Experimental cookery

"2. Those electrolytes which bring about an irreversible precipita- tion 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 > CI > NO3 > Br > I > SCN with the re- verse 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 A'^ — 1 A^, in higher concentrations the precipitate redissolves, a precipitate appearing again at a concentration of 6 A^. Zinc salts show maximal precipitation at 0.01 N — 0.5 N and again at 4 AT." 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 ( — NH2), and carboxyl ( — COOH), groups. The — NHo groups combine with acids; the — COOH groups combine with alkalies. Most of the — NH2 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 — NH2 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 iso- electric point certain characteristic properties of the protein are at a mini- mal, 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 solu- tion. Other minimum points at higher acidity or alkalinity than the isoelec- tric point are not considered in this discussion, for they are found less frequently in food preparation. 12 RELATION OF COOKERY TO COLLOID CHEMISTRY Combinations o£ proteins with alkalies. At a pR above its iso- electric point the protein combines with alkalies to form such salts as sodium proteinate, calcium proteinate, etc. NH2 NH2 R-C -f NaOH > R-C + H2O COOH COONa protein sodium sodium proteinate water hydroxide Combination o£ 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. NHo NH3CI / / R-C + HCl > R-C COOH COOH protein hydro- protein chloride