Experimental cookery

Neurath and Bull state that both heat and surface denaturation proc- esses involve an unfolding of the peptide chains which in the natural state are curled up in the interior of the molecule and become stretched out when the molecule comes in contact with the surface of the bulk of the solution. The polar groups of the protein molecule, the amino, carboxyl, the OH groups of the hydroxy acids, the sulfur-containing groups, and the peptide linkages, have an affinity for water; whereas the non-polar or lyophobic groups, the hydrocarbon residues, tend to be repelled by water. Thinking that an interaction between the amino and carboxjd groups during heat denaturation might diminish the lyophobic or polar properties of natural protein, whereas an unfolding of the peptide chains by surface denaturation might expose lyophobic groups to the surface, which in the native state are buried in the interior, Neurath and Bull measured the volume contraction of native, heat-denatured, and surface-denatured pro- PEPTIZATION OF PROTEINS 21 tei'ns. They found that the native protein had the lowest density, heat- denatured ones were intermediate, and the surface-denatured protein had the highest density. This membrane-forming property of protein through denaturation is important in food preparation, in all products in which beaten egg white is used, in emulsions, and wherever interfacial reactions occur. — — ^_ Membranes form readily on the surface of protoplasm and play Impor- tant parts in cell functions. The presence of calcium has been shown to stiffen the surface membranes in some instances, whereas sodium and potassium in the absence of calcium tend to soften and dissolve the membrane. This suggests that salts may also have some influence in surface denatura- tion and that the salts of flour, egg, and milk used in cooked products may modify the denaturation at surfaces. Clayton states that high concentrations of sugar in egg white will pre- vent surface denaturation, which of course has application in making angel cakes, meringues, and sweetened souffles. Peptization of Proteins Peptization is the reverse process of coagulation. It increases dispersion and solubility. Means of bringing about peptization. Peptization may be brought about by chemical, electrical, and mechanical means or by enzymes. Freund- lich states that the hydroxyl ion is generally a very effective peptizer. Other peptizing ions used in food preparation are the citrate, acetate, and tartrate ions. Peptization brought about by adsorption has been mentioned. Peptization of proteins. The results of Gortner, Hoffman, and Sin- clair show that different salts added to wheat proteins in varying amounts to give the same pH, or in equivalent concentrations, cause peptization and solution of varying amounts of the proteins. Both cations and anions form a lyotropic series. They found the anions arranged in the following order of increasing peptization : F < SO4 < Ql < tartrate < Br < I ; and for the cations the following order of increasing peptization : Na < K < Li < Ba < Sr < Mg < Ca. Most of their salt solution extracts of flour had a />H of 5.0 to 6.0. This would be on the acid side of the isoelectric point of the flour proteins, and they would be positively charged. When the solubility of wheat proteins is increased, the tenderness of the result- ing bread is increased.