Experimental cookery

Particles do not pass through high-grade filter paper Little movement Systems show no measur- able osmotic pressure 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 sys- tems 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. 4 RELATION OF COOKERY TO COLLOID CHEMISTRY 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, elec- trical and other kinds of energy may be used." Water passes from the molec- ular 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, w^hereas with other substances there may be wide degrees of dispersion of the substance within one zone. The properties of the systems vary w^ith the size and degree of dispersion of their particles, which affect the results obtained in cookery. The properties of colloidal particles approaching molecular dis- persion 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 bak- ing qualities of different flours vary with the size of the gluten particles. Particles approaching the limits of the size of one zone may show prop- erties 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. Increasing and Lessening the Degree of Dispersion of Substances in Food Preparation In food preparation many of the methods used and many of the in- gredients added to foods bring about increased or decreased dispersion.