of a true solution are independent of the method of preparation or their previous history. The systems whose properties are dependent upon their previous history are said to show hysteresis.

Colloidal systems are heterogeneous, so that it is necessary to distinguish which is the dispersed phase and which is the dispersing medium. Some of the terms used are dispersed phase and dispersing medium . discontinuous phase and continuous phase; internal phase and external phase; and micelles and intermicellar liquid. The usage of the last group appears to be increasing at the present time.

Table 1 Characteristic Differences of

Particles of colloidal size may be in a gaseous, liquid, or a solid state. These micelles may be dispersed in solids, liquids, or gases; and if systems are classified according to the dispersed and dispersing medium there may be a solid in a solid, ruby glass; liquid in a solid, opals; gas in a solid, pumice; solid in a liquid, gold sol; liquid in a liquid, lyophilic colloids; gas in a liquid, whipped cream; solid in a gas, smoke; liquid in a gas, fog.

Sometimes the classification of colloidal systems is based on the dispersed phase only, and this is designated as a solid, liquid, or a gas, according to the material dispersed. Gortner adds emulsions to the above eight systems to be considered in colloidal systems.

Dispersion of substances. It is possible to change particles from molecular size to suspension particles and vice versa. Ostwald states that "it may be accomplished either through the dispersion of nondispersed or coarsely dispersed substances, or through the condensation of molecularly dispersed systems. To these ends not only chemical but mechanical, electrical and other kinds of energy may be used." Water passes from the molecular through the colloidal and into the suspension state in freezing. Von Weimarn states that all crystalline substances pass through a colloidal zone in going into solution and in crystallizing from solution, for during crystallization the size of the particles increases, passing from molecular, through colloidal, to suspension dimensions. This emphasizes the fact that, within each group or class of substances, there may be a wide variation in the degree of dispersion. This dispersion, as in crystallization, may pass through molecular, colloidal, and suspension zones, whereas with other substances there may be wide degrees of dispersion of the substance within one zone. The properties of the systems vary with the size and degree of dispersion of their particles, which affect the results obtained in cookery. The properties of colloidal particles approaching molecular dispersion are different from those of particles approaching the suspension zone. One illustration will be mentioned. The gluten particles of flour have colloidal dimensions. According to Gortner and Doherty, not all gluten particles from different flours are the same size. The gluten particles in pastry flour are more dispersed or of smaller size than those in bread flour. This is one reason for the different results obtained in baked foods when bread flour is used instead of pastry flour. The properties and baking qualities of different flours vary with the size of the gluten particles.

Particles approaching the limits of the size of one zone may show properties of two zones. Thus sugar has a high molecular weight and in cookery shows both molecular and colloidal properties as if it belonged to an in-between group. In gelatin dishes with a definite concentration it increases the stiffness of the gel; in custards it acts like a protective colloid.