This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Effect of added substances upon swelling of gels. The addition of such substances as acids, alkalies, mineral salts, and sugar may increase or inhibit the degree of swelling. Many such combinations are made in cookery. Lemon juice and vinegar may be added for flavor. Soda, salt, and baking powder may be added to foods. Different proportions of mineral salts are found in foods, and the proportion may vary in the same food owing to many causes. In general, the addition of acids or alkalies increases the swelling of colloidal gels. With acids, this usually continues until a maximum is reached at pH 3.0 to 2.5, when imbibition is decreased with greater acidity. With alkalies the maximum swelling is about pH 10.5, though gluten gels are likely to disintegrate when they become as alkaline as this. Sometimes the addition of acids or alkalies lessens hydration. This depends upon the pH of the substance when the acid or alkali is added. In general, salts lessen the degree of swelling even in the presence of acids or alkalies.
Syneresis. After gels are allowed to stand protected against evaporation for a number of hours there is a tendency for the gel to separate into two phases. A liquid may squeeze out of the gel. A typical example is the separation of the whey from the curd in clabbered milk. It is also noticed in some jellies, cranberry jelly in particular. This separation into a more solid and a more liquid part may take a long time in the case of some food gels, a shorter time in others. In syneresis the liquid part contains a large proportion of the solvent, a smaller proportion of the solid. The more solid part has a high concentration of the solid, a lower one of the solvent.
Hydrophilic and hydrophobic colloids. The terms lyophilic and lyophobic include all dispersing mediums whereas hydrophilic and hydrophobic indicate that the dispersing medium is water. Hydrophilic means "water loving"; hydrophobic means "water hating." The lyophilic colloids belong to the liquid dispersed in liquid systems, though as pointed out by Gortner and by Fischer this terminology is not strictly accurate, for the dispersed phase and dispersing medium are more or less soluble in each other. Thus, with gelatin and water, hydration occurs or the two are mutually more or less soluble in each other. The chief differences between hydrophilic and hydrophobic colloids are their degree of hydration and their reaction to electrolytes. The particles of a hydrophilic colloid require the addition of a large quantity of an electrolyte to bring about coagulation, whereas the hydrophobic colloids are sensitive to, and coagulated by, very small quantities of electrolytes. Gelatin, agar-agar, starch, and protein solutions belong to the hydrophilic group; the metal sols belong to the hydrophobic group. There is no distinct line between the hydrophilic and hydrophobic colloids. Even the particles of the rather typical hydrophilic colloids are not hydrated to the same extent. Thus an agar-agar sol is more strongly hydrated than a gelatin one. Another way of expressing this is to say that about 1 per cent of agar-agar will form a stiff gel, but more than 1 per cent of gelatin is required for a stiff gel.
The charge on colloidal particles. That the micelles possess either a negative or a positive charge is agreed, though the origin of the charge is still disputed. For aqueous solutions the charge is most easily explained on the basis of adsorbed ions. The charge may also come from ionization of the micelle, or by electrification by contact with the dispersing medium, in the same manner that a glass rod becomes charged when rubbed with fur. If the charge on the micelles is reduced to practically zero, the colloidal system becomes unstable. The electrical charge is one important factor in the stabilization of sols. One example in foods is the casein of milk. When the electrical charge of casein reaches zero, the protein flocculates and is precipitated. Kruyt cites it as an example of a protein sol that is not sufficiently hydrated to be stabilized by hydration alone, so that it can exist when negatively or positively charged, but not when the charge is neutralized.
Freundlich uses the term electrokinetic phenomena to designate certain electrical properties of colloidal systems. He also states that these electro-kinetic phenomena are closely associated with the physical properties of interfacial tension, adsorption, colloidal stability, mutual precipitation, and flocculation.
The theories that have been advanced to explain electrokinetic phenomena are based upon the double-layer theory of Helmholtz. This theory is that the micelle is surrounded by a double layer of ions, the inner layer, which may be negative or positive, being closely adsorbed by the micelle, and the outer layer, consisting of ions of opposite charge from those of the inner layer, lying close to the micelles in the intermicellar liquid. If the inner layer of ions is negative, the micelle is negatively charged, the outer layer being positively charged. As the colloid passes through its isoelectric point the charge of each double layer is reversed.
Effect of electrolytes upon hydrophobic colloids. When a hydrophobic colloid is coagulated by an electrolyte its electric charge is removed. The amount of electrolyte required depends upon several factors: (1) The manner of adding. More electrolyte is required if it is added in small portions than if added all at once. (2) The valence of the ion bringing about the coagulation. Coagulation is brought about by the ion having the opposite charge to that of the colloid. As a general rule, the precipitating effect is increased with an increase of valence of the ion bringing about the coagulation. There are exceptions to this rule, as some monovalent ions have greater effect in bringing about coagulation than some polyvalent ions. (3) Concentration of the electrolyte. In many cases there are zones in which a definite concentration of the electrolyte brings about maximum coagulation. Higher or lower concentrations are not so effective or may not bring about coagulation. This is illustrated later in egg cookery. High concentrations of ferric or aluminum chloride do not bring about coagulation of distilled-water custards but small concentrations cause coagulation. (4) The concentration of the colloid also affects the amount of electrolyte required for coagulation. (5) A definite time may be required, depending upon the concentration of the protein and electrolyte.
Effect of electrolytes upon hydrophilic colloids. The effect of electrolytes upon hydrophilic colloids is varied. Kruyt states that the hydro-philic colloids are stabilized by two factors, the electric charge and the strong hydration of the particles, and after the hydrophilic colloid is dehydrated it is as sensitive to electrolytes as the hydrophobic colloids. Dehydration of the hydrophilic colloids may be brought about by different means. Some proteins may be dehydrated by heating. Alcohol may be used to dehydrate hydrophilic colloids, and tannins may also bring about dehydration. This dehydration by heating or other means is called denatur-ation. If an egg white is dialyzed and the electrolytes removed it is not coagulated when heated. But the addition of electrolytes to the heated dialyzed egg white brings about coagulation. Kruyt states that if small quantities of electrolytes are added to a starch or agar-agar sol the electric charge is removed but the colloidal particles do not precipitate. If alcohol, a dehydrating agent, is added to the above starch or agar-agar sol, coagulation occurs. It is immaterial in which order the two stability factors, the electric charge and hydration, are removed. The removal of one has no evident effect, but the removal of both factors causes coagulation. The term denaturation will be used in later references to indicate sensitization of hydrophilic colloids to electrolytes. The term denotes whatever changes are brought about during dehydration of the colloid.