This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
In food preparation many of the methods used and many of the ingredients added to foods bring about increased or decreased dispersion.
Heat. Increasing the temperature may bring about a greater or a lesser degree of dispersion. The heating of water increases its dispersion. The dispersion of fat globules in milk is increased by the application of heat to the milk, but when proteins are coagulated by heat the degree of dispersion is decreased.
Mechanical dispersion. Beating of a food is a mechanical means of bringing about a greater or a lesser degree of dispersion. In cookery, stirring may bring about more uniform distribution of particles in a food, like white sauce. But even this may prevent lumping or clumping and thus is a means of bringing about dispersion, just as beating a curdled custard is a mechanical means of bringing about a greater degree of dispersion of a suspension, but although it reduces the size of the curds it does not reduce them enough to change the suspension to the colloidal state. Homogenization of milk or cream is a mechanical means of increasing the dispersion of fat particles. Beating an egg white is a mechanical means of lessening the degree of dispersion, as it brings about partial coagulation of the egg white in the cell walls surrounding the air bubbles.
Dispersion by acids. The addition or the development of acid in preparation of food may be a chemical means of bringing about a greater or a lesser degree of dispersion. For instance, the development of acid during the fermentation of bread brings about greater dispersion of the gluten. The addition of acid to whole egg tends to curdle the egg or bring about a lessened degree of dispersion. If a large enough quantity of acid is added, the degree of dispersion may be increased after passing through a zone of lesser dispersion. The explanation of the different effects of acid on the gluten and on the egg proteins depends upon the isoelectric point of the gluten and the egg protein and the degree of acidity developed. This will be considered later.
Dispersion by alkalies. The addition of alkalies may also tend to bring about a greater or a lesser degree of dispersion. In quick breads and cakes the addition of soda in excess of the amount required to neutralize the acidity of the mixture may bring about increased dispersion of the gluten particles, which results in a definite grain or crumb.
The different methods of bringing about a lesser or a greater degree of dispersion are applicable to all food products. In some cases the effect may be modified by other factors. It is interesting to trace these through different classes of food products. If alkali is taken as an example, its effects on some foods may be cited. The alkalies, hydroxides of ammonia, sodium, and potassium or their basic salts are the ones considered. Calcium and magnesium salts may have different effects.
Sucrose is little affected by alkalies if they are not stronger than those ordinarily used in cookery. But when alkalies are added to the mono-saccharid sugars they bring about decomposition. The sugar in solution is in the molecular state, and the alkalies cause increased dispersion, even though the sugar and its decomposition products both remain in molecular dispersion. The properties of the decomposition products are different from those of the original sugar.
Vegetables and fruits are softened by the addition of alkalies during cooking, and they may become mushy and disintegrate. This is probably due to the greater dispersion of the cellulose and pectic substances. In dried legumes, alkalies may also increase the disintegration of some of the protein.
Milk is prevented from curdling or coagulating by the addition of alkali. Curdling is a lessened dispersion of the milk protein, casein. The addition of alkalies to eggs elevates the temperature for coagulation. Alkalies added to doughs cause a greater degree of dispersion of the gluten, which results in a dough that is runny and sticky to handle. In larger quantities the baking quality of the flour is partially destroyed. Alkalies added to gelatin tend to prevent its setting, and they may cause greater dispersion in emulsions.
Dispersion by enzymes. Enzymes may also cause an increased or lessened degree of dispersion in foods. The clotting of milk upon the addition of rennin is an example of lessened dispersion, but the proteinase enzyme in flour increases the dispersion of the gluten.