This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
"1. Those electrolytes which bring about a reversible precipitation in high concentration. These include the salts of the alkalis, K, Na, NH4, Li and possibly Mg. Ammonium sulphate is commonly used in 'salting out' of proteins. The precipitate so formed will be redis-solved on dilution, i.e., the process is reversible.
"When the protein is on the alkaline side of its isoelectric point (i.e., negatively charged), the order of effectiveness of the salt is on the basis of cations: Li > K > Na > NH4 > Mg and of anions, citrate > tartrate > SO4 > acetate > CL > NO3 > ClO3 > I > SCN.
"2. Those electrolytes which bring about an irreversible precipitation in concentrated solutions. These include the salts of alkaline earths, Sr, Ba, Ca, and possibly Mg.
"The order of effectiveness for negatively charged protein is Ba > Ca > Sr and acetate > CL > NO3 > Br > I > SCN with the reverse order for the positively charged protein.
"3. Those electrolytes which in low concentrations bring about an irreversible precipitation. These include the salts of the heavy metals such as Ag, Hg, Fe, Cu. An interesting characteristic of this group is the fact that two optimal zones may be found. For instance, copper sulphate solutions precipitate albumin in concentrations from 0.001 N - 12V, in higher concentrations the precipitate redissolves, a precipitate appearing again at a concentration of 6 N. Zinc salts show maximal precipitation at 0.01 N - 0.5 N and again at 42V."
Protective and denaturating colloids. A substance that tends to prevent coagulation of micelles is designated as a protector; if it is in the colloidal state it is called a protective colloid. Sometimes small amounts of a colloid sensitize instead of protecting. The latter are sometimes called denaturating colloids.
Amphoteric colloids. Substances that combine with either acids or bases are known as amphoteric substances. Proteins belong to this group. They are composed of amino acids. The amino acids contain amine (- NH1), and carboxyl (- COOH), groups. The - NH1 groups combine with acids; the - COOH groups combine with alkalies. Most of the - NH1 and - COOH groups are linked or bound in forming the protein molecule, but some are free, and combinations with acids and bases are formed with these free groups.
Isoelectric point. At a definite acidity or pH for each protein, there is a point called the isoelectric point. The pH of different proteins at the isoelectric point varies because of the different amino-acid content of each protein, which results in a larger or smaller number of - NH1 or - COOH groups. At the isoelectric point the protein is combined with neither anions nor cations or else it is combined with both equally, for the charge is neutral. Thus at the isoelectric point in a cataphoresis experiment the protein does not migrate to either the anode or cathode. At the isoelectric point certain characteristic properties of the protein are at a minimal, i.e., it is most easily precipitated by electrolytes, is least soluble, shows the least viscosity, is also less dispersed, and least stable as a colloidal solution. Other minimum points at higher acidity or alkalinity than the isoelectric point are not considered in this discussion, for they are found less frequently in food preparation.
Combinations of proteins with alkalies. At a pH above its isoelectric point the protein combines with alkalies to form such salts as sodium proteinate, calcium proteinate, etc.
Combination of proteins with acids. At a pH below the isoelectric point or on the acid side the protein combines with acids to form salts such as protein chlorides. Here the effect is additive and similar to the addition of hydrochloric acid to ammonia to form ammonium chloride.
Combinations of proteins with acids or alkalies in food preparation. Many combinations of proteins with acids or alkalies are formed in food preparation. Most alkaline salts of the proteins are soluble. Some of the acid salts are soluble; others are difficultly soluble. Casein, the protein present in milk in the largest quantity, has a pH of 4.7 at its isoelectric point. Casein in sweet milk is found as an alkaline salt. Fresh milk has a pH of 6 to 7. If an acid is added to the milk the casein will be precipitated when the reaction of the milk reaches the isoelectric point of the casein, pH 4.7. This occurs in natural souring by the formation of lactic acid in the milk. Familiar examples of combinations of acid with milk are the addition of lemon juice to milk for sherbet or the addition of tomatoes to milk for cream of tomato soup. If enough acid is added to lower the reaction of the milk below the isoelectric point of the casein, an acid salt is formed. If this salt is soluble, the curds of casein will dissolve. This change of the protein from an alkaline to an acid salt often occurs in making mayonnaise and other salad dressings. The addition of a small amount of acid to egg yolk will curdle it, but upon the addition of a little more acid the curd may dissolve.
Stoichiometrical combination. Stoichiometrical combination means that the reaction between compounds is carried out according to the laws of valence. Loeb and others working with dilute solutions of proteins, acids, alkalies, and salts showed that proteins combine with acids and alkalies in stoichiometrical relationship. But Hoffman and Gortner have shown that proteins in stronger concentrations of acids or alkalies adsorb acid or alkali. This means that owing to the surface area and the physical property of adsorption the proteins can combine with larger quantities of acids or alkalies than is possible in stoichiometrical combination alone.